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

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Date:	Thu, 22 Jan 2015 11:21:51 +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 13:48, Daniel Thompson wrote:
> 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.

Just to update the record...

The above paragraph is wrong in every possible way. It is a livelock
(and I'm working on it).



> 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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