linux-kernel - Re: [PATCH][GIT PULL] tracing: Fix compile issue for trace_sched

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Message-ID: <1287659008.3488.102.camel@twins>
Date:	Thu, 21 Oct 2010 13:03:28 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Jason Baron <jbaron@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	2nddept-manager@....hitachi.co.jp
Subject: Re: [PATCH][GIT PULL] tracing: Fix compile issue for
 trace_sched_wakeup.c

On Thu, 2010-10-21 at 07:01 -0400, Steven Rostedt wrote:
> On Thu, 2010-10-21 at 09:22 +0200, Peter Zijlstra wrote:
> > On Thu, 2010-10-21 at 11:58 +0900, Masami Hiramatsu wrote:
> > 
> > > It seems there can be a bug in stop_machine() routine under
> > > heavy use. usually that is called just once at a time, but jump
> > > label and optprobe might call it heavily (thousands times?).
> > > So some racy situation can be happen easily.
> > 
> > There are people doing hotplug stress testing, that too results in heavy
> > stop_machine usage.
> 
> But with hotplug, isn't there a bit more time between stop machine
> calls? That is, you need to do a bit of work to bring down or up a CPU,
> and that will slow down the number of stop machine calls together.
> 
> Here, we do a simple change and call stop machine() several times.
> 
> Although, I agree, I do not think the bug is in stop machine itself, but
> perhaps the way we are using it might have some niche anomaly that we
> are hitting.

Possibly, but wouldn't it make sense to batch up the work and simply
call stop_machine only once? I mean, if you already know you're going to
do this...

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