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. Until then... Acked-by: Rik van Riel <riel@xxxxxxxxxx> -- All Rights Reversed.
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