linux-kernel - Re: [PATCH v3 2/2] sched: consider missed ticks when updating global cpu load

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Date:	Fri, 2 Oct 2015 17:59:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	byungchul.park@....com
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, fweisbec@...il.com,
	tglx@...utronix.de
Subject: Re: [PATCH v3 2/2] sched: consider missed ticks when updating global
 cpu load

On Fri, Oct 02, 2015 at 04:46:14PM +0900, byungchul.park@....com wrote:
> From: Byungchul Park <byungchul.park@....com>
> 
> in hrtimer_interrupt(), the first tick_program_event() can be failed
> because the next timer could be already expired due to,
> (see the comment in hrtimer_interrupt())
> 
> - tracing
> - long lasting callbacks

If anything keeps interrupts disabled for longer than 1 tick, you'd
better go fix that.

> - being scheduled away when running in a VM

Not sure how much I should care about that, and this patch is completely
wrong for that anyhow.

And this case in hrtimer_interrupt() is basically a fail case, if you
hit that, you've got bigger problems. The solution is to rework things
so you don't get there.


> in the case that the first tick_program_event() is failed, the second
> tick_program_event() set the expired time to more than one tick later.
> then next tick can happen after more than one tick, even though tick is
> not stopped by e.g. NOHZ.
> 
> when the next tick occurs, update_process_times() -> scheduler_tick()
> -> update_cpu_load_active() is performed, assuming the distance between
> last tick and current tick is 1 tick! it's wrong in this case. thus,
> this abnormal case should be considered in update_cpu_load_active().

Everything in update_process_times() assumes 1 tick, just fixing up 
one function inside that callchain is wrong -- I've already told you
that.


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