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

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Date:   Thu, 22 Feb 2018 11:33:53 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Saravana Kannan <skannan@...eaurora.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>, will.deacon@....com,
        robh@...nel.org, sudeep.holla@....com, mathieu.poirier@...aro.org,
        peterz@...radead.org, jonathan.cameron@...wei.com,
        linux-kernel@...r.kernel.org, marc.zyngier@....com,
        leo.yan@...aro.org, frowand.list@...il.com,
        linux-arm-kernel@...ts.infradead.org, avilaj@...eaurora.org
Subject: Re: [PATCH v11 8/8] perf: ARM DynamIQ Shared Unit PMU support

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.

[...]

> > +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.

> 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.

> 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.

Thanks,
Mark.