Re: [PATCH 10/11] perf/x86/intel: Support task events with Intel CQM

[Peter Zijlstra]

On Wed, Oct 08, 2014 at 01:07:43PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 24, 2014 at 03:04:14PM +0100, Matt Fleming wrote:
> > From: Matt Fleming <matt.fleming@xxxxxxxxx>
> > 
> > Add support for task events as well as system-wide events. This change
> > has a big impact on the way that we gather LLC occupancy values in
> > intel_cqm_event_read().
> > 
> > Currently, for system-wide (per-cpu) events we defer processing to
> > userspace which knows how to discard all but one cpu result per package.
> > 
> > Things aren't so simple for task events because we need to do the value
> > aggregation ourselves. To do this, we defer updating the LLC occupancy
> > value in event->count from intel_cqm_event_read() and do an SMP
> > cross-call to read values for all packages in intel_cqm_event_count().
> > We need to ensure that we only do this for one task event per cache
> > group, otherwise we'll report duplicate values.
> > 
> > If we're a system-wide event we want to fallback to the default
> > perf_event_count() implementation. Refactor this into a common function
> > so that we don't duplicate the code.
> 
> So it looks like these events will be classified as regular HW events,
> this means they'll be mixed with the other HW events, and we'll stop
> scheduling the moment either one returns a fail.
> 
> There are two alternatives;
>  1) create an extra task context to keep them in
>  2) pretend to be a software event and do the scheduling yourself
> 
> I think my initial proposal was 2, can you clarify why you've changed
> that? Lemme go read the next patch though, maybe that'll clarify things
> further.

As you rightly pointed out on IRC, the following (as found in patch 8):

+static struct pmu intel_cqm_pmu = {
+       .attr_groups    = intel_cqm_attr_groups,
+       .task_ctx_nr    = perf_sw_context,

Does indeed make it a software event, so then need to do all the
scheduling outselves.
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