On Sun, 4 Aug 2019 at 04:21, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > > On Thu, Aug 01, 2019 at 06:54:49PM +0200, Paolo Bonzini wrote: > > On 01/08/19 18:51, Rafael J. Wysocki wrote: > > > On 8/1/2019 9:06 AM, Wanpeng Li wrote: > > >> From: Wanpeng Li <wanpengli@xxxxxxxxxxx> > > >> > > >> The downside of guest side polling is that polling is performed even > > >> with other runnable tasks in the host. However, even if poll in kvm > > >> can aware whether or not other runnable tasks in the same pCPU, it > > >> can still incur extra overhead in over-subscribe scenario. Now we can > > >> just enable guest polling when dedicated pCPUs are available. > > >> > > >> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> > > >> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > > >> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> > > >> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx> > > >> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx> > > > > > > Paolo, Marcelo, any comments? > > > > Yes, it's a good idea. > > > > Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> > > > > Paolo > Hi Marcelo, Sorry for the late response. > I think KVM_HINTS_REALTIME is being abused somewhat. > It has no clear meaning and used in different locations > for different purposes. ================== ============ ================================= KVM_HINTS_REALTIME 0 guest checks this feature bit to determine that vCPUs are never preempted for an unlimited time allowing optimizations ================== ============ ================================= Now it disables pv queued spinlock, pv tlb shootdown, pv sched yield which are not expected present in vCPUs are never preempted for an unlimited time scenario. > > For example, i think that using pv queued spinlocks and > haltpoll is a desired scenario, which the patch below disallows. So even if dedicated pCPU is available, pv queued spinlocks should still be chose if something like vhost-kthreads are used instead of DPDK/vhost-user. kvm adaptive halt-polling will compete with vhost-kthreads, however, poll in guest unaware other runnable tasks in the host which will defeat vhost-kthreads. Regards, Wanpeng Li
