linux-kernel - Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples

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Date:	Wed, 3 Apr 2013 16:00:05 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	David Ahern <dsahern@...il.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	Pawel Moll <pawel.moll@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	Will Deacon <Will.Deacon@....com>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Pekka Enberg <penberg@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Robert Richter <robert.richter@....com>
Subject: Re: [RFC] perf: need to expose sched_clock to correlate user samples
 with kernel samples

On Wed, Apr 3, 2013 at 3:55 PM, David Ahern <dsahern@...il.com> wrote:
> On 4/3/13 3:17 AM, Stephane Eranian wrote:
>>
>> I haven't done any specific testing with either approach yet. The goal is
>> to
>> use this perf timestamp to correlate user level events to hardware
>> events recorded
>> by the kernel. I would assume there would be situations where those user
>> events
>> could be on the critical path, and thus the timestamp operation would have
>> to be
>> as efficient as possible. The vdso approach would be ideal.
>>
>>>
>>> The performance/speed depends on how often is called. I have no idea what
>>> Stephane's use case is but for me it is to correlate perf_clock
>>> timestamps
>>> to timeofday. In my perf-based daemon that tracks process schedulings, I
>>> update the correlation every 5-10 minutes.
>>>
>> I was more thinking along the lines of runtime environments like Java
>> where
>> a JIT compiler is invoked frequently and you need to correlate samples in
>> the
>> native code with Java source. For that, the JIT compiler has to emit
>> mapping
>> tables which have to be timestamped as address ranges may be re-used.
>
>
> What's the advantage of changing apps -- like the JIT compiler -- to emit
> perf based timestamps versus having perf emit existing timestamps? ie.,
> monotonic and realtime clocks already have vdso mappings for userspace with
> well known performance characteristics. Why not have perf convert its
> perf_clock timestamps into monotonic or realtime when dumping events?
>
Can monotonic timestamps be obtained from NMI context in the kernel?
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