[External] Re: [v2 0/6] KVM: arm64: implement vcpu_is_preempted check

[Usama Arif]

On 18/11/2022 00:20, Marc Zyngier wrote:
> On Mon, 07 Nov 2022 12:00:44 +0000,
> Usama Arif <usama.arif at bytedance.com> wrote:
>>
>>
>>
>> On 06/11/2022 16:35, Marc Zyngier wrote:
>>> On Fri, 04 Nov 2022 06:20:59 +0000,
>>> Usama Arif <usama.arif at bytedance.com> wrote:
>>>>
>>>> This patchset adds support for vcpu_is_preempted in arm64, which
>>>> allows the guest to check if a vcpu was scheduled out, which is
>>>> useful to know incase it was holding a lock. vcpu_is_preempted can
>>>> be used to improve performance in locking (see owner_on_cpu usage in
>>>> mutex_spin_on_owner, mutex_can_spin_on_owner, rtmutex_spin_on_owner
>>>> and osq_lock) and scheduling (see available_idle_cpu which is used
>>>> in several places in kernel/sched/fair.c for e.g. in wake_affine to
>>>> determine which CPU can run soonest):
>>>
>>> [...]
>>>
>>>> pvcy shows a smaller overall improvement (50%) compared to
>>>> vcpu_is_preempted (277%).  Host side flamegraph analysis shows that
>>>> ~60% of the host time when using pvcy is spent in kvm_handle_wfx,
>>>> compared with ~1.5% when using vcpu_is_preempted, hence
>>>> vcpu_is_preempted shows a larger improvement.
>>>
>>> And have you worked out *why* we spend so much time handling WFE?
>>>
>>> 	M.
>>
>> Its from the following change in pvcy patchset:
>>
>> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
>> index e778eefcf214..915644816a85 100644
>> --- a/arch/arm64/kvm/handle_exit.c
>> +++ b/arch/arm64/kvm/handle_exit.c
>> @@ -118,7 +118,12 @@ static int kvm_handle_wfx(struct kvm_vcpu *vcpu)
>>          }
>>
>>          if (esr & ESR_ELx_WFx_ISS_WFE) {
>> -               kvm_vcpu_on_spin(vcpu, vcpu_mode_priv(vcpu));
>> +               int state;
>> +               while ((state = kvm_pvcy_check_state(vcpu)) == 0)
>> +                       schedule();
>> +
>> +               if (state == -1)
>> +                       kvm_vcpu_on_spin(vcpu, vcpu_mode_priv(vcpu));
>>          } else {
>>                  if (esr & ESR_ELx_WFx_ISS_WFxT)
>>                          vcpu_set_flag(vcpu, IN_WFIT);
>>
>>
>> If my understanding is correct of the pvcy changes, whenever pvcy
>> returns an unchanged vcpu state, we would schedule to another
>> vcpu. And its the constant scheduling where the time is spent. I guess
>> the affects are much higher when the lock contention is very
>> high. This can be seem from the pvcy host side flamegraph as well with
>> (~67% of the time spent in the schedule() call in kvm_handle_wfx), For
>> reference, I have put the graph at:
>> https://uarif1.github.io/pvlock/perf_host_pvcy_nmi.svg
> 
> The real issue here is that we don't try to pick the right vcpu to
> run, and strictly rely on schedule() to eventually pick something that
> can run.
> 
> An interesting to do would be to try and fit the directed yield
> mechanism there. It would be a lot more interesting than the one-off
> vcpu_is_preempted hack, as it gives us a low-level primitive on which
> to construct things (pvcy is effectively a mwait-like primitive).

We could use kvm_vcpu_yield_to to yield to a specific vcpu, but how 
would we determine which vcpu to yield to?

IMO vcpu_is_preempted is very well integrated in a lot of core kernel 
code, i.e. mutex, rtmutex, rwsem and osq_lock. It is also used in 
scheduler to determine better which vCPU we can run on soonest, select 
idle core, etc. I am not sure if all of these cases will be optimized by 
pvcy? Also, with vcpu_is_preempted, some of the lock heavy benchmarks 
come down from spending around 50% of the time in lock to less than 1% 
(so not sure how much more room is there for improvement).

We could also use vcpu_is_preempted to optimize IPI performance (along 
with directed yield to target IPI vCPU) similar to how its done in x86 
(https://lore.kernel.org/all/1560255830-8656-2-git-send-email-wanpengli@tencent.com/). 
This case definitely wont be covered by pvcy.

Considering all the above, i.e. the core kernel integration already 
present and possible future usecases of vcpu_is_preempted, maybe its 
worth making vcpu_is_preempted work on arm independently of pvcy?

Thanks,
Usama

> 
> 	M.
>