On 06/11/2022 16:35, Marc Zyngier wrote:
On Fri, 04 Nov 2022 06:20:59 +0000,
Usama Arif <usama.arif@xxxxxxxxxxxxx> 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
