linux-kernel - Re: [PATCH Resend 1/3] sched: fix nr_busy

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Date:	Thu, 24 Jan 2013 18:55:53 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, linaro-dev@...ts.linaro.org,
	peterz@...radead.org, mingo@...nel.org, ccross@...roid.com
Subject: Re: [PATCH Resend 1/3] sched: fix nr_busy_cpus with coupled cpuidle

On 24 January 2013 17:44, Frederic Weisbecker <fweisbec@...il.com> wrote:
> 2012/12/3 Vincent Guittot <vincent.guittot@...aro.org>:
>> With the coupled cpuidle driver (but probably also with other drivers),
>> a CPU loops in a temporary safe state while waiting for other CPUs of its
>> cluster to be ready to enter the coupled C-state. If an IRQ or a softirq
>> occurs, the CPU will stay in this internal loop if there is no need
>> to resched. The SCHED softirq clears the NOHZ and increases
>> nr_busy_cpus. If there is no need to resched, we will not call
>> set_cpu_sd_state_idle because of this internal loop in a cpuidle state.
>> We have to call set_cpu_sd_state_idle in tick_nohz_irq_exit which is used
>> to handle such situation.
>
> I'm a bit confused with this.
>
> set_cpu_sd_state_busy() is only called from nohz_kick_needed(). And it
> checks idle_cpu() before doing anything. So if no task is going to be
> scheduled, idle_cpu() prevents from calling set_cpu_sd_state_busy().
>
> I'm probably missing something.

Hi Frederic

I can't find back the trace that i had saved with the issue but IIRC
the sequence is:
The CPU is kicked for ILB
The wake_list of the CPU becomes not empty so cpu id not idle
CPU wakes up, updates is timer framework and call nohz_kick_needed the
execute the ILB sequence
we don't go out of the cpuidle driver function because we don't need
to resched so we don't clear the busy state

I'm going to look for the saved trace to check the sequence above

Vincent

>
> Thanks.
>
>>
>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>> ---
>>  kernel/time/tick-sched.c |    2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>> index 955d35b..b8d74ea 100644
>> --- a/kernel/time/tick-sched.c
>> +++ b/kernel/time/tick-sched.c
>> @@ -570,6 +570,8 @@ void tick_nohz_irq_exit(void)
>>         if (!ts->inidle)
>>                 return;
>>
>> +       set_cpu_sd_state_idle();
>> +
>>         /* Cancel the timer because CPU already waken up from the C-states*/
>>         menu_hrtimer_cancel();
>>         __tick_nohz_idle_enter(ts);
>> --
>> 1.7.9.5
>>
>> --
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