linux-kernel - Re: [PATCH v3 4/4] nmi_backtrace: generate one-line reports for idle cpus

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Message-ID: <1714182.jSNjvC6qiX@vostro.rjw.lan>
Date:	Tue, 22 Mar 2016 23:28:03 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chris Metcalf <cmetcalf@...lanox.com>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Aaron Tomlin <atomlin@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Andrew Morton <akpm@...l.org>,
	Daniel Thompson <daniel.thompson@...aro.org>, x86@...nel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH v3 4/4] nmi_backtrace: generate one-line reports for idle cpus

On Tuesday, March 22, 2016 06:30:05 PM Peter Zijlstra wrote:
> On Tue, Mar 22, 2016 at 01:19:39PM -0400, Chris Metcalf wrote:
> > When doing an nmi backtrace of many cores, most of which are idle,
> > the output is a little overwhelming and very uninformative.  Suppress
> > messages for cpus that are idling when they are interrupted and just
> > emit one line, "NMI backtrace for N skipped: idling at pc 0xNNN".
> > 
> > We do this by grouping all the cpuidle code together into a new
> > .cpuidle.text section, and then checking the address of the
> > interrupted PC to see if it lies within that section.
> > 
> > This commit suitably tags x86, arm64, and tile idle routines,
> > and only adds in the minimal framework for other architectures.
> > 
> > Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Tested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Signed-off-by: Chris Metcalf <cmetcalf@...lanox.com>
> 
> For some reason I found a few CPUs using poll_idle().
> 
> Rafael, when and why would that ever get selected as a useful idle
> state? When the predicted idle time is so short even C1 isn't worth it?

Yes, that's the case.