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

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Date:	Wed, 10 Apr 2013 09:05:12 +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 2/7] sched: Update rq clock on nohz CPU before setting
 fair group shares

On Tue, 2013-04-09 at 15:21 +0200, Frederic Weisbecker wrote:
> 
> 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

Right.. I was thinking of something like function-graph trace, where
we'd mark the context (fgraph_data I think its called) of the function
calling update_rq_clock() (and inherit this state on all child
contexts) and then verify the current context has this marker set when
we read ->clock.

Now clearly that would not work for something like:

  a()
   b()
    update_rq_clock()
   c()
    read_rq_clock()

since we'd loose the marker once we pop b, but I'm not sure how common
this is.

Also, we could issue a warning when calling update_rq_clock() and the
marker is already set.

There's a further issue where we need to track the rq we updated vs the
rq we're reading, but I think even without that this might be of some
help.

I think Mike once tried something along the lines of keeping a per rq
state that got cleared at the end of schedule() but that doesn't catch
things like the migrate handlers I think.

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