linux-kernel - Re: [PATCH] sched: account system time properly

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Date:	Wed, 29 Apr 2009 10:08:40 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrew Gallatin <gallatin@...i.com>,
	linux-kernel@...r.kernel.org, rick.jones2@...com, brice@...i.com,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: account system time properly

On Wed, 29 Apr 2009 09:46:17 +0200
Eric Dumazet <dada1@...mosbay.com> wrote:

> Eric Dumazet a écrit :
> > Andrew Morton a écrit :
> > 
> > So, if IRQs are interrupting idle task, I guess if (p != rq->idle) will be false.
> > 

If an IRQ interrupts the idle task the tick is supposed to be accounted
as an idle tick. Only if the IRQ interrupted the system while it has
been in hardirq or softirq processing then it should be accounted as
system tick.

> 
> Maybe following patch is needed ?
> 
> [PATCH] sched: account system time properly
> 
> When idle task is interrupted by an IRQ, time accounting considers CPU is idle,
> even while it should account for hard or softirq.
> 
> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index b902e58..26efa47 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4732,7 +4732,7 @@ void account_process_tick(struct task_struct *p, int user_tick)
>  
>  	if (user_tick)
>  		account_user_time(p, one_jiffy, one_jiffy_scaled);
> -	else if (p != rq->idle)
> +	else if ((p != rq->idle) || (irq_count() != HARDIRQ_OFFSET))
>  		account_system_time(p, HARDIRQ_OFFSET, one_jiffy,
>  				    one_jiffy_scaled);
>  	else

That patch makes a lot of sense to me. Does it fix the problem?

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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