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From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Jayachandran Chandrasekharan Nair <jnair@marvell.com>
Cc: "catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	Jan Glauber <jglauber@marvell.com>,
	Will Deacon <will.deacon@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC] Disable lockref on arm64
Date: Sat, 18 May 2019 12:00:34 +0200
Message-ID: <CAKv+Gu9U9z3iAuz4V1c5zTHuz1As8FSNGY-TJon4OLErB8ts8Q@mail.gmail.com> (raw)
In-Reply-To: <20190518042424.GA28517@dc5-eodlnx05.marvell.com>

On Sat, 18 May 2019 at 06:25, Jayachandran Chandrasekharan Nair
<jnair@marvell.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/

> > Implementing yield in the CPU may generally be beneficial for SMT designs so
> > that the hardware resources aren't wasted when spinning round a busy loop.
>
> Yield is probably used in sub-optimal implementations of delay or wait.
> It is going to be different across multiple implementations and
> revisions (given the description in ARM spec). Having a more yielding(?)
> implementation would be equally problematic especially in the lockref
> case.
>
> > For this particular discussion (i.e. lockref), however, it seems as though
> > the cpu_relax() call is questionable to start with.
>
> In case of lockref, taking out the yield/pause and dropping to queued
> spinlock after some cycles appears to me to be a better approach.
> Relying on the quality of cpu_relax() on the specific processor to
> mitigate against contention is going to be tricky anyway.
>
> We will do some more work here, but would appreciate any pointers
> based on your experience here.
>
> Thanks,
> JC
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-29 14:52 Jan Glauber
2019-05-01 16:01 ` Will Deacon
2019-05-02  8:38   ` Jan Glauber
2019-05-01 16:41 ` Linus Torvalds
2019-05-02  8:27   ` Jan Glauber
2019-05-02 16:12     ` Linus Torvalds
2019-05-02 23:19       ` Jayachandran Chandrasekharan Nair
2019-05-03 19:40         ` Linus Torvalds
2019-05-06  6:13           ` [EXT] " Jayachandran Chandrasekharan Nair
2019-05-06 17:13             ` Linus Torvalds
2019-05-06 18:10             ` Will Deacon
2019-05-18  4:24               ` Jayachandran Chandrasekharan Nair
2019-05-18 10:00                 ` Ard Biesheuvel [this message]
2019-05-22 16:04                   ` Will Deacon
2019-06-12  4:10                     ` Jayachandran Chandrasekharan Nair
2019-06-12  9:31                       ` Will Deacon
2019-06-14  7:09                         ` Jayachandran Chandrasekharan Nair
2019-06-14  9:58                           ` Will Deacon
2019-06-14 10:24                             ` Ard Biesheuvel
2019-06-14 10:38                               ` Will Deacon
2019-06-15  4:21                                 ` Kees Cook
2019-06-15  8:47                                   ` Ard Biesheuvel
2019-06-15 13:59                                     ` Kees Cook
2019-06-15 14:18                                       ` Ard Biesheuvel
2019-06-16 21:31                                         ` Kees Cook
2019-06-17 11:33                                           ` Ard Biesheuvel
2019-06-17 17:26                                             ` Will Deacon
2019-06-17 20:07                                               ` Jayachandran Chandrasekharan Nair
2019-06-18  5:41                                               ` Kees Cook
2019-06-13  9:53                       ` Hanjun Guo
2019-06-05 13:48   ` [PATCH] lockref: Limit number of cmpxchg loop retries Jan Glauber
2019-06-05 20:16     ` Linus Torvalds
2019-06-06  8:03       ` Jan Glauber
2019-06-06  9:41         ` Will Deacon
2019-06-06 10:28           ` Jan Glauber
2019-06-07  7:27             ` Jan Glauber
2019-06-07 20:14               ` Linus Torvalds

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