linux-kernel - Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task enqueue/dequeue

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Message-ID: <20160119131708.GF6344@twins.programming.kicks-ass.net>
Date:	Tue, 19 Jan 2016 14:17:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>,
	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: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task
 enqueue/dequeue

On Wed, Jan 13, 2016 at 05:01:31PM +0100, Frederic Weisbecker wrote:
> The full nohz CPU load is currently accounted on tick restart only.
> But there are a few issues with this model:
> 
> _ On tick restart, if cpu_load[0] doesn't contain the load of the actual
>   tickless load that just ran, we are going to account a wrong value.
>   And it is very likely to be so given that cpu_load[0] doesn't have
>   an opportunity to be updated between tick stop and tick restart.
> 
> _ If the runqueue had updates that didn't trigger a tick restart, we
>   are going to miss those CPU load changes.
> 
> A solution to fix this is to update the CPU load everytime we enqueue
> or dequeue a task in the fair runqueue and more than a jiffy occured
> since the last update.

Would not a much better solution be to do this remotely instead of from
one of the hottest functions in the scheduler?