linux-kernel - Re: [PATCH v5 2/3] sched/cputime: Fix prev steal time accouting during cpu hotplug

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Message-ID: <c06f0947-cf14-88c2-f5a1-8328f93e1dbd@redhat.com>
Date:	Wed, 8 Jun 2016 12:01:44 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v5 2/3] sched/cputime: Fix prev steal time accouting
 during cpu hotplug



On 08/06/2016 05:05, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> Commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug") 
> set rq->prev_* to 0 after a cpu hotplug comes back in order to fix the scenario:
> 
> | 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.
> 
> However, it is still buggy.
> 
> rq->prev_steal_time = 0:
> 
> As Rik pointed out:
> 
> | setting rq->prev_irq_time to 0 in the guest, and then getting a giant value from 
> | the host, could result in a very large of steal_jiffies.
> 
> rq->prev_steal_time_rq = 0:
> 
> | steal = paravirt_steal_clock(cpu_of(rq));
> | steal -= rq->prev_steal_time_rq;
> |
> | if (unlikely(steal > delta))
> |	 steal = delta;
> |
> | rq->prev_steal_time_rq += steal;
> | delta -= steal;
> |
> | rq->clock_task += delta;
> 
> steal is a giant value and rq->prev_steal_time_rq is 0, rq->prev_steal_time_rq 
> grows in delta granularity, rq->clock_task can't ramp up until rq->prev_steal_time_rq 
> catches up steal clock since delta value will be 0 after reducing steal time from 
> normal execution time. That's why I obersved that cpuhg/1-12 continue running 
> until rq->prev_steal_time_rq catches up steal clock timestamp.
> 
> I believe rq->prev_irq_time has similar issue. So this patch fix it by reverting  
> commit e9532e69b8d1.

This is still very hard to read.  I think the justification for this
patch is much simpler:

-------
Commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU
hotplug") set rq->prev_* to 0 after a cpu hotplug comes back in order to
fix the case where (after CPU hotplug) steal is smaller than
rq->prev_steal_time.

However, this should never happen. steal was only smaller because of the
KVM-specific bug fixed by the previous patch.  Worse, the previous patch
triggers a bug on CPU hot-unplug/plug operation: because
rq->prev_steal_time is cleared, all of the CPU's past steal time will be
accounted again on hot-plug.

Since the root cause has been fixed, we can just revert commit e9532e69b8d1.
-------

Since things are currently pretty broken, I think it's fair to keep the
two patches separate instead of squashing them.

Paolo