Re: [PATCH V4 07/14] perf/x86/intel: Support hardware TopDown metrics

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Sep 30, 2019 at 03:06:15PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 16, 2019 at 06:41:21AM -0700, kan.liang@linux.intel.com wrote:

> > +static bool is_first_topdown_event_in_group(struct perf_event *event)
> > +{
> > +	struct perf_event *first = NULL;
> > +
> > +	if (is_topdown_event(event->group_leader))
> > +		first = event->group_leader;
> > +	else {
> > +		for_each_sibling_event(first, event->group_leader)
> > +			if (is_topdown_event(first))
> > +				break;
> > +	}
> > +
> > +	if (event == first)
> > +		return true;
> > +
> > +	return false;
> > +}
> 
> > +static u64 icl_update_topdown_event(struct perf_event *event)
> > +{
> > +	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> > +	struct perf_event *other;
> > +	u64 slots, metrics;
> > +	int idx;
> > +
> > +	/*
> > +	 * Only need to update all events for the first
> > +	 * slots/metrics event in a group
> > +	 */
> > +	if (event && !is_first_topdown_event_in_group(event))
> > +		return 0;
> 
> This is pretty crap and approaches O(n^2); let me think if there's
> anything saner to do here.

This is also really complicated in the case where we do
perf_remove_from_context() in the 'wrong' order.

In that case we get detached events that are not up-to-date (and never
will be). It doesn't look like that matters, but it is weird.