linux-kernel - Re: [PATCH 2/7] sched: Update rq clock on nohz CPU before setting fair group shares

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Message-ID: <CAFTL4hxcQyv_J4ky8wL0rzVemWPpBcjuCV=FeHBvXn5Bht-3OA@mail.gmail.com>
Date:	Tue, 9 Apr 2013 15:21:49 +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 2/7] sched: Update rq clock on nohz CPU before setting
 fair group shares

2013/4/9 Peter Zijlstra <peterz@...radead.org>:
> On Sat, 2013-04-06 at 18:45 +0200, Frederic Weisbecker wrote:
>> Because we may update the execution time (sched_group_set_shares()->
>>         update_cfs_shares()->reweight_entity()->update_curr()) before
>> reweighting the entity after updating the group shares and this
>> requires
>> an uptodate version of the runqueue clock. Let's update it on the
>> target
>> CPU if it runs tickless because scheduler_tick() is not there to
>> maintain
>> it.
>
> Same as the last comment, we should never rely on the tick to update
> ->clock except for the work done by the tick itself.

Ok!

>
> Therefore you seem to have found another missing clock update.

Yep, as in the other patches I believe, I'll reiterate that by
removing that rely-on-tick assumption.

> The problem seems to be that we haven't been able to come up with a
> sane debug framework for the ->clock updates. But we should have at
> least one (and preferably no more) update_sched_clock() calls per
> scheduler entry point.

I'll check if I can factorize some update_rq_clock() calls on some
upper sched_class callback wrappers. Many of them have that update
before calling the callbacks already. But there may be a few missing.
I'll check the other sched entrypoints as well.

Also please check those two patches in my series, it's a draft for an
rq clock debug framework. For now it's just a brainless stale clock
check but that's a start:

* http://thread.gmane.org/gmane.linux.kernel/1470769/focus=1470786
* http://thread.gmane.org/gmane.linux.kernel/1470769/focus=1470750
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