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

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Message-Id: <1213869682.10476.2.camel@twins>
Date:	Thu, 19 Jun 2008 12:01:22 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	bharathravi1@...il.com
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org,
	dhaval@...ux.vnet.ibm.com, vatsa@...ibm.com, balbir@...ibm.com,
	Madhava K R <madhavakr@...il.com>
Subject: Re: [PATCH] Delay accounting, fix incorrect delay time when
	constantly waiting on runqueue

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.
> 
> Signed-off-by: Bharath Ravi <bharathravi1@...il.com>
> Signed-off-by: Madhava K R  <madhavakr@...il.com>

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

> ---
>  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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