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SubjectRe: [PATCH] sched: account system time properly
Ingo Molnar a écrit :
> * Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
>
>> On Wed, 29 Apr 2009 11:20:03 +0200
>> Eric Dumazet <dada1@cosmosbay.com> wrote:
>>
>>> Martin Schwidefsky a écrit :
>>>> On Wed, 29 Apr 2009 09:46:17 +0200
>>>> Eric Dumazet <dada1@cosmosbay.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@cosmosbay.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@de.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.
>

No problem, here it is :

[PATCH] sched: account system time properly

Andrew Gallatin reported that IRQ and SOFTIRQ times were sometime not reported
correctly on recent kernels, and even bisected to commit
457533a7d3402d1d91fbc125c8bd1bd16dcd3cd4 ([PATCH] fix scaled & unscaled cputime
accounting) first bad commit.

Further analysis pointed that commit 79741dd35713ff4f6fd0eafd59fa94e8a4ba922d
([PATCH] idle cputime accounting) was the real cause of the problem.

account_process_tick() was not taking into account timer IRQ interrupting
the idle task servicing a hard or soft irq.

On mostly idle cpu, irqs were thus not accounted and top or mpstat could tell
user/admin that cpu was 100 % idle, 0.00 % irq, 0.00 % softirq, while it was not.

Reported-by: Andrew Gallatin <gallatin@myri.com>
Re-reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Tested-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.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


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