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

[Catalin Marinas]

On Tue, Oct 15, 2024 at 10:17:13AM -0700, Christoph Lameter (Ampere) wrote:
> On Tue, 15 Oct 2024, Catalin Marinas wrote:
> > > 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).
> 
> Hmm... I have WFET implementation here without smp_cond modelled after
> the delay() implementation ARM64 (but its not generic and there is
> an additional patch required to make this work. Intermediate patch
> attached)

At least one additional patch ;). But yeah, I suggested hiding all this
behind something like smp_cond_load_timeout() which would wait on
current_thread_info()->flags but with a timeout. The arm64
implementation would follow some of the logic in __delay(). Others may
simply poll with cpu_relax().

Alternatively, if we get an IPI anyway, we can avoid smp_cond_load() and
rely on need_resched() and some new delay/cpu_relax() API that waits for
a timeout or an IPI, whichever comes first. E.g. cpu_relax_timeout()
which on arm64 it's just a simplified version of __delay() without the
'while' loops.

-- 
Catalin