Re: [PATCH 1/5] perf: Provide a proper stop action for software events

[Frederic Weisbecker <fweisbec@gmail.com>]

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.