Re: [RFC][PATCH] perf_events: added new start/stop PMU callbacks

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2010-02-08 at 17:06 +0200, Stephane Eranian wrote:
> 	In certain situations, the kernel may need to stop and start the
> 	same event rapidly. The current PMU callbacks do not distinguish
> 	between stop and release  (i.e., stop + free the resource). Thus,
> 	a counter may be released, then it will be immediately re-acquired.
> 	Event scheduling will again take place with no guarantee to assign
> 	the same counter. On some processors, this may event yield to failure
> 	to assign the event back due to competion between cores.
> 
> 	This patch is adding a new pair of callback to stop and restart a
> 	counter without actually release the underlying counter resource.
> 	On stop, the counter is stopped, its values saved and that's it.
> 	On start, the value is reloaded and counter is restarted (on x86,
> 	actual restart is delayed until perf_enable()).
> 	
> 	Note this patch does not provide support for non-X86 PMU. This needs
> 	to be added.
> 
> 	Signed-off-by: Stephane Eranian <eranian@google.com>
> --

I think we can do this much easier by adding a list_head to
hw_perf_event and make event_list into a proper list, then we can remove
that funny loop on remove and instead move the event to a remove_list
when there's a put_event_constraint() method and iterate that list on
hw_perf_enable().

But before we do that, I think we need to look at the /* hardware */
part of struct hw_perf_event, and make that arch specific, we've been
growing that a lot lately and I don't think !x86 uses any of that.