On Tue, Nov 22, 2022 at 11:59 AM Paolo Abeni <pabeni@xxxxxxxxxx> wrote: > > Hello, > > Thank you for the prompt feedback! > > On Tue, 2022-11-22 at 11:18 -0500, Soheil Hassas Yeganeh wrote: > > On Tue, Nov 22, 2022 at 10:43 AM Paolo Abeni <pabeni@xxxxxxxxxx> wrote: > > > > > > We are observing huge contention on the epmutex during an http > > > connection/rate test: > > > > > > 83.17% 0.25% nginx [kernel.kallsyms] [k] entry_SYSCALL_64_after_hwframe > > > [...] > > > |--66.96%--__fput > > > |--60.04%--eventpoll_release_file > > > |--58.41%--__mutex_lock.isra.6 > > > |--56.56%--osq_lock > > > > > > The application is multi-threaded, creates a new epoll entry for > > > each incoming connection, and does not delete it before the > > > connection shutdown - that is, before the connection's fd close(). > > > > > > Many different threads compete frequently for the epmutex lock, > > > affecting the overall performance. > > > > > > To reduce the contention this patch introduces explicit reference counting > > > for the eventpoll struct. Each registered event acquires a reference, > > > and references are released at ep_remove() time. ep_free() doesn't touch > > > anymore the event RB tree, it just unregisters the existing callbacks > > > and drops a reference to the ep struct. The struct itself is freed when > > > the reference count reaches 0. The reference count updates are protected > > > by the mtx mutex so no additional atomic operations are needed. > > > > > > Since ep_free() can't compete anymore with eventpoll_release_file() > > > for epitems removal, we can drop the epmutex usage at disposal time. > > > > > > With the patched kernel, in the same connection/rate scenario, the mutex > > > operations disappear from the perf report, and the measured connections/rate > > > grows by ~60%. > > > > I locally tried this patch and I can reproduce the results. Thank you > > for the nice optimization! > > > > > Tested-by: Xiumei Mu <xmu@xxxxxxxxxx> > > > Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx> > > > --- > > > This is just a repost reaching out for more recipents, > > > as suggested by Carlos. > > > > > > Previous post at: > > > > > > https://lore.kernel.org/linux-fsdevel/20221122102726.4jremle54zpcapia@andromeda/T/#m6f98d4ccbe0a385d10c04fd4018e782b793944e6 > > > --- > > > fs/eventpoll.c | 113 ++++++++++++++++++++++++++++--------------------- > > > 1 file changed, 64 insertions(+), 49 deletions(-) > > > > > > diff --git a/fs/eventpoll.c b/fs/eventpoll.c > > > index 52954d4637b5..6e415287aeb8 100644 > > > --- a/fs/eventpoll.c > > > +++ b/fs/eventpoll.c > > > @@ -226,6 +226,12 @@ struct eventpoll { > > > /* tracks wakeup nests for lockdep validation */ > > > u8 nests; > > > #endif > > > + > > > + /* > > > + * protected by mtx, used to avoid races between ep_free() and > > > + * ep_eventpoll_release() > > > + */ > > > + unsigned int refcount; > > > > nitpick: Given that napi_id and nest are both macro protected, you > > might want to pull it right after min_wait_ts. > > Just to be on the same page: the above is just for an aesthetic reason, > right? Is there some functional aspect I don't see? Yes, a nitpick completely for aesthetics. It's also slightly easier to think about the size, alignment and padding of the structure that way. Please feel free to ignore. > [...] > > > > @@ -2165,10 +2174,16 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds, > > > error = -EEXIST; > > > break; > > > case EPOLL_CTL_DEL: > > > - if (epi) > > > - error = ep_remove(ep, epi); > > > - else > > > + if (epi) { > > > + /* > > > + * The eventpoll itself is still alive: the refcount > > > + * can't go to zero here. > > > + */ > > > + WARN_ON_ONCE(ep_remove(ep, epi)); > > > > There are similar examples of calling ep_remove() without checking the > > return value in ep_insert(). > > Yes, the error paths in ep_insert(). I added a comment referring to all > of them, trying to explain that ep_dispose() is not needed there. > > > I believe we should add a similar comment there, and maybe a > > WARN_ON_ONCE. I'm not sure, but it might be worth adding a new helper > > given this repeated pattern? > > I like the idea of such helper. I'll use it in the next iteration, if > there is a reasonable agreement on this patch. > > Whould 'ep_remove_safe()' fit as the helper's name? 'ep_remove_safe()' sounds great to me. Thanks! Soheil > Thanks, > > Paolo >
