On Tue, Aug 13, 2019 at 08:55:29AM +0800, Wanpeng Li wrote: > 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 Unlimited time means infinite time, or unlimited time means 10s ? 1s ? The previous definition was much better IMO: HINTS_DEDICATED. > allowing optimizations > ================== ============ ================================= > > Now it disables pv queued spinlock, OK. > pv tlb shootdown, OK. > pv sched yield "The idea is from Xen, when sending a call-function IPI-many to vCPUs, yield if any of the IPI target vCPUs was preempted. 17% performance increasement of ebizzy benchmark can be observed in an over-subscribe environment. (w/ kvm-pv-tlb disabled, testing TLB flush call-function IPI-many since call-function is not easy to be trigged by userspace workload)." This can probably hurt if vcpus are rarely preempted. > 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. Can't you enable the individual features you need for optimizing the overcommitted case? This is how things have been done historically: If a new feature is available, you enable it to get the desired performance. x2apic, invariant-tsc, cpuidle haltpoll... So in your case: enable pv schedyield, enable pv tlb shootdown. > 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. It depends on how much work vhost-kthreads needs to do, how successful halt-poll in the guest is, and what improvement halt-polling brings. The amount of polling will be reduced to zero if polling is not successful.
