linux-kernel - Re: [PATCH] sched,kvm: Fix KVM preempt

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Date:	Fri, 03 Jul 2015 17:57:17 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Pontus Fuchs <pontus.fuchs@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	gleb@...nel.org
Subject: Re: [PATCH] sched,kvm: Fix KVM preempt_notifier usage

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@...radead.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@...il.com>
> >> Reported-by: Takashi Iwai <tiwai@...e.de>
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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@...e.de>


Thanks!

Takashi
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