Revision - 2ecd9d2 - sched, preempt_notifier: separate notifier registration from [...] - origin: https://github.com/torvalds/linux

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Revision 2ecd9d29abb171d6e97a4f3eb29d7456a11401b7 authored by Peter Zijlstra on 03 July 2015, 16:53:58 UTC, committed by Paolo Bonzini on 03 July 2015, 16:55:00 UTC

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>
Tested-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 parent 9bdc771

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Tip revision: 2ecd9d29abb171d6e97a4f3eb29d7456a11401b7 authored by Peter Zijlstra on 03 July 2015, 16:53:58 UTC
sched, preempt_notifier: separate notifier registration from static_key inc/dec

Tip revision: 2ecd9d2

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sched, preempt_notifier: separate notifier registration from static_key inc/dec

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