Re: [PATCH 0/4] perf_event: introduce 'perf timer' to analyze timer's behavior

Ingo Molnar
2009-12-16T07:33:13Z

* Xiao Guangrong <xiaoguangrong@cn.fujitsu.com> wrote:

>
>
> Frederic Weisbecker wrote:
> > On Tue, Dec 15, 2009 at 07:17:03PM +0800, Xiao Guangrong wrote:
> >> Hi,
> >>
> >> We introduce 'perf timer' in this patchset, it can analyze timer
> >> latency and timer function handle time, the usage and result is
> >> like below:
> >>
> >> # perf timer record
> >> # perf timer lat --print-lat --print-handle
> >> -------------------------------------------------------------------------------------------------------
> >> | Timer | TYPE | Avg-latency | Max-latency | Max-latency-at-TS |Max-lat-at-Task |
> >> |0xf7ad1f5c |hrtimer |996068.500 ns|1607650 ns|10270128658526 |init |
> >> |0xf7903f04 |timer |0.625 HZ|2 HZ|10270344082394 |swapper |
> >> |0xf787a05c |hrtimer |200239.500 ns|359929 ns|10269316024808 |main |
> >> |main :[ PROF]|itimer |0.000 HZ|0 HZ|10237021270557 |main |
> >> |main :[VIRTUAL]|itimer |0.000 HZ|0 HZ|10257314773501 |main |
> >
> >
> >
> > Cool! This is really a good work and a good idea.
> >
> > Just have some neats in mind. hrtimers and timers don't have the same latency and
> > granularity requirements.
> >
> > As you show it, timers have an HZ granularity and hrtimers are about nanoseconds,
> > and mixing them up in the same array of latency report is too messy.
> > They don't have the same granularity/latency scope so they should
> > be reported separately.
> >
>
> Yeah, it has different unit and looks ugly :-(
>
> but, the problem is we can't get HZ in userspace now, i'll export HZ by proc
> or other way and rectify 'perf timer' output in my next work

We could export HZ as part of the ftrace event format file, in
/debug/tracing/events/timers/ or so. Not sure what the best way would be to do
it (without causing runtime overhead in event processing), but we should
reduce our reliance on /proc and export such instrumentation information via a
single, coherent interface.

There's a couple of other 'system settings' values that would be useful to
expose in such a way - for example the number of online CPUs. (in fact that
could be exposed as a set of online/offline events plus a way to read the
current count as well)

Ingo