Do not waste cache lines with unnecessary paddings.

Make sure paddings are GPR_CACHELINE_SIZE minus the
size of fields in that cache line. Otherwise, we will waste an
additional cache line for no good reason.

src/core/lib/gpr/mpscq.h

@@ -40,7 +40,7 @@ typedef struct gpr_mpscq_node {
 typedef struct gpr_mpscq {
   gpr_atm head;
   // make sure head & tail don't share a cacheline
-  char padding[GPR_CACHELINE_SIZE];
+  char padding[GPR_CACHELINE_SIZE - sizeof(head)];
   gpr_mpscq_node* tail;
   gpr_mpscq_node stub;
 } gpr_mpscq;

src/core/lib/iomgr/ev_epoll1_linux.cc

@@ -195,7 +195,7 @@ struct grpc_pollset_worker {
 typedef struct pollset_neighborhood {
   gpr_mu mu;
   grpc_pollset* active_root;
-  char pad[GPR_CACHELINE_SIZE];
+  char pad[GPR_CACHELINE_SIZE - sizeof(mu) - sizeof(active_root)];
 } pollset_neighborhood;
 
 struct grpc_pollset {