Re: [PATCH] kvm: x86: make lapic hrtimer pinned

[Wanpeng Li]

2016-04-22 21:12 GMT+08:00 Luiz Capitulino <lcapitulino@xxxxxxxxxx>:
> On Fri, 22 Apr 2016 07:12:51 +0800
> Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>
>> 2016-04-05 20:40 GMT+08:00 Luiz Capitulino <lcapitulino@xxxxxxxxxx>:
>> > On Tue, 5 Apr 2016 14:18:01 +0800
>> > Yang Zhang <yang.zhang.wz@xxxxxxxxx> wrote:
>> >
>> >> On 2016/4/5 5:00, Rik van Riel wrote:
>> >> > On Mon, 2016-04-04 at 16:46 -0400, Luiz Capitulino wrote:
>> >> >> When a vCPU runs on a nohz_full core, the hrtimer used by
>> >> >> the lapic emulation code can be migrated to another core.
>> >> >> When this happens, it's possible to observe milisecond
>> >> >> latency when delivering timer IRQs to KVM guests.
>> >> >>
>> >> >> The huge latency is mainly due to the fact that
>> >> >> apic_timer_fn() expects to run during a kvm exit. It
>> >> >> sets KVM_REQ_PENDING_TIMER and let it be handled on kvm
>> >> >> entry. However, if the timer fires on a different core,
>> >> >> we have to wait until the next kvm exit for the guest
>> >> >> to see KVM_REQ_PENDING_TIMER set.
>> >> >>
>> >> >> This problem became visible after commit 9642d18ee. This
>> >> >> commit changed the timer migration code to always attempt
>> >> >> to migrate timers away from nohz_full cores. While it's
>> >> >> discussable if this is correct/desirable (I don't think
>> >> >> it is), it's clear that the lapic emulation code has
>> >> >> a requirement on firing the hrtimer in the same core
>> >> >> where it was started. This is achieved by making the
>> >> >> hrtimer pinned.
>> >> >
>> >> > Given that delivering a timer to a guest seems to
>> >> > involve trapping from the guest to the host, anyway,
>> >> > I don't see a downside to your patch.
>> >> >
>> >> > If that is ever changed (eg. allowing delivery of
>> >> > a timer interrupt to a VCPU without trapping to the
>> >> > host), we may want to revisit this.
>> >>
>> >>
>> >> Posted interrupt helps in this case. Currently, KVM doesn't use PI for
>> >> lapic timer is due to same affinity for lapic timer and VCPU. Now, we
>> >> can change to use PI for lapic timer. The only concern is what's
>> >> frequency of timer migration in upstream Linux? If it is frequently,
>> >> will it bring additional cost?
>> >
>> > I can't answer this questions.
>> >
>> >> BTW, in what case the migration of timers during VCPU scheduling will fail?
>> >
>> > For hrtimers (which is the lapic emulation case), it only succeeds if
>> > the destination core has a hrtimer expiring before the hrtimer being
>> > migrated.
>>
>> Interesting, did you figure out why this happen? Actually the clock
>> event device will be reprogrammed if the expire time of the new
>> enqueued hrtimer is earlier than the left most(earliest expire time)
>> hrtimer in hrtimer rb tree.
>
> Unless the code has changed very recently, what you describe is
> what happens when queueing a hrtimer in the same core. Migrating a
> hrtimer to a different core is a different case.

You are right!

Regards,
Wanpeng Li