Re: [PATCH] sched,kvm: Fix KVM preempt_notifier usage

Takashi Iwai
2015-07-03T15:57:29Z

At Fri, 3 Jul 2015 17:46:06 +0200,
Paolo Bonzini wrote:
>
>
>
> On 03/07/2015 17:42, Peter Zijlstra wrote:
> > On Fri, Jul 03, 2015 at 05:38:42PM +0200, Paolo Bonzini wrote:
> >> So basically this. Can you reply with SoB and maybe Acked-by?
> >
> > Ah, thanks for doing that!
> >
> >> ------------- 8< ---------------
> >> From: Peter Zijlstra <peterz@infradead.org>
> >> Subject: [PATCH] sched, preempt_notifier: separate notifier registration from static_key inc/dec
> >>
> >> Commit 1cde2930e154 ("sched/preempt: Add static_key() to preempt_notifiers")
> >> had two problems. First, the preempt-notifier API needs to sleep with the
> >> addition of the static_key, we do however need to hold off preemption
> >> while modifying the preempt notifier list, otherwise a preemption could
> >> observe an inconsistent list state. KVM correctly registers and
> >> unregisters preempt notifiers with preemption disabled, so the sleep
> >> caused dmesg splats.
> >>
> >> Second, KVM registers and unregisters preemption notifiers very often
> >> (in vcpu_load/vcpu_put). With a single uniprocessor guest the static key
> >> would move between 0 and 1 continuously, hitting the slow path on every
> >> userspace exit.
> >>
> >> To fix this, wrap the static_key inc/dec in a new API, and call it from
> >> KVM.
> >>
> >> Fixes: 1cde2930e154 ("sched/preempt: Add static_key() to preempt_notifiers")
> >> Reported-by: Pontus Fuchs <pontus.fuchs@gmail.com>
> >> Reported-by: Takashi Iwai <tiwai@suse.de>
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>
> Ok, I'm crossing fingers and including this in my pull request in order
> to preserve bisectability. Thanks.

I checked the patch now and confirmed that it fixes the regressions
(the warnings and the sluggish mouse pointer). Feel free to my
tested-by tag, if any.

Tested-by: Takashi Iwai <tiwai@suse.de>

Thanks!

Takashi