Re: Fw: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting during cpu hotplug

From	Paolo Bonzini <pbonzini@redhat.com>
Newsgroups	linux.kernel
Subject	Re: Fw: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting during cpu hotplug
Date	2016-06-07 14:00 +0200
Message-ID	<rHnPR-2e7-61@gated-at.bofh.it> (permalink)
References	<rHmAp-1vx-31@gated-at.bofh.it> <rHmAp-1vx-33@gated-at.bofh.it> <rHmAp-1vx-29@gated-at.bofh.it> <rHnPQ-2e7-23@gated-at.bofh.it> <rHnPQ-2e7-21@gated-at.bofh.it>
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On 07/06/2016 13:50, Wanpeng Li wrote:
>> > I'm not sure this patch is necessary.  Instead you could just revert
>> > commit e9532e69b8d1.  The previous patch obviously makes it unnecessary
>> > to reset rq->prev_steal_time and rq->prev_steal_time_rq, and the reset
>> > of rq->prev_irq_time looks like a no-op to me.
> The reason why I'm not just simple revert it is that commit mentioned
> "steal is smaller than rq->prev_steal_time we end up with an insane
> large value which then gets added to rq->prev_steal_time, resulting in
> a permanent wreckage of the accounting."

With this patch, you go back to having underflow if steal is smaller
than rq->prev_steal_time.  The point is that it should never be smaller;
it was only smaller because of the bug that you are fixing in patch 1.

Thanks,

Paolo

 Though I didn't meet such
> scenario. So I just do what that commit really want to do.

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Re: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting  during cpu hotplug Paolo Bonzini <pbonzini@redhat.com> - 2016-06-07 12:40 +0200
  Re: Fw: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting  during cpu hotplug Wanpeng Li <kernellwp@gmail.com> - 2016-06-07 14:00 +0200
    Re: Fw: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting  during cpu hotplug Paolo Bonzini <pbonzini@redhat.com> - 2016-06-07 14:00 +0200

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