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

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Message-ID: <CAFTL4hzEM2cSX=Y5Z5AnsK3ZdrvFEWt4WwvHxx5vt4dOoWkJkQ@mail.gmail.com>
Date:	Tue, 9 Apr 2013 15:11:56 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
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

2013/4/9 Peter Zijlstra <peterz@...radead.org>:
> 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.

Ah I made that big mis-assumption then ;)

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

So most of those fixup need to be made unconditional I think.
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