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

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Date:   Thu, 22 Feb 2018 12:38:39 -0800
From:   Saravana Kannan <skannan@...eaurora.org>
To:     Mark Rutland <mark.rutland@....com>
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 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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