linux-kernel - Re: [lkp] [sched/fair] 41e0d37f7a: divide error: 0000 [#1] SMP

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Message-ID: <CAJZ5v0hR-N+kUtv5nqWJ7oTE3dtnw5kQux9Y2tgXRk11fZg4ZQ@mail.gmail.com>
Date:	Wed, 4 May 2016 13:56:35 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	kernel test robot <ying.huang@...ux.intel.com>,
	Steve Muckle <steve.muckle@...aro.org>, lkp@...org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Mike Galbraith <efault@....de>,
	Michael Turquette <mturquette@...libre.com>,
	Juri Lelli <Juri.Lelli@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steve Muckle <smuckle@...aro.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [lkp] [sched/fair] 41e0d37f7a: divide error: 0000 [#1] SMP

On Wed, May 4, 2016 at 1:51 PM, Wanpeng Li <kernellwp@...il.com> wrote:
> 2016-05-04 19:44 GMT+08:00 Rafael J. Wysocki <rafael@...nel.org>:
>> On Wed, May 4, 2016 at 2:58 AM, Wanpeng Li <kernellwp@...il.com> wrote:
>>> 2016-05-03 20:15 GMT+08:00 Rafael J. Wysocki <rafael@...nel.org>:
>>>> On Tue, May 3, 2016 at 10:32 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>>>> On Tue, May 03, 2016 at 09:10:51AM +0800, kernel test robot wrote:
>>>>>> FYI, we noticed the following commit:
>>>>>>
>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git sched/core
>>>>>> commit 41e0d37f7ac81297c07ba311e4ad39465b8c8295 ("sched/fair: Do not call cpufreq hook unless util changed")
>>>>>
>>
>> [cut]
>>
>>>>>
>>>>> That's intel_pstate.c:get_avg_frequency(), which assumes mperf != 0. It
>>>>> being 0 seems to suggest intel_pstate_sample() hasn't been called yet or
>>>>> so.
>>>>
>>>> Well, what's the tree based on?
>>>>
>>>> The mainline does this:
>>>>
>>>> bool sample_taken = intel_pstate_sample(cpu, time);
>>>>
>>>> if (sample_taken && !hwp_active)
>>>>         intel_pstate_adjust_busy_pstate(cpu);
>>>>
>>>> and (the mainline version of) intel_pstate_sample() returns false when
>>>> it is called for the first time after setting the update_util hook.
>>>
>>> The callsites in scheduler will set time to rq_clock(rq) when trigger
>>> sample, so when time 0 will be used even if it is set just before
>>> setting the update_util hook?
>>
>> I'm not sure what you mean.
>>
>> time=0 is special as it will cause intel_pstate_sample() to return
>> false on the next invocation.
>
> Sample is driven by cpufreq_update_util() which uses rq_clock(rq) as
> time parameter, so there is no opportunity to pass time 0 to
> intel_pstate_sample().

Right.

So I should have said that had time=0 been passed to
intel_pstate_sample(), it would have caused it to return false on the
next invocation. :-)

The way it works is that sample.time is 0 initially, so
intel_pstate_sample() returns false first time it is called and the
second invocation gets all of the deltas as needed.