Messages in this thread | | | Date | Thu, 10 Jun 2010 21:54:53 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 1/5] perf: Provide a proper stop action for software events |
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On Thu, Jun 10, 2010 at 06:16:16PM +0200, Peter Zijlstra wrote: > On Thu, 2010-06-10 at 18:12 +0200, Frederic Weisbecker wrote: > > On Thu, Jun 10, 2010 at 01:10:42PM +0200, Peter Zijlstra wrote: > > > On Thu, 2010-06-10 at 12:46 +0200, Peter Zijlstra wrote: > > > > > > > > Something like the below would work, the only 'problem' is that it grows > > > > hw_perf_event. > > > > > > If we do the whole PAUSEd thing right, we'd not need this I think. > > > > > > It's not needed, and moreover software_pmu:stop/start() can be the same > > than software:pmu:disable/enable() without the need to add another check > > in the fast path. > > > > But we need perf_event_stop/start() to work on software events. And in fact > > now that we use the hlist_del_init, it's safe, but a bit wasteful in > > the period reset path. That's another problem that is not critical, but > > if you want to solve this by ripping the differences between software and > > hardware (which I agree with), we need a ->reset_period callback. > > > Why? ->start() should reprogram the hardware, so a > ->stop()/poke-at-state/->start() cycle is much more flexible.
Reconsidering the situation after remembering the race with software events on period adjusting:
In fact, if we want to support start/stop on software events, we still need the if (!software event) in perf_adjust_period(), otherwise start and stop may race on a software event with the hlist ops.
So it's now both useless and dangerous.
What about keeping this software event check for now? Once we'll have a pmu:disable_all()/enable_all(), this can serve as a more appropriate check later.
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