Re: [PATCH 7/9] perf/x86/intel: Disable sampling read slots and topdown

[Liang, Kan]

On 5/28/2019 9:52 AM, Peter Zijlstra wrote:
> On Tue, May 21, 2019 at 02:40:53PM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> >
> > To get correct PERF_METRICS value, the fixed counter 3 must start from
> > 0. It would bring problems when sampling read slots and topdown events.
> > For example,
> >          perf record -e '{slots, topdown-retiring}:S'
> > The slots would not overflow if it starts from 0.
> >
> > Add specific validate_group() support to reject the case and error out
> > for Icelake.
> >
> > Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/events/core.c       |  2 ++
> >   arch/x86/events/intel/core.c | 20 ++++++++++++++++++++
> >   arch/x86/events/perf_event.h |  2 ++
> >   3 files changed, 24 insertions(+)
> >
> > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> > index 07ecfe75f0e6..a7eb842f8651 100644
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -2065,6 +2065,8 @@ static int validate_group(struct perf_event *event)
> >   	fake_cpuc->n_events = 0;
> >   	ret = x86_pmu.schedule_events(fake_cpuc, n, NULL);
> >   
> > +	if (x86_pmu.validate_group)
> > +		ret = x86_pmu.validate_group(fake_cpuc, n);
> >   out:
> >   	free_fake_cpuc(fake_cpuc);
> >   	return ret;
> > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> > index 79e9d05e047d..2bb90d652a35 100644
> > --- a/arch/x86/events/intel/core.c
> > +++ b/arch/x86/events/intel/core.c
> > @@ -4410,6 +4410,25 @@ static int icl_set_period(struct perf_event *event)
> >   	return 1;
> >   }
> >   
> > +static int icl_validate_group(struct cpu_hw_events *cpuc, int n)
> > +{
> > +	bool has_sampling_slots = false, has_metrics = false;
> > +	struct perf_event *e;
> > +	int i;
> > +
> > +	for (i = 0; i < n; i++) {
> > +		e = cpuc->event_list[i];
> > +		if (is_slots_event(e) && is_sampling_event(e))
> > +			has_sampling_slots = true;
> > +
> > +		if (is_perf_metrics_event(e))
> > +			has_metrics = true;
> > +	}
> > +	if (unlikely(has_sampling_slots && has_metrics))
> > +		return -EINVAL;
> > +	return 0;
> > +}
>
> Why this special hack, why not disallow sampling on SLOTS on creation?

You mean unconditionally disable SLOTS sampling?

The SLOTS doesn't have to be with Topdown metrics event.
I think users may want to only sampling slot events. We should allow 
this usage.

Thanks,
Kan