linux-kernel - Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load

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Date:   Thu, 30 Nov 2023 22:59:45 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     "Christoph Lameter (Ampere)" <cl@...ux.com>
Cc:     Mihai Carabas <mihai.carabas@...cle.com>,
        linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, will@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com,
        pbonzini@...hat.com, wanpengli@...cent.com, vkuznets@...hat.com,
        rafael@...nel.org, daniel.lezcano@...aro.org,
        akpm@...ux-foundation.org, pmladek@...e.com, peterz@...radead.org,
        dianders@...omium.org, npiggin@...il.com,
        rick.p.edgecombe@...el.com, joao.m.martins@...cle.com,
        juerg.haefliger@...onical.com, mic@...ikod.net, arnd@...db.de,
        ankur.a.arora@...cle.com
Subject: Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with
 smp_cond_load_relaxed


Christoph Lameter (Ampere) <cl@...ux.com> writes:

> On Wed, 22 Nov 2023, Mihai Carabas wrote:
>
>> La 22.11.2023 22:51, Christoph Lameter a scris:
>>> On Mon, 20 Nov 2023, Mihai Carabas wrote:
>>>
>>>> cpu_relax on ARM64 does a simple "yield". Thus we replace it with
>>>> smp_cond_load_relaxed which basically does a "wfe".
>>> Well it clears events first (which requires the first WFE) and then does a
>>> WFE waiting for any events if no events were pending.
>>> WFE does not cause a VMEXIT? Or does the inner loop of
>>> smp_cond_load_relaxed now do 2x VMEXITS?
>>> KVM ARM64 code seems to indicate that WFE causes a VMEXIT. See
>>> kvm_handle_wfx().
>>
>> In KVM ARM64 the WFE traping is dynamic: it is enabled only if there are more
>> tasks waiting on the same core (e.g. on an oversubscribed system).
>>
>> In arch/arm64/kvm/arm.c:
>>
>>  457 >-------if (single_task_running())
>>  458 >------->-------vcpu_clear_wfx_traps(vcpu);
>>  459 >-------else
>>  460 >------->-------vcpu_set_wfx_traps(vcpu);
>
> Ahh. Cool did not know about that. But still: Lots of VMEXITs once the load has
> to be shared.

Yeah, anytime there's more than one runnable process. Another, more
critical place where we will vmexit is the qspinlock slowpath which
uses smp_cond_load.

>> This of course can be improved by having a knob where you can completly
>> disable wfx traping by your needs, but I left this as another subject to
>> tackle.

Probably needs to be adaptive since we use WFE in error paths as well
(for instance to park the CPU.)


Ankur