linux-kernel - Re: [PATCH 2/2] tracing, sched: Add a new tracepoint for sleeptime

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Message-ID: <4F18998D.30503@fb.com>
Date:	Thu, 19 Jan 2012 14:30:37 -0800
From:	Arun Sharma <asharma@...com>
To:	<avagin@...nvz.org>
CC:	<linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/2] tracing, sched: Add a new tracepoint for sleeptime

On 12/21/11 2:43 AM, Andrey Vagin wrote:
>
>>
>> +#ifdef CREATE_TRACE_POINTS
>> +static inline u64 trace_get_sleeptime(struct task_struct *tsk)
>> +{
>> +#ifdef CONFIG_SCHEDSTATS
>> + u64 block, sleep;
>> +
>> + block = tsk->se.statistics.block_start;
>> + sleep = tsk->se.statistics.sleep_start;
> Arun, probably you have missed one of my comments.
> block_start and sleep_start should be zeroized here.
>
> tsk->se.statistics.block_start = 0;
> tsk->se.statistics.sleep_start = 0;

This still doesn't solve one minor problem: the first sample we get 
might be bad. Here's the sequence that could trigger it:

t1: task goes to sleep. sleep_start=t1
t2: task gets woken up. sleep_start is still t1
t3: context switch. trace_get_sleeptime() is not active. sleep_start is 
still t1
t4: trace_get_sleeptime() is activated
t5: task gets context switched out involuntarily.
t6: task gets context switched in. first sample from the task.

We compute sleeptime=t6-t1. The correct answer should be sleeptime=0.

We should set {sleep,block}_start to 0 regardless of whether the 
tracepoint is active or not.

I'll post a patch shortly.

  -Arun
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