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

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Date:	Fri, 22 Jan 2016 17:40:24 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	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 09:42:16AM +0100, Thomas Gleixner wrote:
> 
> The above changelog is just crap and doesnt make any sense at all. And the
> patch is fixing symptoms not the root cause.

IMHO, the root cause is the "tick" definition. Am I only one confused?
I am confused now. What is tick? The timer interrupt for handling rcu
callback or irq work while the (perioid) tick was stoped, is a tick?

If it is true, then many code including scheduler assuming tick happens
periodically must be fixed to be able to handle the non-periodic tick, esp.
scheduler_tick(). And we have to focus that. Then we don't need something
accounting a cpu load remotely and periodically, because the local tick
handler can account it well locally.

If it is not true, that is, the timer interrupt for handling rcu callback
or irq work during tick-stopped, is not a tick but just a interrupt by which
we want something to be done, then I think we need to make the handler do
only its purpose. In this case, tick related handling has to be deferred to
the tick-restart point. And we have to focus it, so that it can be done.

Regardless of the answer, true or not true, if there is something the
housekeeper must do periodically, then it should be done even remotely. But
I think accounting cpu load is not the case. The way to implement it
depends on the above answer. IMHO, the letter direction is better.

Just a my opinion, and I maybe lack kernel knowledge compared with you.
Please let me know if I am wrong. Or please let me know your opinions.

> 
> Thanks,
> 
> 	tglx
>