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| author |   Davidlohr Bueso <dave@stgolabs.net> | 2019-01-03 15:27:09 -0800 |
|---|---|---|
| committer |   Linus Torvalds <torvalds@linux-foundation.org> | 2019-01-04 13:13:46 -0800 |
| commit | 76699a67f3041ff4c7af6d6ee9be2bfbf1ffb671 (patch) | |
| tree | 5d42b50be630e856b4e91c3109b56d8e4612a594 /fs | |
| parent | fs/epoll: simplify ep_send_events_proc() ready-list loop (diff) | |
| download | linux-dev-76699a67f3041ff4c7af6d6ee9be2bfbf1ffb671.tar.xz linux-dev-76699a67f3041ff4c7af6d6ee9be2bfbf1ffb671.zip | |
fs/epoll: drop ovflist branch prediction
The ep->ovflist is a secondary ready-list to temporarily store events
that might occur when doing sproc without holding the ep->wq.lock. This
accounts for every time we check for ready events and also send events
back to userspace; both callbacks, particularly the latter because of
copy_to_user, can account for a non-trivial time.
As such, the unlikely() check to see if the pointer is being used, seems
both misleading and sub-optimal. In fact, we go to an awful lot of
trouble to sync both lists, and populating the ovflist is far from an
uncommon scenario.
For example, profiling a concurrent epoll_wait(2) benchmark, with
CONFIG_PROFILE_ANNOTATED_BRANCHES shows that for a two threads a 33%
incorrect rate was seen; and when incrementally increasing the number of
epoll instances (which is used, for example for multiple queuing load
balancing models), up to a 90% incorrect rate was seen.
Similarly, by deleting the prediction, 3% throughput boost was seen
across incremental threads.
Link: http://lkml.kernel.org/r/20181108051006.18751-4-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jason Baron <jbaron@akamai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'fs')
| -rw-r--r-- | fs/eventpoll.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/fs/eventpoll.c b/fs/eventpoll.c index aaf614ee08e4..fb7f05f4099d 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1153,7 +1153,7 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v * semantics). All the events that happen during that period of time are * chained in ep->ovflist and requeued later on. */ - if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) { + if (ep->ovflist != EP_UNACTIVE_PTR) { if (epi->next == EP_UNACTIVE_PTR) { epi->next = ep->ovflist; ep->ovflist = epi; |