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From:   John Stultz <jstultz@google.com>
Date:   Mon, 17 Apr 2023 18:22:14 +0200
Message-ID: <CANDhNCp2WEAMjK1DUVKCen05-EdwVBYZxxLSP3ZSZvRh1ayAhQ@mail.gmail.com>
Subject: Re: [PATCH v2] locking/rwsem: Add __always_inline annotation to __down_read_common()
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Minchan Kim <minchan@kernel.org>,
        Tim Murray <timmurray@google.com>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>, kernel-team@android.com,
        stable@vger.kernel.org
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On Mon, Apr 17, 2023 at 1:19=E2=80=AFPM Peter Zijlstra <peterz@infradead.or=
g> wrote:
>
> On Wed, Apr 12, 2023 at 03:59:05AM +0000, John Stultz wrote:
> > Apparently despite it being marked inline, the compiler
> > may not inline __down_read_common() which makes it difficult
> > to identify the cause of lock contention, as the blocked
> > function will always be listed as __down_read_common().
> >
> > So this patch adds __always_inline annotation to the
> > function to force it to be inlines so the calling function
> > will be listed.
>
> I'm a wee bit confused; what are you looking at? Wchan?

Apologies! Yes, traceevent data via wchan, sorry I didn't make that clear.

> What is stopping
> the compiler from now handing you
> __down_read{,_interruptible,_killable}() instead? Is that fine?

No, we want to make the blocked calling function, rather than the
locking functions, visible in the tracepoints captured. That said, the
other __down_read* functions seem to be properly inlined in practice
(Waiman's theory as to why sounds convincing to me).

If you'd like I can add those as well to be always_inline, as well so
it's more consistent?

thanks
-john