Re: [RFC] Disable lockref on arm64

[Ard Biesheuvel]

On Sat, 18 May 2019 at 06:25, Jayachandran Chandrasekharan Nair
<jnair@xxxxxxxxxxx> 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@xxxxxxxxxx/

> > 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
>
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