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

[Peter Zijlstra <peterz@infradead.org>]

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>