linux-kernel - Re: [PATCH 1/7] sched: Update rq clock on nohz CPU before migrating tasks

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Message-ID: <1365498784.2609.165.camel@laptop>
Date:	Tue, 09 Apr 2013 11:13:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Mike Galbraith <efault@....de>
Subject: Re: [PATCH 1/7] sched: Update rq clock on nohz CPU before migrating
 tasks

On Sat, 2013-04-06 at 18:45 +0200, Frederic Weisbecker wrote:
> Because the sched_class::put_prev_task() callback of rt and fair
> classes are referring to the rq clock to update their runtime
> statistics. A CPU running in tickless mode may carry a stale value.
> We need to update it there.

I'm failing to see how tickless makes a difference here.. we should
never rely on a ->clock set at the last tick, that's wrong.

So either explain which/how clock update gets lost by tickless or make
it unconditional.

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