On Wed, 2024-10-16 at 10:04 -0700, Ankur Arora wrote:
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> Okanovic, Haris <harisokn@xxxxxxxxxx> writes:
>
> > On Tue, 2024-10-15 at 13:04 +0100, Catalin Marinas wrote:
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> > >
> > >
> > >
> > > On Wed, Sep 25, 2024 at 04:24:15PM -0700, Ankur Arora wrote:
> > > > diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> > > > index 9b6d90a72601..fc1204426158 100644
> > > > --- a/drivers/cpuidle/poll_state.c
> > > > +++ b/drivers/cpuidle/poll_state.c
> > > > @@ -21,21 +21,20 @@ static int __cpuidle poll_idle(struct cpuidle_device *dev,
> > > >
> > > > raw_local_irq_enable();
> > > > if (!current_set_polling_and_test()) {
> > > > - unsigned int loop_count = 0;
> > > > u64 limit;
> > > >
> > > > limit = cpuidle_poll_time(drv, dev);
> > > >
> > > > while (!need_resched()) {
> > > > - cpu_relax();
> > > > - if (loop_count++ < POLL_IDLE_RELAX_COUNT)
> > > > - continue;
> > > > -
> > > > - loop_count = 0;
> > > > + unsigned int loop_count = 0;
> > > > if (local_clock_noinstr() - time_start > limit) {
> > > > dev->poll_time_limit = true;
> > > > break;
> > > > }
> > > > +
> > > > + smp_cond_load_relaxed(¤t_thread_info()->flags,
> > > > + VAL & _TIF_NEED_RESCHED ||
> > > > + loop_count++ >= POLL_IDLE_RELAX_COUNT);
> > >
> > > The above is not guaranteed to make progress if _TIF_NEED_RESCHED is
> > > never set. With the event stream enabled on arm64, the WFE will
> > > eventually be woken up, loop_count incremented and the condition would
> > > become true. However, the smp_cond_load_relaxed() semantics require that
> > > a different agent updates the variable being waited on, not the waiting
> > > CPU updating it itself. Also note that the event stream can be disabled
> > > on arm64 on the kernel command line.
> >
> > Alternately could we condition arch_haltpoll_want() on
> > arch_timer_evtstrm_available(), like v7?
>
> Yes, I'm thinking of staging it somewhat like that. First an
> smp_cond_load_relaxed() which gets rid of this issue, followed by
> one based on smp_cond_load_relaxed_timeout().
>
> That said, conditioning just arch_haltpoll_want() won't suffice since
> what Catalin pointed out affects all users of poll_idle(), not just
> haltpoll.
The only other users I see today are apm_init() and
acpi_processor_setup_cstates(), both in x86 path. Perhaps not ideal,
but should be sufficient.
>
> Right now there's only haltpoll but there are future users like
> zhenglifeng with a patch for acpi-idle here:
>
> https://lore.kernel.org/all/f8a1f85b-c4bf-4c38-81bf-728f72a4f2fe@xxxxxxxxxx/
>
> > > Does the code above break any other architecture? I'd say if you want
> > > something like this, better introduce a new smp_cond_load_timeout()
> > > API. The above looks like a hack that may only work on arm64 when the
> > > event stream is enabled.
> > >
> > > A generic option is udelay() (on arm64 it would use WFE/WFET by
> > > default). Not sure how important it is for poll_idle() but the downside
> > > of udelay() that it won't be able to also poll need_resched() while
> > > waiting for the timeout. If this matters, you could instead make smaller
> > > udelay() calls. Yet another problem, I don't know how energy efficient
> > > udelay() is on x86 vs cpu_relax().
> > >
> > > So maybe an smp_cond_load_timeout() would be better, implemented with
> > > cpu_relax() generically and the arm64 would use LDXR, WFE and rely on
> > > the event stream (or fall back to cpu_relax() if the event stream is
> > > disabled).
> > >
> > > --
> > > Catalin
>
>
> --
> ankur
