Re: [PATCH v4 7/8] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed

[Ankur Arora <ankur.a.arora@oracle.com>]

Okanovic, Haris <harisokn@amazon.com> writes:

> On Fri, 2024-04-05 at 16:14 -0700, Ankur Arora wrote:
>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>>
>>
>>
>> Okanovic, Haris <harisokn@amazon.com> writes:
>>
>> > On Thu, 2024-02-15 at 09:41 +0200, Mihai Carabas wrote:
>> > > cpu_relax on ARM64 does a simple "yield". Thus we replace it with
>> > > smp_cond_load_relaxed which basically does a "wfe".
>> > >
>> > > Suggested-by: Peter Zijlstra <peterz@infradead.org>
>> > > Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
>> > > ---
>> > >  drivers/cpuidle/poll_state.c | 15 ++++++++++-----
>> > >  1 file changed, 10 insertions(+), 5 deletions(-)
>> > >
>> > > diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> > > index 9b6d90a72601..1e45be906e72 100644
>> > > --- a/drivers/cpuidle/poll_state.c
>> > > +++ b/drivers/cpuidle/poll_state.c
>> > > @@ -13,6 +13,7 @@
>> > >  static int __cpuidle poll_idle(struct cpuidle_device *dev,
>> > >                             struct cpuidle_driver *drv, int index)
>> > >  {
>> > > +    unsigned long ret;
>> > >      u64 time_start;
>> > >
>> > >      time_start = local_clock_noinstr();
>> > > @@ -26,12 +27,16 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
>> > >
>> > >              limit = cpuidle_poll_time(drv, dev);
>> > >
>> > > -            while (!need_resched()) {
>> > > -                    cpu_relax();
>> > > -                    if (loop_count++ < POLL_IDLE_RELAX_COUNT)
>> > > -                            continue;
>> > > -
>> > > +            for (;;) {
>> > >                      loop_count = 0;
>> > > +
>> > > +                    ret = smp_cond_load_relaxed(&current_thread_info()->flags,
>> > > +                                                VAL & _TIF_NEED_RESCHED ||
>> > > +                                                loop_count++ >= POLL_IDLE_RELAX_COUNT);
>> >
>> > Is it necessary to repeat this 200 times with a wfe poll?
>>
>> The POLL_IDLE_RELAX_COUNT is there because on x86 each cpu_relax()
>> iteration is much shorter.
>>
>> With WFE, it makes less sense.
>>
>> > Does kvm not implement a timeout period?
>>
>> Not yet, but it does become more useful after a WFE haltpoll is
>> available on ARM64.
>
> Note that kvm conditionally traps WFE and WFI based on number of host
> CPU tasks. VMs will sometimes see hardware behavior - potentially
> polling for a long time before entering WFI.
>
> https://elixir.bootlin.com/linux/latest/source/arch/arm64/kvm/arm.c#L459

Yeah. There was a discussion on this
https://lore.kernel.org/lkml/871qc6qufy.fsf@oracle.com/.

>> Haltpoll does have a timeout, which you should be able to tune via
>> /sys/module/haltpoll/parameters/ but that, of course, won't help here.
>>
>> > Could you make it configurable? This patch improves certain workloads
>> > on AWS Graviton instances as well, but blocks up to 6ms in 200 * 30us
>> > increments before going to wfi, which is a bit excessive.
>>
>> Yeah, this looks like a problem. We could solve it by making it an
>> architectural parameter. Though I worry about ARM platforms with
>> much smaller default timeouts.
>> The other possibility is using WFET in the primitive, but then we
>> have that dependency and that's a bigger change.
>
> See arm64's delay() for inspiration:
>
> https://elixir.bootlin.com/linux/v6.9-rc2/source/arch/arm64/lib/delay.c#L26

Sure, that part is straight-forward enough. However, this will need a fallback
the case when WFET is not available. And, because this path is used on x86,
so we need a cross platform smp_cond*timeout(). Though given that the x86
version is based on cpu_relax() then that could just fold the sched_clock()
check in.

Maybe another place to do this would be by KVM forcing a WFE timeout. Arguably
that is needed regardless of whether we use a smp_cond*timeout() or not.

--
ankur