linux-kernel - Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates

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Message-ID: <535AAFD6.9050900@redhat.com>
Date:	Fri, 25 Apr 2014 20:56:22 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates

On 04/24/2014 09:18 PM, Peter Zijlstra wrote:
> On Thu, Apr 24, 2014 at 08:45:58PM +0200, Denys Vlasenko wrote:
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 268a45e..ffea757 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -4218,7 +4218,14 @@ void __sched io_schedule(void)
>>  	current->in_iowait = 1;
>>  	schedule();
>>  	current->in_iowait = 0;
>> +#ifdef CONFIG_NO_HZ_COMMON
>> +	if (atomic_dec_and_test(&rq->nr_iowait)) {
>> +		if (raw_smp_processor_id() != cpu_of(rq))
>> +			tick_nohz_iowait_to_idle(cpu_of(rq));
>> +	}
>> +#else
>>  	atomic_dec(&rq->nr_iowait);
>> +#endif
>>  	delayacct_blkio_end();
>>  }
> 
> You're really refusing to collapse that stuff eh?

I'm sending two patches on top of my last patch set
which tidies up a few such aspects (another one
is where we fetch a percpu variable before knowing
that we'll need it, potentially wasting a few cycles).

>> +void tick_nohz_iowait_to_idle(int cpu)
>> +{
>> +	struct tick_sched *ts = tick_get_tick_sched(cpu);
>> +	ktime_t now = ktime_get();
>> +
>> +	write_seqcount_begin(&ts->idle_sleeptime_seq);
>> +	ts->iowait_exittime = now;
>> +	write_seqcount_end(&ts->idle_sleeptime_seq);
>> +}
>
>
> So what again was wrong with this one?
>
>   http://marc.info/?l=linux-kernel&m=139772917211023

That code has no provision to record when last iowait task
left the rq.

Therefore it can undercount iowait - it's very similar
to the problem I had before patch #4 in my patch series.

My patches 1-3 can overcount iowait because they consider
the entire idle period "iowait" if nr_iowait_cpu() != 0
at the *beginning*.

Hidetoshi's patches consider the entire idle period "iowait"
if nr_iowait_cpu() != 0 at the *end*.

He needs to code carefully so that this delayed decision
doesn't make reader functions return wrong results.

However, if nr_iowait_cpu() was 0 at the end it does not mean
that most of this time period it was also 0. It could have been
mostly !0 - and in this case iowait will be undercounted.

I personally thought that both over- or undercounting iowait
might be acceptable.

If not, then *some* form of recording and accounting
for exact moment when last iowait task left the rq is necessary.
That's what I did in patch #4.

-- 
vda

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