On Tue, Sep 25, 2012 at 10:54:21AM +0200, 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. > Can user return notifier can be used instead? Set bit in kvm->preempted_vcpus on return to userspace. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
