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

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Date:	Tue, 19 Feb 2013 10:25:51 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Stephane Eranian <eranian@...gle.com>,
	Pawel Moll <pawel.moll@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	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 02/18/2013 12:35 PM, Thomas Gleixner wrote:
> On Tue, 5 Feb 2013, John Stultz wrote:
>> On 02/05/2013 02:13 PM, Stephane Eranian wrote:
>>> But if people are strongly opposed to the clock_gettime() approach, then
>>> I can go with the ioctl() because the functionality is definitively needed
>>> ASAP.
>> I prefer the ioctl method, since its less likely to be re-purposed/misused.
> Urgh. No! With a dedicated CLOCK_PERF we might have a decent chance to
> put this into a vsyscall. With an ioctl not so much.
>   
>> Though I'd be most comfortable with finding some way for perf-timestamps to be
>> CLOCK_MONOTONIC based (or maybe CLOCK_MONOTONIC_RAW if it would be easier),
>> and just avoid all together adding another time domain that doesn't really
>> have clear definition (other then "what perf uses").
> What's wrong with that. We already have the infrastructure to create
> dynamic time domains which can be completely disconnected from
> everything else.

Right, but those are for actual hardware domains that we had no other 
way of interacting with.

> Tracing/perf/instrumentation is a different domain and the main issue
> there is performance. So going for a vsyscall enabled clock_gettime()
> approach is definitely the best thing to do.

So describe how the perf time domain is different then CLOCK_MONOTONIC_RAW.

My concern here is that we're basically creating a kernel interface that 
exports implementation-defined semantics (again: whatever perf does 
right now). And I think folks want to do this, because adding CLOCK_PERF 
is easier then trying to:

1) Get a lock-free method for accessing CLOCK_MONOTONIC_RAW

2) Having perf interpolate its timestamps to CLOCK_MONOTONIC, or 
CLOCKMONOTONIC_RAW when it exports the data

The semantics on sched_clock() have been very flexible and hand-wavy in 
the past. And I agree with the need for the kernel to have a 
"fast-and-loose" clock as well as the benefits to that flexibility as 
the scheduler code has evolved.  But non-the-less, the changes in its 
semantics have bitten us badly a few times.

So I totally understand why the vsyscall is attractive. I'm just very 
cautious about exporting a similarly fuzzily defined interface to 
userland. So until its clear what the semantics will need to be going 
forward (forever!), my preference will be that we not add it.

thanks
-john

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