[PATCH] perf print-events: make is_event_supported() more robust

[Mark Rutland]

On Thu, Jan 18, 2024 at 09:57:32PM -0800, Namhyung Kim wrote:
> Hello,
> 
> Adding Ravi to CC.
> 
> On Wed, Jan 17, 2024 at 4:12 AM Mark Rutland <mark.rutland at arm.com> wrote:
> >
> > On Wed, Jan 17, 2024 at 09:05:25AM +0000, Marc Zyngier wrote:
> > > Hi Mark,
> > >
> > > On Tue, 16 Jan 2024 17:03:48 +0000,
> > > Mark Rutland <mark.rutland at arm.com> wrote:
> > > >
> > > > Currently the perf tool doesn't deteect support for extneded event types
> > > > on Apple M1/M2 systems, and will not auto-expand plain PERF_EVENT_TYPE
> > > > hardware events into per-PMU events. This is due to the detection of
> > > > extended event types not handling mandatory filters required by the
> > > > M1/M2 PMU driver.
> > >
> > > Thanks for looking into this.
> > >
> > > I've given your patch a go on my M1 box, and it indeed makes things
> > > substantially better:
> > >
> > > $ sudo ./perf stat -e cycles ~/hackbench 100 process 1000
> > > Running with 100*40 (== 4000) tasks.
> > > Time: 3.419
> > >
> > >  Performance counter stats for '/home/maz/hackbench 100 process 1000':
> > >
> > >    174,783,472,090      apple_firestorm_pmu/cycles/                                             (93.10%)
> > >     39,134,744,813      apple_icestorm_pmu/cycles/                                              (71.86%)
> > >
> > >        3.568145595 seconds time elapsed
> > >
> > >       12.203084000 seconds user
> > >       55.135271000 seconds sys
> >
> > Thanks for giving that a spin!
> >
> > > However, I'm seeing some slightly odd behaviours:
> > >
> > > $ sudo ./perf stat -e cycles:k ~/hackbench 100 process 1000
> > > Running with 100*40 (== 4000) tasks.
> > > Time: 3.313
> > >
> > >  Performance counter stats for '/home/maz/hackbench 100 process 1000':
> > >
> > >    <not supported>      apple_firestorm_pmu/cycles:k/
> > >    <not supported>      apple_icestorm_pmu/cycles:k/
> 
> Hmm.. I guess this should look like apple_firestorm_pmu/cycles/k.

Yeah, the ":k" within the slashes isn't right, and I suspect that's due to the
way the event gets expanded.

Ian, is that something you're aware of already?

> IIRC there was a thread for this, right?

I'm not aware of a thread for the way filters get applied to expanded events.

> > >
> > >        3.467568841 seconds time elapsed
> > >
> > >       13.080111000 seconds user
> > >       53.162099000 seconds sys
> > >
> > > I would have expected it to count, but it didn't. For that to work, I
> > > have to add the 'H' modifier:
> >
> > Ok, so that'll have something to do with the way the tool chooses which
> > perf_evant_attr::exclude_* bits to set. I thought that was the same for plain
> > events and pmu_name/event/ events, but I could be mistaken.
> 
> I think it sets the attr.exclude_guest by event_attr_init().  Maybe
> it's deleted during the missing feature detection logic.  But IIUC
> it should work on each PMU separately.

I'll try to look into this a bit more.

> By the way, I really hope the kernel exports caps/exclude_bits
> for PMUs so that tools can see which bits are supported.  For
> example AMD IBS has CAP_NO_EXCLUDE so setting exclude_guest
> will fail to open.  Then it disables the new features added after
> that in the missing feature detection logic.

I'm ok in principle with exposing some info on the supported exclude_*
configuration, but it's worth noting that event where a PMU supports specific
filters, it might not support all combinations of filters for all events. For
example, s390 doesn't support exclude_* filters on RAW events, and doesn't
support kernel-only counters. Given that, I'm not sure how we can expose that
in a useful way.

Mark.

> If we know if it doesn't support any exclude bits, then tools can try other
> features after removing the bit first.
> 
> Thanks,
> Namhyung
> 
> >
> > Is that something you had tried prior to this patch, and did that "just work"
> > with the explicit pmu_name/event/ syntax prior to this patch?
> >
> > e.g. did something like:
> >
> >         perf stat -e apple_firestorm_pmu/cycles/k -e apple_icestorm_pmu/cycles/k ./workload
> >
> > ... happen to work withiout requiring the addition of 'H'?
> >
> > If so, does that behave the same before/after this patch?
> >
> > ... and could you run that with '-vvv' and dump the output for comparison?
> >
> > Thanks,
> > Mark.