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

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Message-ID: <CAHk-=whGK712fPqmQ3FSHxqe3Aqny4bEeWEvfaytLeLV2+ijCQ@mail.gmail.com>
Date:   Mon, 9 Mar 2020 08:52:41 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jeff Layton <jlayton@...nel.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, 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..

Hmm?

                    Linus

View attachment "patch.diff" of type "text/x-patch" (1935 bytes)