Re: [PATCH v8 01/11] cpuidle/poll_state: poll via smp_cond_load_relaxed()

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Oct 15, 2024 at 09:42:56AM -0700, Christoph Lameter (Ampere) wrote:
> On Tue, 15 Oct 2024, Catalin Marinas wrote:
> > > +			unsigned int loop_count = 0;
> > >  			if (local_clock_noinstr() - time_start > limit) {
> > >  				dev->poll_time_limit = true;
> > >  				break;
> > >  			}
> > > +
> > > +			smp_cond_load_relaxed(&current_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.
> 
> Setting of need_resched() from another processor involves sending an IPI
> after that was set. I dont think we need to smp_cond_load_relaxed since
> the IPI will cause an event. For ARM a WFE would be sufficient.

I'm not worried about the need_resched() case, even without an IPI it
would still work.

The loop_count++ side of the condition is supposed to timeout in the
absence of a need_resched() event. You can't do an smp_cond_load_*() on
a variable that's only updated by the waiting CPU. Nothing guarantees to
wake it up to update the variable (the event stream on arm64, yes, but
that's generic code).

-- 
Catalin