linux-kernel - Re: [PATCH] Delay accounting, fix incorrect delay time when constantly waiting on runqueue

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Date:	Mon, 16 Jun 2008 17:30:26 +0530
From:	"Madhava K R" <madhavakr@...il.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	bharathravi1@...il.com, mingo@...e.hu,
	linux-kernel@...r.kernel.org, dhaval@...ux.vnet.ibm.com,
	vatsa@...ibm.com, balbir@...ibm.com,
	"Ankita Garg" <ankita@...ibm.com>
Subject: Re: [PATCH] Delay accounting, fix incorrect delay time when constantly waiting on runqueue

Hello,

On Mon, Jun 16, 2008 at 3:25 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2008-06-16 at 15:11 +0530, bharathravi1@...il.com wrote:
>> From: Bharath Ravi <bharathravi1@...il.com>
>>
>> This patch corrects the incorrect value of per process run-queue wait time
>> reported by delay statistics. The anomaly was due to the following reason.
>> When a process leaves the CPU and immediately starts waiting for CPU on
>> the runqueue (which means it remains in the TASK_RUNNABLE state), the time
>> of re-entry into the run-queue is never recorded. Due to this, the waiting time
>> on the runqueue from this point of re-entry upto the next time it hits the CPU
>> is not accounted for. This is solved by recording the time of re-entry of a
>> process leaving the CPU in the sched_info_depart() function IF the process will
>> go back to waiting on the run-queue. This IF condition is verified by checking
>> whether the process is still in the TASK_RUNNABLE state.
>>
>> The patch was tested on 2.6.26-rc6 using two simple CPU hog programs. The
>> values noted prior to the fix did not account for the time spent on the
>> runqueue waiting. After the fix, the correct values were reported back
>> to user space.
>
>
> Have you considered: http://lkml.org/lkml/2008/6/5/10
>
> I'm sad to say it is still pending in my todo list :-( - sorry Ankita.
>

It seems that Ankita's patch addresses the scenario where a process is
already on the run-queue, and is shuffled about CPUs. This patch
addresses the scenario where a process is pre-empted and returns to
the run-queue, where the last_queued value is not recorded.

I tried our test case with Ankita's patch, and the problem remains.
Our test case involves running two tight loops on an idle CPU.
Ideally, both should experience a run time of 50% and a delay time of
50%. But the results show negligible delay time for both processes.

The problems appear mutually exclusive...

>> Signed-off-by: Bharath Ravi <bharathravi1@...il.com>
>> Signed-off-by: Madhava K R  <madhavakr@...il.com>
>> ---
>>  kernel/sched_stats.h |    6 ++++++
>>  1 files changed, 6 insertions(+), 0 deletions(-)
>>
>> diff --git a/kernel/sched_stats.h b/kernel/sched_stats.h
>> index a38878e..80179ef 100644
>> --- a/kernel/sched_stats.h
>> +++ b/kernel/sched_stats.h
>> @@ -198,6 +198,9 @@ static inline void sched_info_queued(struct task_struct *t)
>>  /*
>>   * Called when a process ceases being the active-running process, either
>>   * voluntarily or involuntarily.  Now we can calculate how long we ran.
>> + * Also, if the process is still in the TASK_RUNNING state, call
>> + * sched_info_queued() to mark that it has now again started waiting on
>> + * the runqueue.
>>   */
>>  static inline void sched_info_depart(struct task_struct *t)
>>  {
>> @@ -206,6 +209,9 @@ static inline void sched_info_depart(struct task_struct *t)
>>
>>       t->sched_info.cpu_time += delta;
>>       rq_sched_info_depart(task_rq(t), delta);
>> +
>> +     if (t->state == TASK_RUNNING)
>> +             sched_info_queued(t);
>>  }
>>
>>  /*
>
>
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