linux-kernel - Re: [locks] 6d390e4b5d: will-it-scale.per_process

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Date:   Mon, 09 Mar 2020 15:09:15 -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 13:22 -0400, Jeff Layton wrote:
> 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?
>   

How about something like this instead? (untested other than for
compilation)

Basically, this just switches the waiters over to wait for
fl_blocked_member to go empty. That still happens before the wakeup, so
it should be ok to wait on that.

I think we can also eliminate the lockless list_empty check in
locks_delete_block, as the fl_blocker check should be sufficient now.
-- 
Jeff Layton <jlayton@...nel.org>

View attachment "0001-locks-reinstate-locks_delete_lock-optimization.patch" of type "text/x-patch" (4917 bytes)