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

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Message-ID: <20090429102409.GB2373@elte.hu>
Date:	Wed, 29 Apr 2009 12:24:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	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>
Subject: Re: [PATCH] sched: account system time properly


* Martin Schwidefsky <schwidefsky@...ibm.com> wrote:

> On Wed, 29 Apr 2009 11:20:03 +0200
> Eric Dumazet <dada1@...mosbay.com> wrote:
> 
> > Martin Schwidefsky a écrit :
> > > 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?
> > > 
> > 
> > Yes it does, on my machine at least :
> > 
> > 11:18:48 AM  CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest   %idle
> > 11:18:58 AM  all    0.00    0.00    0.00    0.00    0.21    0.69    0.00    0.00   99.10
> > 11:18:58 AM    0    0.00    0.00    0.00    0.00    1.70    5.50    0.00    0.00   92.80  << HERE >>
> > 11:18:58 AM    1    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00  100.00
> > 11:18:58 AM    2    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00  100.00
> > 11:18:58 AM    3    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00  100.00
> > 11:18:58 AM    4    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00  100.00
> > 11:18:58 AM    5    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00  100.00
> > 11:18:58 AM    6    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00  100.00
> > 11:18:58 AM    7    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00  100.00
> 
> Very good. Acked-by: Martin Schwidefsky <schwidefsky@...ibm.com> 

Thanks.

Eric, mind (re-)sending the patch with Martin's ack included, and 
with either a suitable impact-line footer or an extra paragraph that 
describes the bug you found (and how it shows up in practice) and 
how the patch fixed that problem.

Thanks,

	Ingo
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