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

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Message-ID: <87zfn2o9tu.fsf@oracle.com>
Date: Thu, 17 Oct 2024 11:36:45 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>,
        Catalin Marinas
 <catalin.marinas@....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()


Christoph Lameter (Ampere) <cl@...two.org> writes:

> On Wed, 16 Oct 2024, Ankur Arora wrote:
>
>> > The other core will wake our core up by sending an IPI. The IPI will
>> > invoke a scheduler function on our core and the WFE will continue.
>>
>> Why? The target core is not sleeping. It is *polling* on a memory
>> address (on arm64, via LDXR; WFE). Ergo an IPI is not needed to tell
>> it that a need-resched bit is set.
>
> The IPI is sent to interrupt the process that is not sleeping. This is
> done so the busy processor can reschedule the currently running process
> and respond to the event.

The scheduler treats idle specially (if the architecture defines
TIF_POLLING_NRFLAG). There's also the sched_wake_idle_without_ipi
tracepoint for this path.

$ sudo perf stat -e sched:sched_wake_idle_without_ipi perf bench sched pipe
# Running 'sched/pipe' benchmark:
# Executed 1000000 pipe operations between two processes

     Total time: 5.173 [sec]

       5.173613 usecs/op
         193288 ops/sec

 Performance counter stats for 'perf bench sched pipe':

         1,992,368      sched:sched_wake_idle_without_ipi

       5.178976487 seconds time elapsed

       0.396076000 seconds user
       6.999566000 seconds sys

--
ankur