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

["Okanovic, Haris" <harisokn@amazon.com>]

On Fri, 2024-04-05 at 16:14 -0700, Ankur Arora wrote:
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> 
> 
> 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

> 
> 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

> 
> Will address this in the next version.
> 
> Thanks for pointing this out.
> 
> --
> ankur