linux-kernel - Re: [PATCH] locking/atomic: Make test_and_*

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Message-ID: <CABdtJHuwGQ1Vj+HVfkhp=JN_hsFjJeK0-nfj+Ys1LXZrTKUaZg@mail.gmail.com>
Date:   Wed, 17 Aug 2022 07:40:29 +0200
From:   Jon Nettleton <jon@...id-run.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Will Deacon <will@...nel.org>, Hector Martin <marcan@...can.st>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Mark Rutland <mark.rutland@....com>,
        Jonathan Corbet <corbet@....net>, Tejun Heo <tj@...nel.org>,
        jirislaby@...nel.org, Marc Zyngier <maz@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Oliver Neukum <oneukum@...e.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Asahi Linux <asahi@...ts.linux.dev>, stable@...r.kernel.org
Subject: Re: [PATCH] locking/atomic: Make test_and_*_bit() ordered on failure

On Tue, Aug 16, 2022 at 8:02 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Tue, Aug 16, 2022 at 10:49 AM Jon Nettleton <jon@...id-run.com> wrote:
> >
> > It is moot if Linus has already taken the patch, but with a stock
> > kernel config I am
> > still seeing a slight performance dip but only ~1-2% in the specific
> > tests I was running.
>
> It would be interesting to hear if you can pinpoint in the profiles
> where the time is spent.
>
> It might be some random place that really doesn't care about ordering
> at all, and then we could easily rewrite _that_ particular case to do
> the unordered test explicitly, ie something like
>
> -        if (test_and_set_bit()) ...
> +       if (test_bit() || test_and_set_bit()) ...
>
> or even introduce an explicitly unordered "test_and_set_bit_relaxed()" thing.
>
>                  Linus

This is very interesting, the additional performance overhead doesn't seem
to be coming from within the kernel but from userspace. Comparing patched
and unpatched kernels I am seeing more cycles being taken up by glibc
atomics like __aarch64_cas4_acq  and __aarch64_ldadd4_acq_rel.

I need to test further to see if there is less effect on a system with
less cores,
This is a 16-core Cortex-A72, it is possible this is less of an issue on 4 core
A72's and A53's.

-Jon