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

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Date:   Mon, 27 Nov 2023 12:17:43 -0800 (PST)
From:   "Christoph Lameter (Ampere)" <cl@...ux.com>
To:     Mihai Carabas <mihai.carabas@...cle.com>
cc:     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

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.

> 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.

kvm_arch_vcpu_load() looks strange. On the one hand we pass a cpu 
number into it and then we use functions that only work if we are running 
on that cpu?

It would be better to use smp_processor_id() in the function 
and not pass the cpu number to it.