Messages in this thread | | | Date | Tue, 19 Feb 2013 10:25:51 -0800 | From | John Stultz <> | 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
| |