Messages in this thread | | | Date | Tue, 25 Sep 2012 19:19:03 +0530 | From | Raghavendra K T <> | Subject | Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler |
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On 09/25/2012 02:24 PM, Avi Kivity wrote: > On 09/25/2012 10:09 AM, Raghavendra K T wrote: >> On 09/24/2012 09:36 PM, Avi Kivity wrote: >>> On 09/24/2012 05:41 PM, Avi Kivity wrote: >>>> >>>>> >>>>> case 2) >>>>> rq1 : vcpu1->wait(lockA) (spinning) >>>>> rq2 : vcpu3 (running) , vcpu2->holding(lockA) [scheduled out] >>>>> >>>>> I agree that checking rq1 length is not proper in this case, and as >>>>> you >>>>> rightly pointed out, we are in trouble here. >>>>> nr_running()/num_online_cpus() would give more accurate picture here, >>>>> but it seemed costly. May be load balancer save us a bit here in not >>>>> running to such sort of cases. ( I agree load balancer is far too >>>>> complex). >>>> >>>> In theory preempt notifier can tell us whether a vcpu is preempted or >>>> not (except for exits to userspace), so we can keep track of whether >>>> it's we're overcommitted in kvm itself. It also avoids false positives >>>> from other guests and/or processes being overcommitted while our vm >>>> is fine. >>> >>> It also allows us to cheaply skip running vcpus. >> >> Hi Avi, >> >> Could you please elaborate on how preempt notifiers can be used >> here to keep track of overcommit or skip running vcpus? >> >> Are we planning set some flag in sched_out() handler etc? >> > > Keep a bitmap kvm->preempted_vcpus. > > In sched_out, test whether we're TASK_RUNNING, and if so, set a vcpu > flag and our bit in kvm->preempted_vcpus. On sched_in, if the flag is > set, clear our bit in kvm->preempted_vcpus. We can also keep a counter > of preempted vcpus. > > We can use the bitmap and the counter to quickly see if spinning is > worthwhile (if the counter is zero, better to spin). If not, we can use > the bitmap to select target vcpus quickly. > > The only problem is that in order to keep this accurate we need to keep > the preempt notifiers active during exits to userspace. But we can > prototype this without this change, and add it later if it works. >
Avi, Thanks for the idea.. I want to try this some time soon.
So ideally it means if we are under-committed the counter/ bitmap effective value is zero.
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