linux-kernel - Re: [PATCH v2] locking/rwsem: Add __always_inline annotation to __down_read

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Date:   Tue, 18 Apr 2023 12:30:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Stultz <jstultz@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Minchan Kim <minchan@...nel.org>,
        Tim Murray <timmurray@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>, kernel-team@...roid.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] locking/rwsem: Add __always_inline annotation to
 __down_read_common()

On Mon, Apr 17, 2023 at 06:22:14PM +0200, John Stultz wrote:
> On Mon, Apr 17, 2023 at 1:19 PM Peter Zijlstra <peterz@...radead.org> 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.

No worries; good addition to the v3 Changelog ;-)

> > 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).

Right, but we should not rely on the compiler heuristics for correctness
:-)

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

Yes please. I'm not sure I care much about the whole 'inline __sched' vs
'__always_inline' thing, but I do feel it should all be consistently
applied.