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

[David Ahern <dsahern@gmail.com>]

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.

>
> 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.

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.

>
> 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