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

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Message-ID: <49F81BC3.2050805@cosmosbay.com>
Date:	Wed, 29 Apr 2009 11:20:03 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.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

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



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