linux-kernel - Re: [PATCH 1/4] sched: Don't account tickless CPU load on tick

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Message-ID: <20160120102614.GA5310@X58A-UD3R>
Date:	Wed, 20 Jan 2016 19:26:14 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/4] sched: Don't account tickless CPU load on tick

On Wed, Jan 20, 2016 at 02:43:35PM +0900, Byungchul Park wrote:
> 
> It looks very tricky. I have a question. Do we have to call the
> scheduler_tick() even while the tick is stopped? IMHO, it seems to be
> ok even if we won't call it while the tick is stopped. Wrong? I mean,
> 

The reason why I asked is that, scheduler_tick() looks to be a
scheduler callback for *periodic tick*. IMHO, we need to choose one of
these two.

1) Make scheduler_tick() can handle it, not only for the periodic tick
but also for the tick-like event during tick-stopped. But I am not sure
if this is the right way.

2) Distinguish the periodic tick from the tick-like event by which we
can handle rcu callback, irq work and so on, so that the periodic tick
handler only handles periodic stuff either locally or remotely, while
the tick-like event handler only does its purpose. I think this is
better, I am sure though.

> ---
> 
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index bbc5d11..774adc2 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1422,7 +1422,8 @@ void update_process_times(int user_tick)
>  	if (in_irq())
>  		irq_work_tick();
>  #endif
> -	scheduler_tick();
> +	if (!tick_nohz_tick_stopped())
> +		scheduler_tick();
>  	run_posix_cpu_timers(p);
>  }
>  
> ---
> 
> hm ???