Re: [PATCH v7] soc: qcom: add l2 cache perf events driver

[Leeder, Neil]

Hi Will,

On 11/9/2016 1:16 PM, Will Deacon wrote:
> On Wed, Nov 09, 2016 at 05:54:13PM +0000, Mark Rutland wrote:
> > On Fri, Oct 28, 2016 at 04:50:13PM -0400, Neil Leeder wrote:
> > > +	struct perf_event *events[MAX_L2_CTRS];
> > > +	struct l2cache_pmu *l2cache_pmu;
> > > +	DECLARE_BITMAP(used_counters, MAX_L2_CTRS);
> > > +	DECLARE_BITMAP(used_groups, L2_EVT_GROUP_MAX + 1);
> > > +	int group_to_counter[L2_EVT_GROUP_MAX + 1];
> > > +	int irq;
> > > +	/* The CPU that is used for collecting events on this cluster */
> > > +	int on_cpu;
> > > +	/* All the CPUs associated with this cluster */
> > > +	cpumask_t cluster_cpus;
> >
> > I'm still uncertain about aggregating all cluster PMUs into a larger
> > PMU, given the cluster PMUs are logically independent (at least in terms
> > of the programming model).
> >
> > However, from what I understand the x86 uncore PMU drivers aggregate
> > symmetric instances of uncore PMUs (and also aggregate across packages
> > to the same logical PMU).
> >
> > Whatever we do, it would be nice for the uncore drivers to align on a
> > common behaviour (and I think we're currently going the oppposite route
> > with Cavium's uncore PMU). Will, thoughts?
>
> I'm not a big fan of aggregating this stuff. Ultimately, the user in the
> driving seat of perf is going to need some knowledge about the toplogy of
> the system in order to perform sensible profiling using an uncore PMU.
> If the kernel tries to present a single, unified PMU then we paint ourselves
> into a corner when the hardware isn't as symmetric as we want it to be
> (big/little on the CPU side is the extreme example of this). If we want
> to be consistent, then exposing each uncore unit as a separate PMU is
> the way to go. That doesn't mean we can't aggregate the components of a
> distributed PMU (e.g. the CCN or the SMMU), but we don't want to aggregate
> at the programming interface/IP block level.
>
> We could consider exposing some topology information in sysfs if that's
> seen as an issue with the non-aggregated case.
>
> Will

So is there a use-case for individual uncore PMUs when they can't be 
used in task mode or per-cpu?

The main (only?) use will be in system mode, in which case surely it 
makes sense to provide a single aggregated count?

With individual PMUs exposed there will be potentially dozens of nodes 
for userspace to collect from which would make perf command-line usage 
unwieldy at best.

Neil
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