linux-kernel - Re: [PATCH 2/7] posix-cpu-timers: fix acounting delta

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Date:	Fri, 10 May 2013 22:17:38 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Olivier Langlois <olivier@...llion01.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/7] posix-cpu-timers: fix acounting delta_exec twice

>> @@ -250,8 +250,13 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
>>                * values through the TIMER_ABSTIME flag, therefore we have
>>                * to synchronize the timer to the clock every time we start
>>                * it.
>> +              *
>> +              * Do not add the current delta, because
>> +              * account_group_exec_runtime() will also this delta and we
>> +              * wouldn't want to double account time and get ahead of
>> +              * ourselves.
>>                */
>> -             thread_group_cputime(tsk, &sum);
>> +             thread_group_cputime(tsk, false, &sum);
>>               raw_spin_lock_irqsave(&cputimer->lock, flags);
>
> I wonder if we should move thread_group_cputime() inside this lock.
> Otherwise we can miss some updates in-between.

Hmm..

I don't agree with this. Right, we can miss some updates. But 1)
cputimer->lock doesn't
prevent any update update_curr() only take rq_lock, and 2) POSIX timer
and sleeping
semantics allow longer sleep than an argument. Then, the missing is
safe, nobody
can observe which of the timer_setime() syscall and update_curr()
happened earlier.

Ah, I'm now finding when update_gt_cputime() effectively work. It
helps to avoid timer_settime() vs timer_settime() mess.
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