Re: [PATCH v11 8/8] perf: ARM DynamIQ Shared Unit PMU support

[Saravana Kannan]

On 02/22/2018 03:33 AM, Mark Rutland wrote:
> On Wed, Feb 21, 2018 at 06:32:46PM -0800, Saravana Kannan wrote:
> > On 01/02/2018 03:25 AM, Suzuki K Poulose wrote:
> > > +static int dsu_pmu_event_init(struct perf_event *event)
> > > +{
> > > +	struct dsu_pmu *dsu_pmu = to_dsu_pmu(event->pmu);
> > > +
> > > +	if (event->attr.type != event->pmu->type)
> > > +		return -ENOENT;
> >
> > You are checking if the caller set the attr.type "correctly".
>
> This is necessary for the case where perf_init_event() falls back to
> iterating over the list of PMUs, if event->attr.type wasn't found in the
> idr.
>
> Without this, we'd erroneously check events intended for other PMUs.
> So this is correct, and necessary.

Right, I'm aware of this. Which is why I also mentioned below that we 
can't just blindly delete this.

> > > +static int dsu_pmu_device_probe(struct platform_device *pdev)
>
> > > +	rc = perf_pmu_register(&dsu_pmu->pmu, name, -1);
> >
> > You are passing in -1 here. Which means the event type is assigned by the
> > perf framework. perf framework uses idr_alloc(&pmu_idr, ...) to get the id.
> > So the id assigned is going to depend on the probe order among the different
> > PMU drivers in the board/platform. So, this seems pretty random.
>
> The dynamic IDs are supposed to by looked up by name.
>
> Each PMU has a folder: /sys/bus/event_source/devices/$PMU
>
> ... with /sys/bus/event_source/devices/$PMU/type giving the type.
>
> > How is the caller supposed to know what to set the "type" to?
>
> The perf tools understand this already. If you do:
>
> perf stat -e $PMU/config=0xf00/
>
> ... they will look up the type for that PMU and use it automatically.

Ah, thanks! This finally explains how this is supposed to work from 
userspace.

> > You also can't just delete the check in dsu_pmu_event_init() because the
> > event numbers you expose overlap with the per-CPU event numbers.
>
> The type check is necessary and cannot be deleted. It provides a
> namespace for the event IDs.

Right. Which is my point too.

> > I'm not exactly sure if we can add entries to perf_type_id. If that's
> > allowed maybe we need to add something line PERF_TYPE_DSU and use that?
> >
> > Or if that's not allowed then would it be better to offset the DSU PMU
> > events by some number (say 0x1000) and then delete the event type check or
> > pass PERF_TYPE_RAW to perf_pmu_register()?
>
> As above, neither of these should be necessary.

For the userspace interface. How about the kernel interface though?
perf_event_create_kernel_counter() takes attr.type as an input. But 
there's no way to look up the DSU PMU's "type".

Thanks,
Saravana

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