Merge pull request #19296 from arjunroy/grpc_zcp

gRPC TCP Send Zerocopy

include/grpc/impl/codegen/grpc_types.h

@@ -323,6 +323,20 @@ typedef struct {
   "grpc.experimental.tcp_min_read_chunk_size"
 #define GRPC_ARG_TCP_MAX_READ_CHUNK_SIZE \
   "grpc.experimental.tcp_max_read_chunk_size"
+/* TCP TX Zerocopy enable state: zero is disabled, non-zero is enabled. By
+   default, it is disabled. */
+#define GRPC_ARG_TCP_TX_ZEROCOPY_ENABLED \
+  "grpc.experimental.tcp_tx_zerocopy_enabled"
+/* TCP TX Zerocopy send threshold: only zerocopy if >= this many bytes sent. By
+   default, this is set to 16KB. */
+#define GRPC_ARG_TCP_TX_ZEROCOPY_SEND_BYTES_THRESHOLD \
+  "grpc.experimental.tcp_tx_zerocopy_send_bytes_threshold"
+/* TCP TX Zerocopy max simultaneous sends: limit for maximum number of pending
+   calls to tcp_write() using zerocopy. A tcp_write() is considered pending
+   until the kernel performs the zerocopy-done callback for all sendmsg() calls
+   issued by the tcp_write(). By default, this is set to 4. */
+#define GRPC_ARG_TCP_TX_ZEROCOPY_MAX_SIMULT_SENDS \
+  "grpc.experimental.tcp_tx_zerocopy_max_simultaneous_sends"
 /* Timeout in milliseconds to use for calls to the grpclb load balancer.
    If 0 or unset, the balancer calls will have no deadline. */
 #define GRPC_ARG_GRPCLB_CALL_TIMEOUT_MS "grpc.grpclb_call_timeout_ms"

src/core/lib/iomgr/socket_utils_common_posix.cc

@@ -50,6 +50,20 @@
 #include "src/core/lib/iomgr/sockaddr.h"
 #include "src/core/lib/iomgr/sockaddr_utils.h"
 
+/* set a socket to use zerocopy */
+grpc_error* grpc_set_socket_zerocopy(int fd) {
+#ifdef GRPC_LINUX_ERRQUEUE
+  const int enable = 1;
+  auto err = setsockopt(fd, SOL_SOCKET, SO_ZEROCOPY, &enable, sizeof(enable));
+  if (err != 0) {
+    return GRPC_OS_ERROR(errno, "setsockopt(SO_ZEROCOPY)");
+  }
+  return GRPC_ERROR_NONE;
+#else
+  return GRPC_OS_ERROR(ENOSYS, "setsockopt(SO_ZEROCOPY)");
+#endif
+}
+
 /* set a socket to non blocking mode */
 grpc_error* grpc_set_socket_nonblocking(int fd, int non_blocking) {
   int oldflags = fcntl(fd, F_GETFL, 0);

src/core/lib/iomgr/socket_utils_posix.h

@@ -31,10 +31,22 @@
 #include "src/core/lib/iomgr/socket_factory_posix.h"
 #include "src/core/lib/iomgr/socket_mutator.h"
 
+#ifdef GRPC_LINUX_ERRQUEUE
+#ifndef SO_ZEROCOPY
+#define SO_ZEROCOPY 60
+#endif
+#ifndef SO_EE_ORIGIN_ZEROCOPY
+#define SO_EE_ORIGIN_ZEROCOPY 5
+#endif
+#endif /* ifdef GRPC_LINUX_ERRQUEUE */
+
 /* a wrapper for accept or accept4 */
 int grpc_accept4(int sockfd, grpc_resolved_address* resolved_addr, int nonblock,
                  int cloexec);
 
+/* set a socket to use zerocopy */
+grpc_error* grpc_set_socket_zerocopy(int fd);
+
 /* set a socket to non blocking mode */
 grpc_error* grpc_set_socket_nonblocking(int fd, int non_blocking);
 

src/core/lib/iomgr/tcp_posix.cc

@@ -36,6 +36,7 @@
 #include <sys/types.h>
 #include <unistd.h>
 #include <algorithm>
+#include <unordered_map>
 
 #include <grpc/slice.h>
 #include <grpc/support/alloc.h>
@@ -49,9 +50,11 @@
 #include "src/core/lib/debug/trace.h"
 #include "src/core/lib/gpr/string.h"
 #include "src/core/lib/gpr/useful.h"
+#include "src/core/lib/gprpp/sync.h"
 #include "src/core/lib/iomgr/buffer_list.h"
 #include "src/core/lib/iomgr/ev_posix.h"
 #include "src/core/lib/iomgr/executor.h"
+#include "src/core/lib/iomgr/socket_utils_posix.h"
 #include "src/core/lib/profiling/timers.h"
 #include "src/core/lib/slice/slice_internal.h"
 #include "src/core/lib/slice/slice_string_helpers.h"
@@ -71,6 +74,15 @@
 #define SENDMSG_FLAGS 0
 #endif
 
+// TCP zero copy sendmsg flag.
+// NB: We define this here as a fallback in case we're using an older set of
+// library headers that has not defined MSG_ZEROCOPY. Since this constant is
+// part of the kernel, we are guaranteed it will never change/disagree so
+// defining it here is safe.
+#ifndef MSG_ZEROCOPY
+#define MSG_ZEROCOPY 0x4000000
+#endif
+
 #ifdef GRPC_MSG_IOVLEN_TYPE
 typedef GRPC_MSG_IOVLEN_TYPE msg_iovlen_type;
 #else
@@ -79,6 +91,264 @@ typedef size_t msg_iovlen_type;
 
 extern grpc_core::TraceFlag grpc_tcp_trace;
 
+namespace grpc_core {
+
+class TcpZerocopySendRecord {
+ public:
+  TcpZerocopySendRecord() { grpc_slice_buffer_init(&buf_); }
+
+  ~TcpZerocopySendRecord() {
+    AssertEmpty();
+    grpc_slice_buffer_destroy_internal(&buf_);
+  }
+
+  // Given the slices that we wish to send, and the current offset into the
+  //   slice buffer (indicating which have already been sent), populate an iovec
+  //   array that will be used for a zerocopy enabled sendmsg().
+  msg_iovlen_type PopulateIovs(size_t* unwind_slice_idx,
+                               size_t* unwind_byte_idx, size_t* sending_length,
+                               iovec* iov);
+
+  // A sendmsg() may not be able to send the bytes that we requested at this
+  // time, returning EAGAIN (possibly due to backpressure). In this case,
+  // unwind the offset into the slice buffer so we retry sending these bytes.
+  void UnwindIfThrottled(size_t unwind_slice_idx, size_t unwind_byte_idx) {
+    out_offset_.byte_idx = unwind_byte_idx;
+    out_offset_.slice_idx = unwind_slice_idx;
+  }
+
+  // Update the offset into the slice buffer based on how much we wanted to sent
+  // vs. what sendmsg() actually sent (which may be lower, possibly due to
+  // backpressure).
+  void UpdateOffsetForBytesSent(size_t sending_length, size_t actually_sent);
+
+  // Indicates whether all underlying data has been sent or not.
+  bool AllSlicesSent() { return out_offset_.slice_idx == buf_.count; }
+
+  // Reset this structure for a new tcp_write() with zerocopy.
+  void PrepareForSends(grpc_slice_buffer* slices_to_send) {
+    AssertEmpty();
+    out_offset_.slice_idx = 0;
+    out_offset_.byte_idx = 0;
+    grpc_slice_buffer_swap(slices_to_send, &buf_);
+    Ref();
+  }
+
+  // References: 1 reference per sendmsg(), and 1 for the tcp_write().
+  void Ref() { ref_.FetchAdd(1, MemoryOrder::RELAXED); }
+
+  // Unref: called when we get an error queue notification for a sendmsg(), if a
+  //  sendmsg() failed or when tcp_write() is done.
+  bool Unref() {
+    const intptr_t prior = ref_.FetchSub(1, MemoryOrder::ACQ_REL);
+    GPR_DEBUG_ASSERT(prior > 0);
+    if (prior == 1) {
+      AllSendsComplete();
+      return true;
+    }
+    return false;
+  }
+
+ private:
+  struct OutgoingOffset {
+    size_t slice_idx = 0;
+    size_t byte_idx = 0;
+  };
+
+  void AssertEmpty() {
+    GPR_DEBUG_ASSERT(buf_.count == 0);
+    GPR_DEBUG_ASSERT(buf_.length == 0);
+    GPR_DEBUG_ASSERT(ref_.Load(MemoryOrder::RELAXED) == 0);
+  }
+
+  // When all sendmsg() calls associated with this tcp_write() have been
+  // completed (ie. we have received the notifications for each sequence number
+  // for each sendmsg()) and all reference counts have been dropped, drop our
+  // reference to the underlying data since we no longer need it.
+  void AllSendsComplete() {
+    GPR_DEBUG_ASSERT(ref_.Load(MemoryOrder::RELAXED) == 0);
+    grpc_slice_buffer_reset_and_unref_internal(&buf_);
+  }
+
+  grpc_slice_buffer buf_;
+  Atomic<intptr_t> ref_;
+  OutgoingOffset out_offset_;
+};
+
+class TcpZerocopySendCtx {
+ public:
+  static constexpr int kDefaultMaxSends = 4;
+  static constexpr size_t kDefaultSendBytesThreshold = 16 * 1024;  // 16KB
+
+  TcpZerocopySendCtx(int max_sends = kDefaultMaxSends,
+                     size_t send_bytes_threshold = kDefaultSendBytesThreshold)
+      : max_sends_(max_sends),
+        free_send_records_size_(max_sends),
+        threshold_bytes_(send_bytes_threshold) {
+    send_records_ = static_cast<TcpZerocopySendRecord*>(
+        gpr_malloc(max_sends * sizeof(*send_records_)));
+    free_send_records_ = static_cast<TcpZerocopySendRecord**>(
+        gpr_malloc(max_sends * sizeof(*free_send_records_)));
+    if (send_records_ == nullptr || free_send_records_ == nullptr) {
+      gpr_free(send_records_);
+      gpr_free(free_send_records_);
+      gpr_log(GPR_INFO, "Disabling TCP TX zerocopy due to memory pressure.\n");
+      memory_limited_ = true;
+    } else {
+      for (int idx = 0; idx < max_sends_; ++idx) {
+        new (send_records_ + idx) TcpZerocopySendRecord();
+        free_send_records_[idx] = send_records_ + idx;
+      }
+    }
+  }
+
+  ~TcpZerocopySendCtx() {
+    if (send_records_ != nullptr) {
+      for (int idx = 0; idx < max_sends_; ++idx) {
+        send_records_[idx].~TcpZerocopySendRecord();
+      }
+    }
+    gpr_free(send_records_);
+    gpr_free(free_send_records_);
+  }
+
+  // True if we were unable to allocate the various bookkeeping structures at
+  // transport initialization time. If memory limited, we do not zerocopy.
+  bool memory_limited() const { return memory_limited_; }
+
+  // TCP send zerocopy maintains an implicit sequence number for every
+  // successful sendmsg() with zerocopy enabled; the kernel later gives us an
+  // error queue notification with this sequence number indicating that the
+  // underlying data buffers that we sent can now be released. Once that
+  // notification is received, we can release the buffers associated with this
+  // zerocopy send record. Here, we associate the sequence number with the data
+  // buffers that were sent with the corresponding call to sendmsg().
+  void NoteSend(TcpZerocopySendRecord* record) {
+    record->Ref();
+    AssociateSeqWithSendRecord(last_send_, record);
+    ++last_send_;
+  }
+
+  // If sendmsg() actually failed, though, we need to revert the sequence number
+  // that we speculatively bumped before calling sendmsg(). Note that we bump
+  // this sequence number and perform relevant bookkeeping (see: NoteSend())
+  // *before* calling sendmsg() since, if we called it *after* sendmsg(), then
+  // there is a possible race with the release notification which could occur on
+  // another thread before we do the necessary bookkeeping. Hence, calling
+  // NoteSend() *before* sendmsg() and implementing an undo function is needed.
+  void UndoSend() {
+    --last_send_;
+    if (ReleaseSendRecord(last_send_)->Unref()) {
+      // We should still be holding the ref taken by tcp_write().
+      GPR_DEBUG_ASSERT(0);
+    }
+  }
+
+  // Simply associate this send record (and the underlying sent data buffers)
+  // with the implicit sequence number for this zerocopy sendmsg().
+  void AssociateSeqWithSendRecord(uint32_t seq, TcpZerocopySendRecord* record) {
+    MutexLock guard(&lock_);
+    ctx_lookup_.emplace(seq, record);
+  }
+
+  // Get a send record for a send that we wish to do with zerocopy.
+  TcpZerocopySendRecord* GetSendRecord() {
+    MutexLock guard(&lock_);
+    return TryGetSendRecordLocked();
+  }
+
+  // A given send record corresponds to a single tcp_write() with zerocopy
+  // enabled. This can result in several sendmsg() calls to flush all of the
+  // data to wire. Each sendmsg() takes a reference on the
+  // TcpZerocopySendRecord, and corresponds to a single sequence number.
+  // ReleaseSendRecord releases a reference on TcpZerocopySendRecord for a
+  // single sequence number. This is called either when we receive the relevant
+  // error queue notification (saying that we can discard the underlying
+  // buffers for this sendmsg()) is received from the kernel - or, in case
+  // sendmsg() was unsuccessful to begin with.
+  TcpZerocopySendRecord* ReleaseSendRecord(uint32_t seq) {
+    MutexLock guard(&lock_);
+    return ReleaseSendRecordLocked(seq);
+  }
+
+  // After all the references to a TcpZerocopySendRecord are released, we can
+  // add it back to the pool (of size max_sends_). Note that we can only have
+  // max_sends_ tcp_write() instances with zerocopy enabled in flight at the
+  // same time.
+  void PutSendRecord(TcpZerocopySendRecord* record) {
+    GPR_DEBUG_ASSERT(record >= send_records_ &&
+                     record < send_records_ + max_sends_);
+    MutexLock guard(&lock_);
+    PutSendRecordLocked(record);
+  }
+
+  // Indicate that we are disposing of this zerocopy context. This indicator
+  // will prevent new zerocopy writes from being issued.
+  void Shutdown() { shutdown_.Store(true, MemoryOrder::RELEASE); }
+
+  // Indicates that there are no inflight tcp_write() instances with zerocopy
+  // enabled.
+  bool AllSendRecordsEmpty() {
+    MutexLock guard(&lock_);
+    return free_send_records_size_ == max_sends_;
+  }
+
+  bool enabled() const { return enabled_; }
+
+  void set_enabled(bool enabled) {
+    GPR_DEBUG_ASSERT(!enabled || !memory_limited());
+    enabled_ = enabled;
+  }
+
+  // Only use zerocopy if we are sending at least this many bytes. The
+  // additional overhead of reading the error queue for notifications means that
+  // zerocopy is not useful for small transfers.
+  size_t threshold_bytes() const { return threshold_bytes_; }
+
+ private:
+  TcpZerocopySendRecord* ReleaseSendRecordLocked(uint32_t seq) {
+    auto iter = ctx_lookup_.find(seq);
+    GPR_DEBUG_ASSERT(iter != ctx_lookup_.end());
+    TcpZerocopySendRecord* record = iter->second;
+    ctx_lookup_.erase(iter);
+    return record;
+  }
+
+  TcpZerocopySendRecord* TryGetSendRecordLocked() {
+    if (shutdown_.Load(MemoryOrder::ACQUIRE)) {
+      return nullptr;
+    }
+    if (free_send_records_size_ == 0) {
+      return nullptr;
+    }
+    free_send_records_size_--;
+    return free_send_records_[free_send_records_size_];
+  }
+
+  void PutSendRecordLocked(TcpZerocopySendRecord* record) {
+    GPR_DEBUG_ASSERT(free_send_records_size_ < max_sends_);
+    free_send_records_[free_send_records_size_] = record;
+    free_send_records_size_++;
+  }
+
+  TcpZerocopySendRecord* send_records_;
+  TcpZerocopySendRecord** free_send_records_;
+  int max_sends_;
+  int free_send_records_size_;
+  Mutex lock_;
+  uint32_t last_send_ = 0;
+  Atomic<bool> shutdown_;
+  bool enabled_ = false;
+  size_t threshold_bytes_ = kDefaultSendBytesThreshold;
+  std::unordered_map<uint32_t, TcpZerocopySendRecord*> ctx_lookup_;
+  bool memory_limited_ = false;
+};
+
+}  // namespace grpc_core
+
+using grpc_core::TcpZerocopySendCtx;
+using grpc_core::TcpZerocopySendRecord;
+
 namespace {
 struct grpc_tcp {
   grpc_endpoint base;
@@ -142,6 +412,8 @@ struct grpc_tcp {
   bool ts_capable;        /* Cache whether we can set timestamping options */
   gpr_atm stop_error_notification; /* Set to 1 if we do not want to be notified
                                       on errors anymore */
+  TcpZerocopySendCtx tcp_zerocopy_send_ctx;
+  TcpZerocopySendRecord* current_zerocopy_send = nullptr;
 };
 
 struct backup_poller {
@@ -151,6 +423,8 @@ struct backup_poller {
 
 }  // namespace
 
+static void ZerocopyDisableAndWaitForRemaining(grpc_tcp* tcp);
+
 #define BACKUP_POLLER_POLLSET(b) ((grpc_pollset*)((b) + 1))
 
 static gpr_atm g_uncovered_notifications_pending;
@@ -339,6 +613,7 @@ static void tcp_handle_write(void* arg /* grpc_tcp */, grpc_error* error);
 
 static void tcp_shutdown(grpc_endpoint* ep, grpc_error* why) {
   grpc_tcp* tcp = reinterpret_cast<grpc_tcp*>(ep);
+  ZerocopyDisableAndWaitForRemaining(tcp);
   grpc_fd_shutdown(tcp->em_fd, why);
   grpc_resource_user_shutdown(tcp->resource_user);
 }
@@ -357,6 +632,7 @@ static void tcp_free(grpc_tcp* tcp) {
   gpr_mu_unlock(&tcp->tb_mu);
   tcp->outgoing_buffer_arg = nullptr;
   gpr_mu_destroy(&tcp->tb_mu);
+  tcp->tcp_zerocopy_send_ctx.~TcpZerocopySendCtx();
   gpr_free(tcp);
 }
 
@@ -390,6 +666,7 @@ static void tcp_destroy(grpc_endpoint* ep) {
   grpc_tcp* tcp = reinterpret_cast<grpc_tcp*>(ep);
   grpc_slice_buffer_reset_and_unref_internal(&tcp->last_read_buffer);
   if (grpc_event_engine_can_track_errors()) {
+    ZerocopyDisableAndWaitForRemaining(tcp);
     gpr_atm_no_barrier_store(&tcp->stop_error_notification, true);
     grpc_fd_set_error(tcp->em_fd);
   }
@@ -652,13 +929,13 @@ static void tcp_read(grpc_endpoint* ep, grpc_slice_buffer* incoming_buffer,
 
 /* A wrapper around sendmsg. It sends \a msg over \a fd and returns the number
  * of bytes sent. */
-ssize_t tcp_send(int fd, const struct msghdr* msg) {
+ssize_t tcp_send(int fd, const struct msghdr* msg, int additional_flags = 0) {
   GPR_TIMER_SCOPE("sendmsg", 1);
   ssize_t sent_length;
   do {
     /* TODO(klempner): Cork if this is a partial write */
     GRPC_STATS_INC_SYSCALL_WRITE();
-    sent_length = sendmsg(fd, msg, SENDMSG_FLAGS);
+    sent_length = sendmsg(fd, msg, SENDMSG_FLAGS | additional_flags);
   } while (sent_length < 0 && errno == EINTR);
   return sent_length;
 }
@@ -671,16 +948,52 @@ ssize_t tcp_send(int fd, const struct msghdr* msg) {
  */
 static bool tcp_write_with_timestamps(grpc_tcp* tcp, struct msghdr* msg,
                                       size_t sending_length,
-                                      ssize_t* sent_length);
+                                      ssize_t* sent_length,
+                                      int additional_flags = 0);
 
 /** The callback function to be invoked when we get an error on the socket. */
 static void tcp_handle_error(void* arg /* grpc_tcp */, grpc_error* error);
 
+static TcpZerocopySendRecord* tcp_get_send_zerocopy_record(
+    grpc_tcp* tcp, grpc_slice_buffer* buf);
+
 #ifdef GRPC_LINUX_ERRQUEUE
+static bool process_errors(grpc_tcp* tcp);
+
+static TcpZerocopySendRecord* tcp_get_send_zerocopy_record(
+    grpc_tcp* tcp, grpc_slice_buffer* buf) {
+  TcpZerocopySendRecord* zerocopy_send_record = nullptr;
+  const bool use_zerocopy =
+      tcp->tcp_zerocopy_send_ctx.enabled() &&
+      tcp->tcp_zerocopy_send_ctx.threshold_bytes() < buf->length;
+  if (use_zerocopy) {
+    zerocopy_send_record = tcp->tcp_zerocopy_send_ctx.GetSendRecord();
+    if (zerocopy_send_record == nullptr) {
+      process_errors(tcp);
+      zerocopy_send_record = tcp->tcp_zerocopy_send_ctx.GetSendRecord();
+    }
+    if (zerocopy_send_record != nullptr) {
+      zerocopy_send_record->PrepareForSends(buf);
+      GPR_DEBUG_ASSERT(buf->count == 0);
+      GPR_DEBUG_ASSERT(buf->length == 0);
+      tcp->outgoing_byte_idx = 0;
+      tcp->outgoing_buffer = nullptr;
+    }
+  }
+  return zerocopy_send_record;
+}
+
+static void ZerocopyDisableAndWaitForRemaining(grpc_tcp* tcp) {
+  tcp->tcp_zerocopy_send_ctx.Shutdown();
+  while (!tcp->tcp_zerocopy_send_ctx.AllSendRecordsEmpty()) {
+    process_errors(tcp);
+  }
+}
 
 static bool tcp_write_with_timestamps(grpc_tcp* tcp, struct msghdr* msg,
                                       size_t sending_length,
-                                      ssize_t* sent_length) {
+                                      ssize_t* sent_length,
+                                      int additional_flags) {
   if (!tcp->socket_ts_enabled) {
     uint32_t opt = grpc_core::kTimestampingSocketOptions;
     if (setsockopt(tcp->fd, SOL_SOCKET, SO_TIMESTAMPING,
@@ -708,7 +1021,7 @@ static bool tcp_write_with_timestamps(grpc_tcp* tcp, struct msghdr* msg,
   msg->msg_controllen = CMSG_SPACE(sizeof(uint32_t));
 
   /* If there was an error on sendmsg the logic in tcp_flush will handle it. */
-  ssize_t length = tcp_send(tcp->fd, msg);
+  ssize_t length = tcp_send(tcp->fd, msg, additional_flags);
   *sent_length = length;
   /* Only save timestamps if all the bytes were taken by sendmsg. */
   if (sending_length == static_cast<size_t>(length)) {
@@ -722,6 +1035,43 @@ static bool tcp_write_with_timestamps(grpc_tcp* tcp, struct msghdr* msg,
   return true;
 }
 
+static void UnrefMaybePutZerocopySendRecord(grpc_tcp* tcp,
+                                            TcpZerocopySendRecord* record,
+                                            uint32_t seq, const char* tag);
+// Reads \a cmsg to process zerocopy control messages.
+static void process_zerocopy(grpc_tcp* tcp, struct cmsghdr* cmsg) {
+  GPR_DEBUG_ASSERT(cmsg);
+  auto serr = reinterpret_cast<struct sock_extended_err*>(CMSG_DATA(cmsg));
+  GPR_DEBUG_ASSERT(serr->ee_errno == 0);
+  GPR_DEBUG_ASSERT(serr->ee_origin == SO_EE_ORIGIN_ZEROCOPY);
+  const uint32_t lo = serr->ee_info;
+  const uint32_t hi = serr->ee_data;
+  for (uint32_t seq = lo; seq <= hi; ++seq) {
+    // TODO(arjunroy): It's likely that lo and hi refer to zerocopy sequence
+    // numbers that are generated by a single call to grpc_endpoint_write; ie.
+    // we can batch the unref operation. So, check if record is the same for
+    // both; if so, batch the unref/put.
+    TcpZerocopySendRecord* record =
+        tcp->tcp_zerocopy_send_ctx.ReleaseSendRecord(seq);
+    GPR_DEBUG_ASSERT(record);
+    UnrefMaybePutZerocopySendRecord(tcp, record, seq, "CALLBACK RCVD");
+  }
+}
+
+// Whether the cmsg received from error queue is of the IPv4 or IPv6 levels.
+static bool CmsgIsIpLevel(const cmsghdr& cmsg) {
+  return (cmsg.cmsg_level == SOL_IPV6 && cmsg.cmsg_type == IPV6_RECVERR) ||
+         (cmsg.cmsg_level == SOL_IP && cmsg.cmsg_type == IP_RECVERR);
+}
+
+static bool CmsgIsZeroCopy(const cmsghdr& cmsg) {
+  if (!CmsgIsIpLevel(cmsg)) {
+    return false;
+  }
+  auto serr = reinterpret_cast<const sock_extended_err*> CMSG_DATA(&cmsg);
+  return serr->ee_errno == 0 && serr->ee_origin == SO_EE_ORIGIN_ZEROCOPY;
+}
+
 /** Reads \a cmsg to derive timestamps from the control messages. If a valid
  * timestamp is found, the traced buffer list is updated with this timestamp.
  * The caller of this function should be looping on the control messages found
@@ -783,73 +1133,76 @@ struct cmsghdr* process_timestamp(grpc_tcp* tcp, msghdr* msg,
 /** For linux platforms, reads the socket's error queue and processes error
  * messages from the queue.
  */
-static void process_errors(grpc_tcp* tcp) {
+static bool process_errors(grpc_tcp* tcp) {
+  bool processed_err = false;
+  struct iovec iov;
+  iov.iov_base = nullptr;
+  iov.iov_len = 0;
+  struct msghdr msg;
+  msg.msg_name = nullptr;
+  msg.msg_namelen = 0;
+  msg.msg_iov = &iov;
+  msg.msg_iovlen = 0;
+  msg.msg_flags = 0;
+  /* Allocate enough space so we don't need to keep increasing this as size
+   * of OPT_STATS increase */
+  constexpr size_t cmsg_alloc_space =
+      CMSG_SPACE(sizeof(grpc_core::scm_timestamping)) +
+      CMSG_SPACE(sizeof(sock_extended_err) + sizeof(sockaddr_in)) +
+      CMSG_SPACE(32 * NLA_ALIGN(NLA_HDRLEN + sizeof(uint64_t)));
+  /* Allocate aligned space for cmsgs received along with timestamps */
+  union {
+    char rbuf[cmsg_alloc_space];
+    struct cmsghdr align;
+  } aligned_buf;
+  msg.msg_control = aligned_buf.rbuf;
+  msg.msg_controllen = sizeof(aligned_buf.rbuf);
+  int r, saved_errno;
   while (true) {
-    struct iovec iov;
-    iov.iov_base = nullptr;
-    iov.iov_len = 0;
-    struct msghdr msg;
-    msg.msg_name = nullptr;
-    msg.msg_namelen = 0;
-    msg.msg_iov = &iov;
-    msg.msg_iovlen = 0;
-    msg.msg_flags = 0;
-
-    /* Allocate enough space so we don't need to keep increasing this as size
-     * of OPT_STATS increase */
-    constexpr size_t cmsg_alloc_space =
-        CMSG_SPACE(sizeof(grpc_core::scm_timestamping)) +
-        CMSG_SPACE(sizeof(sock_extended_err) + sizeof(sockaddr_in)) +
-        CMSG_SPACE(32 * NLA_ALIGN(NLA_HDRLEN + sizeof(uint64_t)));
-    /* Allocate aligned space for cmsgs received along with timestamps */
-    union {
-      char rbuf[cmsg_alloc_space];
-      struct cmsghdr align;
-    } aligned_buf;
-    memset(&aligned_buf, 0, sizeof(aligned_buf));
-
-    msg.msg_control = aligned_buf.rbuf;
-    msg.msg_controllen = sizeof(aligned_buf.rbuf);
-
-    int r, saved_errno;
     do {
       r = recvmsg(tcp->fd, &msg, MSG_ERRQUEUE);
       saved_errno = errno;
     } while (r < 0 && saved_errno == EINTR);
 
     if (r == -1 && saved_errno == EAGAIN) {
-      return; /* No more errors to process */
+      return processed_err; /* No more errors to process */
     }
     if (r == -1) {
-      return;
+      return processed_err;
     }
-    if ((msg.msg_flags & MSG_CTRUNC) != 0) {
+    if (GPR_UNLIKELY((msg.msg_flags & MSG_CTRUNC) != 0)) {
       gpr_log(GPR_ERROR, "Error message was truncated.");
     }
 
     if (msg.msg_controllen == 0) {
       /* There was no control message found. It was probably spurious. */
-      return;
+      return processed_err;
     }
     bool seen = false;
     for (auto cmsg = CMSG_FIRSTHDR(&msg); cmsg && cmsg->cmsg_len;
          cmsg = CMSG_NXTHDR(&msg, cmsg)) {
-      if (cmsg->cmsg_level != SOL_SOCKET ||
-          cmsg->cmsg_type != SCM_TIMESTAMPING) {
-        /* Got a control message that is not a timestamp. Don't know how to
-         * handle this. */
+      if (CmsgIsZeroCopy(*cmsg)) {
+        process_zerocopy(tcp, cmsg);
+        seen = true;
+        processed_err = true;
+      } else if (cmsg->cmsg_level == SOL_SOCKET &&
+                 cmsg->cmsg_type == SCM_TIMESTAMPING) {
+        cmsg = process_timestamp(tcp, &msg, cmsg);
+        seen = true;
+        processed_err = true;
+      } else {
+        /* Got a control message that is not a timestamp or zerocopy. Don't know
+         * how to handle this. */
         if (GRPC_TRACE_FLAG_ENABLED(grpc_tcp_trace)) {
           gpr_log(GPR_INFO,
                   "unknown control message cmsg_level:%d cmsg_type:%d",
                   cmsg->cmsg_level, cmsg->cmsg_type);
         }
-        return;
+        return processed_err;
       }
-      cmsg = process_timestamp(tcp, &msg, cmsg);
-      seen = true;
     }
     if (!seen) {
-      return;
+      return processed_err;
     }
   }
 }
@@ -870,18 +1223,28 @@ static void tcp_handle_error(void* arg /* grpc_tcp */, grpc_error* error) {
 
   /* We are still interested in collecting timestamps, so let's try reading
    * them. */
-  process_errors(tcp);
+  bool processed = process_errors(tcp);
   /* This might not a timestamps error. Set the read and write closures to be
    * ready. */
-  grpc_fd_set_readable(tcp->em_fd);
-  grpc_fd_set_writable(tcp->em_fd);
+  if (!processed) {
+    grpc_fd_set_readable(tcp->em_fd);
+    grpc_fd_set_writable(tcp->em_fd);
+  }
   grpc_fd_notify_on_error(tcp->em_fd, &tcp->error_closure);
 }
 
 #else  /* GRPC_LINUX_ERRQUEUE */
+static TcpZerocopySendRecord* tcp_get_send_zerocopy_record(
+    grpc_tcp* tcp, grpc_slice_buffer* buf) {
+  return nullptr;
+}
+
+static void ZerocopyDisableAndWaitForRemaining(grpc_tcp* tcp) {}
+
 static bool tcp_write_with_timestamps(grpc_tcp* /*tcp*/, struct msghdr* /*msg*/,
                                       size_t /*sending_length*/,
-                                      ssize_t* /*sent_length*/) {
+                                      ssize_t* /*sent_length*/,
+                                      int /*additional_flags*/) {
   gpr_log(GPR_ERROR, "Write with timestamps not supported for this platform");
   GPR_ASSERT(0);
   return false;
@@ -907,12 +1270,138 @@ void tcp_shutdown_buffer_list(grpc_tcp* tcp) {
   }
 }
 
-/* returns true if done, false if pending; if returning true, *error is set */
 #if defined(IOV_MAX) && IOV_MAX < 1000
 #define MAX_WRITE_IOVEC IOV_MAX
 #else
 #define MAX_WRITE_IOVEC 1000
 #endif
+msg_iovlen_type TcpZerocopySendRecord::PopulateIovs(size_t* unwind_slice_idx,
+                                                    size_t* unwind_byte_idx,
+                                                    size_t* sending_length,
+                                                    iovec* iov) {
+  msg_iovlen_type iov_size;
+  *unwind_slice_idx = out_offset_.slice_idx;
+  *unwind_byte_idx = out_offset_.byte_idx;
+  for (iov_size = 0;
+       out_offset_.slice_idx != buf_.count && iov_size != MAX_WRITE_IOVEC;
+       iov_size++) {
+    iov[iov_size].iov_base =
+        GRPC_SLICE_START_PTR(buf_.slices[out_offset_.slice_idx]) +
+        out_offset_.byte_idx;
+    iov[iov_size].iov_len =
+        GRPC_SLICE_LENGTH(buf_.slices[out_offset_.slice_idx]) -
+        out_offset_.byte_idx;
+    *sending_length += iov[iov_size].iov_len;
+    ++(out_offset_.slice_idx);
+    out_offset_.byte_idx = 0;
+  }
+  GPR_DEBUG_ASSERT(iov_size > 0);
+  return iov_size;
+}
+
+void TcpZerocopySendRecord::UpdateOffsetForBytesSent(size_t sending_length,
+                                                     size_t actually_sent) {
+  size_t trailing = sending_length - actually_sent;
+  while (trailing > 0) {
+    size_t slice_length;
+    out_offset_.slice_idx--;
+    slice_length = GRPC_SLICE_LENGTH(buf_.slices[out_offset_.slice_idx]);
+    if (slice_length > trailing) {
+      out_offset_.byte_idx = slice_length - trailing;
+      break;
+    } else {
+      trailing -= slice_length;
+    }
+  }
+}
+
+// returns true if done, false if pending; if returning true, *error is set
+static bool do_tcp_flush_zerocopy(grpc_tcp* tcp, TcpZerocopySendRecord* record,
+                                  grpc_error** error) {
+  struct msghdr msg;
+  struct iovec iov[MAX_WRITE_IOVEC];
+  msg_iovlen_type iov_size;
+  ssize_t sent_length = 0;
+  size_t sending_length;
+  size_t unwind_slice_idx;
+  size_t unwind_byte_idx;
+  while (true) {
+    sending_length = 0;
+    iov_size = record->PopulateIovs(&unwind_slice_idx, &unwind_byte_idx,
+                                    &sending_length, iov);
+    msg.msg_name = nullptr;
+    msg.msg_namelen = 0;
+    msg.msg_iov = iov;
+    msg.msg_iovlen = iov_size;
+    msg.msg_flags = 0;
+    bool tried_sending_message = false;
+    // Before calling sendmsg (with or without timestamps): we
+    // take a single ref on the zerocopy send record.
+    tcp->tcp_zerocopy_send_ctx.NoteSend(record);
+    if (tcp->outgoing_buffer_arg != nullptr) {
+      if (!tcp->ts_capable ||
+          !tcp_write_with_timestamps(tcp, &msg, sending_length, &sent_length,
+                                     MSG_ZEROCOPY)) {
+        /* We could not set socket options to collect Fathom timestamps.
+         * Fallback on writing without timestamps. */
+        tcp->ts_capable = false;
+        tcp_shutdown_buffer_list(tcp);
+      } else {
+        tried_sending_message = true;
+      }
+    }
+    if (!tried_sending_message) {
+      msg.msg_control = nullptr;
+      msg.msg_controllen = 0;
+      GRPC_STATS_INC_TCP_WRITE_SIZE(sending_length);
+      GRPC_STATS_INC_TCP_WRITE_IOV_SIZE(iov_size);
+      sent_length = tcp_send(tcp->fd, &msg, MSG_ZEROCOPY);
+    }
+    if (sent_length < 0) {
+      // If this particular send failed, drop ref taken earlier in this method.
+      tcp->tcp_zerocopy_send_ctx.UndoSend();
+      if (errno == EAGAIN) {
+        record->UnwindIfThrottled(unwind_slice_idx, unwind_byte_idx);
+        return false;
+      } else if (errno == EPIPE) {
+        *error = tcp_annotate_error(GRPC_OS_ERROR(errno, "sendmsg"), tcp);
+        tcp_shutdown_buffer_list(tcp);
+        return true;
+      } else {
+        *error = tcp_annotate_error(GRPC_OS_ERROR(errno, "sendmsg"), tcp);
+        tcp_shutdown_buffer_list(tcp);
+        return true;
+      }
+    }
+    tcp->bytes_counter += sent_length;
+    record->UpdateOffsetForBytesSent(sending_length,
+                                     static_cast<size_t>(sent_length));
+    if (record->AllSlicesSent()) {
+      *error = GRPC_ERROR_NONE;
+      return true;
+    }
+  }
+}
+
+static void UnrefMaybePutZerocopySendRecord(grpc_tcp* tcp,
+                                            TcpZerocopySendRecord* record,
+                                            uint32_t seq, const char* tag) {
+  if (record->Unref()) {
+    tcp->tcp_zerocopy_send_ctx.PutSendRecord(record);
+  }
+}
+
+static bool tcp_flush_zerocopy(grpc_tcp* tcp, TcpZerocopySendRecord* record,
+                               grpc_error** error) {
+  bool done = do_tcp_flush_zerocopy(tcp, record, error);
+  if (done) {
+    // Either we encountered an error, or we successfully sent all the bytes.
+    // In either case, we're done with this record.
+    UnrefMaybePutZerocopySendRecord(tcp, record, 0, "flush_done");
+  }
+  return done;
+}
+
 static bool tcp_flush(grpc_tcp* tcp, grpc_error** error) {
   struct msghdr msg;
   struct iovec iov[MAX_WRITE_IOVEC];
@@ -927,7 +1416,7 @@ static bool tcp_flush(grpc_tcp* tcp, grpc_error** error) {
   // buffer as we write
   size_t outgoing_slice_idx = 0;
 
-  for (;;) {
+  while (true) {
     sending_length = 0;
     unwind_slice_idx = outgoing_slice_idx;
     unwind_byte_idx = tcp->outgoing_byte_idx;
@@ -1027,12 +1516,21 @@ static void tcp_handle_write(void* arg /* grpc_tcp */, grpc_error* error) {
   if (error != GRPC_ERROR_NONE) {
     cb = tcp->write_cb;
     tcp->write_cb = nullptr;
+    if (tcp->current_zerocopy_send != nullptr) {
+      UnrefMaybePutZerocopySendRecord(tcp, tcp->current_zerocopy_send, 0,
+                                      "handle_write_err");
+      tcp->current_zerocopy_send = nullptr;
+    }
     grpc_core::Closure::Run(DEBUG_LOCATION, cb, GRPC_ERROR_REF(error));
     TCP_UNREF(tcp, "write");
     return;
   }
 
-  if (!tcp_flush(tcp, &error)) {
+  bool flush_result =
+      tcp->current_zerocopy_send != nullptr
+          ? tcp_flush_zerocopy(tcp, tcp->current_zerocopy_send, &error)
+          : tcp_flush(tcp, &error);
+  if (!flush_result) {
     if (GRPC_TRACE_FLAG_ENABLED(grpc_tcp_trace)) {
       gpr_log(GPR_INFO, "write: delayed");
     }
@@ -1042,6 +1540,7 @@ static void tcp_handle_write(void* arg /* grpc_tcp */, grpc_error* error) {
   } else {
     cb = tcp->write_cb;
     tcp->write_cb = nullptr;
+    tcp->current_zerocopy_send = nullptr;
     if (GRPC_TRACE_FLAG_ENABLED(grpc_tcp_trace)) {
       const char* str = grpc_error_string(error);
       gpr_log(GPR_INFO, "write: %s", str);
@@ -1057,6 +1556,7 @@ static void tcp_write(grpc_endpoint* ep, grpc_slice_buffer* buf,
   GPR_TIMER_SCOPE("tcp_write", 0);
   grpc_tcp* tcp = reinterpret_cast<grpc_tcp*>(ep);
   grpc_error* error = GRPC_ERROR_NONE;
+  TcpZerocopySendRecord* zerocopy_send_record = nullptr;
 
   if (GRPC_TRACE_FLAG_ENABLED(grpc_tcp_trace)) {
     size_t i;
@@ -1073,8 +1573,8 @@ static void tcp_write(grpc_endpoint* ep, grpc_slice_buffer* buf,
   }
 
   GPR_ASSERT(tcp->write_cb == nullptr);
+  GPR_DEBUG_ASSERT(tcp->current_zerocopy_send == nullptr);
 
-  tcp->outgoing_buffer_arg = arg;
   if (buf->length == 0) {
     grpc_core::Closure::Run(
         DEBUG_LOCATION, cb,
@@ -1085,15 +1585,26 @@ static void tcp_write(grpc_endpoint* ep, grpc_slice_buffer* buf,
     tcp_shutdown_buffer_list(tcp);
     return;
   }
-  tcp->outgoing_buffer = buf;
-  tcp->outgoing_byte_idx = 0;
+
+  zerocopy_send_record = tcp_get_send_zerocopy_record(tcp, buf);
+  if (zerocopy_send_record == nullptr) {
+    // Either not enough bytes, or couldn't allocate a zerocopy context.
+    tcp->outgoing_buffer = buf;
+    tcp->outgoing_byte_idx = 0;
+  }
+  tcp->outgoing_buffer_arg = arg;
   if (arg) {
     GPR_ASSERT(grpc_event_engine_can_track_errors());
   }
 
-  if (!tcp_flush(tcp, &error)) {
+  bool flush_result =
+      zerocopy_send_record != nullptr
+          ? tcp_flush_zerocopy(tcp, zerocopy_send_record, &error)
+          : tcp_flush(tcp, &error);
+  if (!flush_result) {
     TCP_REF(tcp, "write");
     tcp->write_cb = cb;
+    tcp->current_zerocopy_send = zerocopy_send_record;
     if (GRPC_TRACE_FLAG_ENABLED(grpc_tcp_trace)) {
       gpr_log(GPR_INFO, "write: delayed");
     }
@@ -1121,6 +1632,7 @@ static void tcp_add_to_pollset_set(grpc_endpoint* ep,
 static void tcp_delete_from_pollset_set(grpc_endpoint* ep,
                                         grpc_pollset_set* pollset_set) {
   grpc_tcp* tcp = reinterpret_cast<grpc_tcp*>(ep);
+  ZerocopyDisableAndWaitForRemaining(tcp);
   grpc_pollset_set_del_fd(pollset_set, tcp->em_fd);
 }
 
@@ -1172,9 +1684,15 @@ static const grpc_endpoint_vtable vtable = {tcp_read,
 grpc_endpoint* grpc_tcp_create(grpc_fd* em_fd,
                                const grpc_channel_args* channel_args,
                                const char* peer_string) {
+  static constexpr bool kZerocpTxEnabledDefault = false;
   int tcp_read_chunk_size = GRPC_TCP_DEFAULT_READ_SLICE_SIZE;
   int tcp_max_read_chunk_size = 4 * 1024 * 1024;
   int tcp_min_read_chunk_size = 256;
+  bool tcp_tx_zerocopy_enabled = kZerocpTxEnabledDefault;
+  int tcp_tx_zerocopy_send_bytes_thresh =
+      grpc_core::TcpZerocopySendCtx::kDefaultSendBytesThreshold;
+  int tcp_tx_zerocopy_max_simult_sends =
+      grpc_core::TcpZerocopySendCtx::kDefaultMaxSends;
   grpc_resource_quota* resource_quota = grpc_resource_quota_create(nullptr);
   if (channel_args != nullptr) {
     for (size_t i = 0; i < channel_args->num_args; i++) {
@@ -1199,6 +1717,23 @@ grpc_endpoint* grpc_tcp_create(grpc_fd* em_fd,
         resource_quota =
             grpc_resource_quota_ref_internal(static_cast<grpc_resource_quota*>(
                 channel_args->args[i].value.pointer.p));
+      } else if (0 == strcmp(channel_args->args[i].key,
+                             GRPC_ARG_TCP_TX_ZEROCOPY_ENABLED)) {
+        tcp_tx_zerocopy_enabled = grpc_channel_arg_get_bool(
+            &channel_args->args[i], kZerocpTxEnabledDefault);
+      } else if (0 == strcmp(channel_args->args[i].key,
+                             GRPC_ARG_TCP_TX_ZEROCOPY_SEND_BYTES_THRESHOLD)) {
+        grpc_integer_options options = {
+            grpc_core::TcpZerocopySendCtx::kDefaultSendBytesThreshold, 0,
+            INT_MAX};
+        tcp_tx_zerocopy_send_bytes_thresh =
+            grpc_channel_arg_get_integer(&channel_args->args[i], options);
+      } else if (0 == strcmp(channel_args->args[i].key,
+                             GRPC_ARG_TCP_TX_ZEROCOPY_MAX_SIMULT_SENDS)) {
+        grpc_integer_options options = {
+            grpc_core::TcpZerocopySendCtx::kDefaultMaxSends, 0, INT_MAX};
+        tcp_tx_zerocopy_max_simult_sends =
+            grpc_channel_arg_get_integer(&channel_args->args[i], options);
       }
     }
   }
@@ -1215,6 +1750,7 @@ grpc_endpoint* grpc_tcp_create(grpc_fd* em_fd,
   tcp->fd = grpc_fd_wrapped_fd(em_fd);
   tcp->read_cb = nullptr;
   tcp->write_cb = nullptr;
+  tcp->current_zerocopy_send = nullptr;
   tcp->release_fd_cb = nullptr;
   tcp->release_fd = nullptr;
   tcp->incoming_buffer = nullptr;
@@ -1228,6 +1764,20 @@ grpc_endpoint* grpc_tcp_create(grpc_fd* em_fd,
   tcp->socket_ts_enabled = false;
   tcp->ts_capable = true;
   tcp->outgoing_buffer_arg = nullptr;
+  new (&tcp->tcp_zerocopy_send_ctx) TcpZerocopySendCtx(
+      tcp_tx_zerocopy_max_simult_sends, tcp_tx_zerocopy_send_bytes_thresh);
+  if (tcp_tx_zerocopy_enabled && !tcp->tcp_zerocopy_send_ctx.memory_limited()) {
+#ifdef GRPC_LINUX_ERRQUEUE
+    const int enable = 1;
+    auto err =
+        setsockopt(tcp->fd, SOL_SOCKET, SO_ZEROCOPY, &enable, sizeof(enable));
+    if (err == 0) {
+      tcp->tcp_zerocopy_send_ctx.set_enabled(true);
+    } else {
+      gpr_log(GPR_ERROR, "Failed to set zerocopy options on the socket.");
+    }
+#endif
+  }
   /* paired with unref in grpc_tcp_destroy */
   new (&tcp->refcount) grpc_core::RefCount(1, &grpc_tcp_trace);
   gpr_atm_no_barrier_store(&tcp->shutdown_count, 0);
@@ -1294,6 +1844,7 @@ void grpc_tcp_destroy_and_release_fd(grpc_endpoint* ep, int* fd,
   grpc_slice_buffer_reset_and_unref_internal(&tcp->last_read_buffer);
   if (grpc_event_engine_can_track_errors()) {
     /* Stop errors notification. */
+    ZerocopyDisableAndWaitForRemaining(tcp);
     gpr_atm_no_barrier_store(&tcp->stop_error_notification, true);
     grpc_fd_set_error(tcp->em_fd);
   }

src/core/lib/iomgr/tcp_server_utils_posix_common.cc

@@ -157,6 +157,14 @@ grpc_error* grpc_tcp_server_prepare_socket(grpc_tcp_server* s, int fd,
     if (err != GRPC_ERROR_NONE) goto error;
   }
 
+#ifdef GRPC_LINUX_ERRQUEUE
+  err = grpc_set_socket_zerocopy(fd);
+  if (err != GRPC_ERROR_NONE) {
+    /* it's not fatal, so just log it. */
+    gpr_log(GPR_DEBUG, "Node does not support SO_ZEROCOPY, continuing.");
+    GRPC_ERROR_UNREF(err);
+  }
+#endif
   err = grpc_set_socket_nonblocking(fd, 1);
   if (err != GRPC_ERROR_NONE) goto error;
   err = grpc_set_socket_cloexec(fd, 1);