linux-kernel - Re: [PATCH tip/core/rcu 11/15] rcu: Avoid spurious RCU CPU stall warnings

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Message-ID: <1346944758.18408.35.camel@twins>
Date:	Thu, 06 Sep 2012 17:19:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 11/15] rcu: Avoid spurious RCU CPU stall
 warnings

On Thu, 2012-09-06 at 11:07 -0400, Steven Rostedt wrote:
> On Thu, 2012-09-06 at 16:56 +0200, Peter Zijlstra wrote:
> > On Thu, 2012-08-30 at 11:56 -0700, Paul E. McKenney wrote:
> > > 
> > > If a given CPU avoids the idle loop but also avoids starting a new
> > > RCU grace period for a full minute, RCU can issue spurious RCU CPU
> > > stall warnings.  This commit fixes this issue by adding a check for
> > > ongoing grace period to avoid these spurious stall warnings. 
> > 
> > How would it avoid starting a new period for over a minute? fqs should
> > happen, right? And holding rcu_read_lock() for over a minute surely is a
> > bug.
> 
> I can see this happening in test cases, but it would seem weird on a
> normal system. That is, for preempt rcu, having a process scheduled out
> holding an rcu_read_lock() for over a minute could happen on a really
> stressed out system. But for such a case, I don't think a warning is out
> of question.

One would hope that fqs would boost things.. but yeah, if your app is
spinning above the rcu boost prio you're still toast. But in that case
you're right, a warning is fully deserved.
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