linux-kernel - Re: [PATCH v8 01/11] cpuidle/poll_state: poll via smp_cond_load

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Message-ID: <086081ed-e2a8-508d-863c-21f2ff7c5490@gentwo.org>
Date: Tue, 15 Oct 2024 09:42:56 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Catalin Marinas <catalin.marinas@....com>
cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-pm@...r.kernel.org, 
    kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
    linux-kernel@...r.kernel.org, will@...nel.org, tglx@...utronix.de, 
    mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, 
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    joao.m.martins@...cle.com, boris.ostrovsky@...cle.com, 
    konrad.wilk@...cle.com
Subject: Re: [PATCH v8 01/11] cpuidle/poll_state: poll via
 smp_cond_load_relaxed()

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.