linux-kernel - Re: [PATCH tip/core/rcu 1/1] Tiny RCU changes for 3.9

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Date:	Mon, 7 Jan 2013 08:57:48 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 1/1] Tiny RCU changes for 3.9

On Mon, Jan 07, 2013 at 07:58:10AM -0800, Josh Triplett wrote:
> On Sat, Jan 05, 2013 at 09:50:59AM -0800, Paul E. McKenney wrote:
> > rcu: Provide RCU CPU stall warnings for tiny RCU
> > 
> > Tiny RCU has historically omitted RCU CPU stall warnings in order to
> > reduce memory requirements, however, lack of these warnings caused Thomas
> > Gleixner some debugging pain recently.  Therefore, this commit adds RCU
> > CPU stall warnings to tiny RCU if RCU_TRACE=y.  This keeps the memory
> > footprint small, while still enabling CPU stall warnings in kernels
> > built to enable them.
> > 
> > This is still a bit on the high-risk side, so running this will likely
> > be a debugging exercise.
> > 
> > Reported-by: Thomas Gleixner <tglx@...utronix.de>
> > Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> Did you generate this patch with something other than git?  The
> formatting seems a bit off: it doesn't have a diffstat or the usual
> "---" line between the commit message and the patch.

Indeed I did -- couldn't see the point of sending a 0/1 and 1/1
series of patches.  ;-)

> This patch seems reasonable to me, but the repeated use of #if
> defined(CONFIG_SMP) || defined(CONFIG_RCU_TRACE) seems somewhat
> annoying, and fragile if you ever decide to change the conditions.  How
> about defining an appropriate symbol in Kconfig for stall warnings, and
> using that?

But I only just removed the config option for SMP RCU stall warnings.  ;-)

But I must agree that "defined(CONFIG_SMP) || defined(CONFIG_RCU_TRACE)"
is a bit obscure.  The rationale is that RCU stall warnings are
unconditionally enabled in SMP kernels, but don't want to be in
TINY_RCU kernels due to size constraints.  I therefore put it under
CONFIG_RCU_TRACE, which also contains other TINY_RCU debugging-style
options.  Would adding a comment to this effect help?

							Thanx, Paul

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