Fix bug in Rcv Lowat (#32081)

* Fix bug in Rcv Lowat

* fix

* update

src/core/lib/iomgr/tcp_posix.cc

@@ -835,17 +835,18 @@ static void update_rcvlowat(grpc_tcp* tcp)
 
   int remaining = std::min(static_cast<int>(tcp->incoming_buffer->length),
                            tcp->min_progress_size);
+
   remaining = std::min(remaining, kRcvLowatMax);
 
   // Setting SO_RCVLOWAT for small quantities does not save on CPU.
-  if (remaining < kRcvLowatThreshold) {
+  if (remaining < 2 * kRcvLowatThreshold) {
     remaining = 0;
   }
 
-  // If zerocopy is off, wake shortly before the full RPC is here. More can
-  // show up partway through recvmsg() since it takes a while to copy data.
-  // So an early wakeup aids latency.
-  if (!tcp->tcp_zerocopy_send_ctx.enabled() && remaining > 0) {
+  // Decrement remaining by kRcvLowatThreshold. This would have the effect of
+  // waking up a little early. It would help with latency because some bytes
+  // may arrive while we execute the recvmsg syscall after waking up.
+  if (remaining > 0) {
     remaining -= kRcvLowatThreshold;
   }
 
@@ -1118,9 +1119,11 @@ static void tcp_handle_read(void* arg /* grpc_tcp */, grpc_error_handle error) {
   if (GPR_LIKELY(error.ok())) {
     maybe_make_read_slices(tcp);
     if (!tcp_do_read(tcp, &tcp_read_error)) {
-      // We've consumed the edge, request a new one
+      // Maybe update rcv lowat value based on the number of bytes read in this
+      // round.
       update_rcvlowat(tcp);
       tcp->read_mu.Unlock();
+      // We've consumed the edge, request a new one
       notify_on_read(tcp);
       return;
     }
@@ -1130,6 +1133,12 @@ static void tcp_handle_read(void* arg /* grpc_tcp */, grpc_error_handle error) {
     grpc_slice_buffer_reset_and_unref(tcp->incoming_buffer);
     grpc_slice_buffer_reset_and_unref(&tcp->last_read_buffer);
   }
+  // Update rcv lowat needs to be called at the end of the current read
+  // operation to ensure the right SO_RCVLOWAT value is set for the next read.
+  // Otherwise the next endpoint read operation may get stuck indefinitely
+  // because the previously set rcv lowat value will persist and the socket may
+  // erroneously considered to not be ready for read.
+  update_rcvlowat(tcp);
   grpc_closure* cb = tcp->read_cb;
   tcp->read_cb = nullptr;
   tcp->incoming_buffer = nullptr;
@@ -1152,14 +1161,12 @@ static void tcp_read(grpc_endpoint* ep, grpc_slice_buffer* incoming_buffer,
   grpc_slice_buffer_swap(incoming_buffer, &tcp->last_read_buffer);
   TCP_REF(tcp, "read");
   if (tcp->is_first_read) {
-    update_rcvlowat(tcp);
     tcp->read_mu.Unlock();
     // Endpoint read called for the very first time. Register read callback with
     // the polling engine
     tcp->is_first_read = false;
     notify_on_read(tcp);
   } else if (!urgent && tcp->inq == 0) {
-    update_rcvlowat(tcp);
     tcp->read_mu.Unlock();
     // Upper layer asked to read more but we know there is no pending data
     // to read from previous reads. So, wait for POLLIN.