Date:	Wed, 3 Apr 2013 11:17:12 +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 Tue, Apr 2, 2013 at 12:29 AM, David Ahern <dsahern@...il.com> wrote:
> On 4/1/13 12:29 PM, John Stultz wrote:
>>>
>>> Any chance a decision can be reached in time for 3.10? Seems like the
>>> simplest option is the perf event based ioctl.
>>
>>
>> I'm still not sold on the CLOCK_PERF posix clock. The semantics are
>> still too hand-wavy and implementation specific.
>>
>> While I'd prefer perf to export some existing semi-sane time domain
>> (using interpolation if necessary), I realize the hardware constraints
>> and performance optimizations make this unlikely (though I'm
>> disappointed I've not seen any attempt or proof point that it won't work).
>>
>> Thus if we must expose this kernel detail to userland, I think we should
>> be careful about how publicly we expose such an interface, as it has the
>> potential for misuse and eventual user-land breakage.
>
>
> But perf_clock timestamps are already exposed to userland. This new API --
> be it a posix clock or an ioctl -- just allows retrieval of a timestamp
> outside of a generated event.
>
Agreed.
>
>>
>> So while having a perf specific ioctl is still exposing what I expect
>> will be non-static kernel internal behavior to userland, it at least it
>> exposes it in a less generic fashion, which is preferable to me.
>>
>>
>>
>> The next point of conflict is likely if the ioctl method will be
>> sufficient given performance concerns. Something I'd be interested in
>> hearing about from the folks pushing this. Right now it seems any method
>> is preferable then not having an interface - but I want to make sure
>> that's really true.
>>
>> For example, if the ioctl interface is really too slow, its likely folks
>> will end up using periodic perf ioctl samples and interpolating using
>> normal vdso clock_gettime() timestamps.
>
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.

>
>>
>> If that is acceptable, then why not invert the solution and just have
>> perf injecting periodic CLOCK_MONOTONIC timestamps into the log, then
>> have perf report fast, but less-accurate sched_clock deltas from that
>> CLOCK_MONOTONIC boundary.
>
>
> Something similar to that approach has been discussed as well. i.e, add a
> realtime clock event and have it injected into the stream e.g.,
> https://lkml.org/lkml/2011/2/27/158
>
> But there are cons to this approach -- e.g, you need that first event
> generated that tells you realtime to perf_clock correlation and you don't
> want to have to scan an unknown length of events looking for the first one
> to get the correlation only to backup and process the events.
>
> And an ioctl to generate that first event was shot down as well...
>   https://lkml.org/lkml/2011/3/1/174
>   https://lkml.org/lkml/2011/3/2/186
>
> David
>
>
>>
>> Another alternative that might be a reasonable compromise: have perf
>> register a dynamic posix clock id, which would be a driver specific,
>> less public interface. That would provide the initial method to access
>> the perf time domain. Then when it came time to optimize further,
>> someone would have to sort out the difficulties of creating a vdso
>> method for accessing dynamic posix clocks. It wouldn't be easy, but it
>> wouldn't be impossible to do.
>>
>>
>>> Converting/correlating perf_clock timestamps to time-of-day is a
>>> feature I have been trying to get into perf for over 2 years. This is
>>> a big piece needed for that goal -- along with the xtime tracepoints:
>>>   https://lkml.org/lkml/2013/3/19/433
>>
>>
>> I sympathize with how long this process can take.  Having maintainers
>> disagree without resolution can be a tar-pit. That said, its only been a
>> few months where this has had proper visibility, and the discussion has
>> paused for months at a time. Despite how long and slow this probably
>> feels, the idea of maintaining a bad interface for the next decade seems
>> much longer. ;)  So don't get discouraged yet.
>>
>> thanks
>> -john
>
>
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