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

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Date:	Tue, 7 Jun 2016 19:50:08 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krcmar <rkrcmar@...hat.com>,
	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>
Subject: Re: Fw: [PATCH v4 2/3] sched/cputime: Fix prev steal time accouting
 during cpu hotplug

2016-06-07 19:41 GMT+08:00 Wanpeng Li <wanpeng.li@...mail.com>:
> On 07/06/2016 10:00, 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
>> setting
>> rq->prev_* to current irq time and steal clock timestamp after a cpu
>> hotplug
>> comes back.
>
> 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." Though I didn't meet such
scenario. So I just do what that commit really want to do.

Regards,
Wanpeng Li