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Message-ID: <277fbd0d-25ea-437e-2ea7-6121c4e269db@linux.com>
Date: Mon, 27 Nov 2023 12:17:43 -0800 (PST)
From: "Christoph Lameter (Ampere)" <cl@...ux.com>
To: Mihai Carabas <mihai.carabas@...cle.com>
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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.
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