linux-kernel - Re: [PATCH 2/2] nohz: Synchronize sleep time stats with memory barriers

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Date:	Wed, 23 Apr 2014 20:59:40 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linux Kernel Mailing List <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 2/2] nohz: Synchronize sleep time stats with memory barriers

On Tue, Apr 15, 2014 at 12:51 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Apr 02, 2014 at 10:17:52PM +0200, Denys Vlasenko wrote:
>> When some call site uses get_cpu_*_time_us() to read a sleeptime
>> stat, it deduces the total sleeptime by adding the pending time
>> to the last sleeptime snapshot if the CPU target is idle.
>>
>> But this only works if idle_sleeptime, idle_entrytime and idle_active are
>> read and updated under some disciplined order.
>>
>> This patch changes updaters to modify idle_entrytime,
>> {idle,iowait}_sleeptime, and idle_active exactly in this order,
>> with write barriers on SMP to ensure other CPUs see then in this order too.
>> Readers are changed read them in the opposite order, with read barriers.
>> When readers detect a race by seeing cleared idle_entrytime,
>> they retry the reads.
>>
>> The "iowait-ness" of every idle period is decided at the moment it starts:
>> if this CPU's run-queue had tasks waiting on I/O, then this idle
>> period's duration will be added to iowait_sleeptime.
>> This, along with proper SMP syncronization, fixes the bug where iowait
>> counts could go backwards.
>
> It also makes for a near infinite source of iowait. Who is to say the
> CPU that started with iowait will ever wake up? The nohz sleep time is
> practically unbounded.

Yes, it does that.
I prepared a new patchset which fixes that.

It also addresses your other points:

>>       /* Updates the per cpu time idle statistics counters */
>>       delta = ktime_sub(now, ts->idle_entrytime);
>> +     ts->idle_entrytime.tv64 = 0;
>
> One must at all times describe the memory ordering and pairing barrier
> in a comment when placing barriers.

Done.

>> +             if (start.tv64 == 0)
>> +                     /* Other CPU is updating the count.
>> +                      * We don't know whether fetched count is valid.
>> +                      */
>> +                     goto again;
>
> This is double wrong; any multi line stmt (even if its a single stmt)
> should have {}. Also, wrong multi line comment style.

Fixed.

> You're not nearly lazy enough; this is a near identical copy of the
> above, two nearly identical copies of 'tricky' code is a bad idea.

True. I was planning to address that, but new logic added
to avoid overcounting iowait made these two functions
not as similar as they used to be.

Please review a newer patchset, I'll send it in a minute.
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