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

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Message-ID: <87r08eo75m.fsf@oracle.com>
Date: Thu, 17 Oct 2024 12:34:29 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: "Christoph Lameter (Ampere)" <cl@...two.org>,
        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,
        x86@...nel.org, hpa@...or.com, pbonzini@...hat.com,
        wanpengli@...cent.com, vkuznets@...hat.com, rafael@...nel.org,
        daniel.lezcano@...aro.org, peterz@...radead.org, arnd@...db.de,
        lenb@...nel.org, mark.rutland@....com, harisokn@...zon.com,
        mtosatti@...hat.com, sudeep.holla@....com, misono.tomohiro@...itsu.com,
        maobibo@...ngson.cn, 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()


Catalin Marinas <catalin.marinas@....com> writes:

> On Thu, Oct 17, 2024 at 09:56:13AM -0700, Christoph Lameter (Ampere) wrote:
>> On Wed, 16 Oct 2024, Catalin Marinas wrote:
>> > The behaviour above is slightly different from the current poll_idle()
>> > implementation. The above is more like poll every timeout period rather
>> > than continuously poll until either the need_resched() condition is true
>> > _or_ the timeout expired. From Ankur's email, an IPI may not happen so
>> > we don't have any guarantee that WFET will wake up before the timeout.
>> > The only way for WFE/WFET to wake up on need_resched() is to use LDXR to
>> > arm the exclusive monitor. That's what smp_cond_load_relaxed() does.
>>
>> Sorry no. The IPI will cause the WFE to continue immediately and not wait
>> till the end of the timeout period.
>
> *If* there is an IPI. The scheduler is not really my area but some
> functions like wake_up_idle_cpu() seem to elide the IPI if
> TIF_NR_POLLING is set.
>
> But even if we had an IPI, it still feels like abusing the semantics of
> smp_cond_load_relaxed() when relying on it to increment a variable in
> the condition check as a result of some unrelated wake-up event. This
> API is meant to wait for a condition on a single variable. It cannot
> wait on multiple variables and especially not one it updates itself
> (even if it happens to work on arm64 under certain conditions).

Yeah that makes sense. smp_cond_load_relaxed() uses two separate
side-effects to make sure things work: the event-stream and the
increment in the conditional.

I do want to thresh out smp_cond_load_timeout() a bit more but let
me reply to your other mail for that.

> My strong preference would be to revive the smp_cond_load_timeout()
> proposal from Ankur earlier in the year.

Ack that.

--
ankur