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Date: Mon, 09 Mar 2020 13:22:28 -0400 From: Jeff Layton <jlayton@...nel.org> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: kernel test robot <rong.a.chen@...el.com>, yangerkun <yangerkun@...wei.com>, LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org, Neil Brown <neilb@...e.de>, Bruce Fields <bfields@...ldses.org>, Al Viro <viro@...iv.linux.org.uk> Subject: Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression On Mon, 2020-03-09 at 08:52 -0700, Linus Torvalds wrote: > On Mon, Mar 9, 2020 at 7:36 AM Jeff Layton <jlayton@...nel.org> wrote: > > On Sun, 2020-03-08 at 22:03 +0800, kernel test robot wrote: > > > FYI, we noticed a -96.6% regression of will-it-scale.per_process_ops due to commit: > > > > This is not completely unexpected as we're banging on the global > > blocked_lock_lock now for every unlock. This test just thrashes file > > locks and unlocks without doing anything in between, so the workload > > looks pretty artificial [1]. > > > > It would be nice to avoid the global lock in this codepath, but it > > doesn't look simple to do. I'll keep thinking about it, but for now I'm > > inclined to ignore this result unless we see a problem in more realistic > > workloads. > > That is a _huge_ regression, though. > > What about something like the attached? Wouldn't that work? And make > the code actually match the old comment about wow "fl_blocker" being > NULL being special. > > The old code seemed to not know about things like memory ordering either. > > Patch is entirely untested, but aims to have that "smp_store_release() > means I'm done and not going to touch it any more", making that > smp_load_acquire() test hopefully be valid as per the comment.. Yeah, something along those lines maybe. I don't think we can use fl_blocker that way though, as the wait_event_interruptible is waiting on it to go to NULL, and the wake_up happens before fl_blocker is cleared. Maybe we need to mix in some sort of FL_BLOCK_ACTIVE flag and use that instead of testing for !fl_blocker to see whether we can avoid the blocked_lock_lock? -- Jeff Layton <jlayton@...nel.org>
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