On Fri, 04 Nov 2022 06:20:59 +0000,
Usama Arif [off-list ref] 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.

-- 
Without deviation from the norm, progress is not possible.

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