linux-kernel - Re: [RFC] Disable lockref on arm64

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Message-ID: <20190522160417.GF7876@fuggles.cambridge.arm.com>
Date:   Wed, 22 May 2019 17:04:17 +0100
From:   Will Deacon <will.deacon@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Jayachandran Chandrasekharan Nair <jnair@...vell.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        Jan Glauber <jglauber@...vell.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC] Disable lockref on arm64

On Sat, May 18, 2019 at 12:00:34PM +0200, Ard Biesheuvel wrote:
> On Sat, 18 May 2019 at 06:25, Jayachandran Chandrasekharan Nair
> <jnair@...vell.com> wrote:
> >
> > On Mon, May 06, 2019 at 07:10:40PM +0100, Will Deacon wrote:
> > > On Mon, May 06, 2019 at 06:13:12AM +0000, Jayachandran Chandrasekharan Nair wrote:
> > > > Perhaps someone from ARM can chime in here how the cas/yield combo
> > > > is expected to work when there is contention. ThunderX2 does not
> > > > do much with the yield, but I don't expect any ARM implementation
> > > > to treat YIELD as a hint not to yield, but to get/keep exclusive
> > > > access to the last failed CAS location.
> > >
> > > Just picking up on this as "someone from ARM".
> > >
> > > The yield instruction in our implementation of cpu_relax() is *only* there
> > > as a scheduling hint to QEMU so that it can treat it as an internal
> > > scheduling hint and run some other thread; see 1baa82f48030 ("arm64:
> > > Implement cpu_relax as yield"). We can't use WFE or WFI blindly here, as it
> > > could be a long time before we see a wake-up event such as an interrupt. Our
> > > implementation of smp_cond_load_acquire() is much better for that kind of
> > > thing, but doesn't help at all for a contended CAS loop where the variable
> > > is actually changing constantly.
> >
> > Looking thru the perf output of this case (open/close of a file from
> > multiple CPUs), I see that refcount is a significant factor in most
> > kernel configurations - and that too uses cmpxchg (without yield).
> > x86 has an optimized inline version of refcount that helps
> > significantly. Do you think this is worth looking at for arm64?
> >
> 
> I looked into this a while ago [0], but at the time, we decided to
> stick with the generic implementation until we encountered a use case
> that benefits from it. Worth a try, I suppose ...
> 
> [0] https://lore.kernel.org/linux-arm-kernel/20170903101622.12093-1-ard.biesheuvel@linaro.org/

If JC can show that we benefit from this, it would be interesting to see if
we can implement the refcount-full saturating arithmetic using the
LDMIN/LDMAX instructions instead of the current cmpxchg() loops.

Will