On Tue, 2019-07-30 at 13:46 +0200, Paolo Bonzini wrote:
> On 30/07/19 11:33, Wanpeng Li wrote:
> > When qemu/other vCPU inject virtual interrupt to guest through
> > waking up one
> > sleeping vCPU, it increases the probability to stack vCPUs/qemu by
> > scheduler
> > wake-affine. vCPU stacking issue can greately inceases the lock
> > synchronization
> > latency in a virtualized environment. This patch disables wake-
> > affine vCPU
> > process to mitigtate lock holder preemption.
>
> There is no guarantee that the vCPU remains on the thread where it's
> created, so the patch is not enough.
>
> If many vCPUs are stacked on the same pCPU, why doesn't the wake_cap
> kick in sooner or later?
>
Assuming it actually is the case that vcpus *do* get stacked *and* that
wake_cap() *doesn't* kick in, maybe it could be because of this check?
/* Minimum capacity is close to max, no need to abort wake_affine */
if (max_cap - min_cap < max_cap >> 3)
return 0;
Regards
--
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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