linux-kernel - Re: [PATCH v7 4/4] nmi_backtrace: generate one-line reports for idle cpus

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Date:	Tue, 16 Aug 2016 10:04:01 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Chris Metcalf <cmetcalf@...lanox.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	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 v7 4/4] nmi_backtrace: generate one-line reports for idle
 cpus

On Mon 2016-08-15 12:41:54, Chris Metcalf wrote:
> On 8/11/2016 11:25 AM, Petr Mladek wrote:
> >On Mon 2016-08-08 12:03:38, 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".
> >Hmm, the problem is that native_safe_halt() is called from default_idle()
> >here. The function is marked as inline but the compiler did not inline
> >it.
> >
> >It helped me to put native_safe_halt() into the __cpuidle_text section:
> 
> Following Peter Z's suggestion, I have added an __always_inline marker
> to native_safe_halt.  For consistency, I also did arch_safe_halt(), since that
> invokes native_safe_halt, and then also native_halt() and halt(), so that
> we're not being weirdly inconsistent with markings for halt inlines.
> 
> There are also the native_irq_enable(), etc., accessors in that same header
> that are still only marked "inline" not "always_inline", but I wanted to stop
> before I was touching too much unrelated code.

Sounds fine.

> >I wonder if it would be possible to detect the idle thread an other
> >way. For example, I wonder if it would be enough to check for the
> >PID 0.
> 
> No, the problem is that pid 0 can also go off and run "interesting" code
> for things like power management, etc., so we really just want to
> focus on being quite sure that the running code isn't interesting before
> we suppress the backtrace information.
> 
> See the thread around here:
> 
> https://lkml.kernel.org/r/20160307204317.GR6344@twins.programming.kicks-ass.net

Makes sense. Thanks for the poitner.

Best Regards,
Petr