linux-kernel - Re: [PATCH 3.19-rc2 v14 0/7] arm: Implement arch_trigger_all_cpu

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Message-ID: <54BFAE3F.7000300@linaro.org>
Date:	Wed, 21 Jan 2015 13:48:47 +0000
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Stephen Boyd <sboyd@...eaurora.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Russell King <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Dirk Behme <dirk.behme@...bosch.com>,
	Daniel Drake <drake@...lessm.com>,
	Dmitry Pervushin <dpervushin@...il.com>,
	Tim Sander <tim@...eglstein.org>
Subject: Re: [PATCH 3.19-rc2 v14 0/7] arm: Implement arch_trigger_all_cpu_backtrace

On 21/01/15 13:06, Steven Rostedt wrote:
> On Wed, 21 Jan 2015 10:47:37 +0000
> Daniel Thompson <daniel.thompson@...aro.org> wrote:
> 
> 
>>> With this patchset, is it possible to call sched_clock() from within NMI
>>> context? I ask because the generic sched_clock() code is not NMI safe
> 
> That's not good. Better not run function tracing, as that could trace
> functions in NMI context (I depend on that it does), and it uses
> sched_clock() as the default clock.

I think sched_clock is unsafe as in "may sometimes give the wrong value"
rather than "can lock up arbitrarily".  Thus the impact is unlikely to
be harmful enough to want to avoid tracing altogether.

It would require special care be taken when interpreting the timestamps
however. Also since update_sched_clock() is a notrace function its very
hard to figure out when timestamps are at risk.

Anyhow, the fix doesn't seem that hard. I can take a look.


Daniel.

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