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

[Usama Arif]

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

Thanks,
Usama

>