Re: [PATCH v2 03/20] perf/core: add PERF_PMU_CAP_EXCLUDE for exclusion capable PMUs

[Andrew Murray <andrew.murray@arm.com>]

On Mon, Nov 26, 2018 at 02:10:24PM +0000, Robin Murphy wrote:
> Hi Andrew,
> 
> On 26/11/2018 11:12, Andrew Murray wrote:
> > Many PMU drivers do not have the capability to exclude counting events
> > that occur in specific contexts such as idle, kernel, guest, etc. These
> > drivers indicate this by returning an error in their event_init upon
> > testing the events attribute flags. This approach is error prone and
> > often inconsistent.
> > 
> > Let's instead allow PMU drivers to advertise their ability to exclude
> > based on context via a new capability: PERF_PMU_CAP_EXCLUDE. This
> > allows the perf core to reject requests for exclusion events where
> > there is no support in the PMU.
> > 
> > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> > ---
> >   include/linux/perf_event.h | 1 +
> >   kernel/events/core.c       | 9 +++++++++
> >   2 files changed, 10 insertions(+)
> > 
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index b2e806f..69b3d65 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -244,6 +244,7 @@ struct perf_event;
> >   #define PERF_PMU_CAP_EXCLUSIVE			0x10
> >   #define PERF_PMU_CAP_ITRACE			0x20
> >   #define PERF_PMU_CAP_HETEROGENEOUS_CPUS		0x40
> > +#define PERF_PMU_CAP_EXCLUDE			0x80
> >   /**
> >    * struct pmu - generic performance monitoring unit
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 5a97f34..9afb33c 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -9743,6 +9743,15 @@ static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)
> >   	if (ctx)
> >   		perf_event_ctx_unlock(event->group_leader, ctx);
> > +	if (!ret) {
> > +		if (!(pmu->capabilities & PERF_PMU_CAP_EXCLUDE) &&
> > +				event_has_any_exclude_flag(event)) {
> 
> Technically this is a bisection-breaker, since no driver has this capability
> yet - ideally, this patch should come after all the ones introducing it to
> the relevant drivers (with the removal of the now-redundant code from the
> other drivers at the end).

Indeed. Thought it is possible to first introduce the capability, update the
relevant drivers to advertise it, then add the change to core.c and finally
remove the unnecessary error checks as a result of using the new capability.
This approach could be bisection-proof.

> 
> Alternatively, since we already have several other negative capabilities,
> unless there's a strong feeling against adding any more then it might work
> out simpler to flip it to PERF_PMU_CAP_NO_EXCLUDE, such that we only need to
> introduce the core check then directly replace the open-coded event checks
> with the capability in the appropriate drivers, and need not touch the
> exclusion-supporting ones at all.

This would certaintly be less risky and invasive (e.g. compare the number of
files touched between this v2 and the previous v1).

I'm happy with either approach.

Thanks,

Andrew Murray

> 
> Robin.
> 
> > +			if (event->destroy)
> > +				event->destroy(event);
> > +			ret = -EINVAL;
> > +		}
> > +	}
> > +
> >   	if (ret)
> >   		module_put(pmu->module);
> >