[patch 40/85] epoll: make sure all elements in ready list are in FIFO order

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From: Roman Penyaev <rpenyaev@xxxxxxx>
Subject: epoll: make sure all elements in ready list are in FIFO order

Patch series "use rwlock in order to reduce ep_poll_callback()
contention", v3.

The last patch targets the contention problem in ep_poll_callback(), which
can be very well reproduced by generating events (write to pipe or eventfd)
from many threads, while consumer thread does polling.

The following are some microbenchmark results based on the test [1] which
starts threads which generate N events each.  The test ends when all events
are successfully fetched by the poller thread:

 spinlock
 ========

 threads  events/ms  run-time ms
       8       6402        12495
      16       7045        22709
      32       7395        43268

 rwlock + xchg
 =============

 threads  events/ms  run-time ms
       8      10038         7969
      16      12178        13138
      32      13223        24199


According to the results bandwidth of delivered events is significantly
increased, thus execution time is reduced.


This patch (of 4):

All coming events are stored in FIFO order and this is also should be
applicable to ->ovflist, which originally is stack, i.e.  LIFO.

Thus to keep correct FIFO order ->ovflist should reversed by adding
elements to the head of the read list but not to the tail.

Link: http://lkml.kernel.org/r/20190103150104.17128-2-rpenyaev@xxxxxxx
Signed-off-by: Roman Penyaev <rpenyaev@xxxxxxx>
Reviewed-by: Davidlohr Bueso <dbueso@xxxxxxx>
Cc: Jason Baron <jbaron@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---


--- a/fs/eventpoll.c~epoll-make-sure-all-elements-in-ready-list-are-in-fifo-order
+++ a/fs/eventpoll.c
@@ -722,7 +722,11 @@ static __poll_t ep_scan_ready_list(struc
 		 * contain them, and the list_splice() below takes care of them.
 		 */
 		if (!ep_is_linked(epi)) {
-			list_add_tail(&epi->rdllink, &ep->rdllist);
+			/*
+			 * ->ovflist is LIFO, so we have to reverse it in order
+			 * to keep in FIFO.
+			 */
+			list_add(&epi->rdllink, &ep->rdllist);
 			ep_pm_stay_awake(epi);
 		}
 	}
_



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