linux-kernel - Re: [PATCH 5/6] perf/core: Use ioctl to communicate driver configuration to kernel

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Date:   Wed, 04 Jul 2018 13:51:22 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>, ast@...nel.org,
        Greg KH <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-s390@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 5/6] perf/core: Use ioctl to communicate driver configuration to kernel

Mathieu Poirier <mathieu.poirier@...aro.org> writes:

> Hi Alex,

Hi Mathieu,

> On Tue, 3 Jul 2018 at 04:03, Alexander Shishkin
> <alexander.shishkin@...ux.intel.com> wrote:
>>
>> On Mon, Jul 02, 2018 at 04:33:29PM -0600, Mathieu Poirier wrote:
>> > This patch follows what has been done for filters by adding an ioctl()
>> > option to communicate to the kernel arbitrary PMU specific configuration
>> > that don't fit in the conventional struct perf_event_attr to the kernel.
>>
>> Ok, so what *is* the PMU specific configuration that doesn't fit in the
>> attribute and needs to be re-configured by the driver using the generation
>> tracking?
>>
>
> In this patchset I'm am after the specification of sink information
> for each event, i.e what sink a CPU is supposed to use for the
> session.  I simply don't see putting something that PMU specific in
> the generic perf_event_attr structure.  I also intend to use the same
> ioctl mechanism to communicate complex tracer configuration for
> sequencers, counters and input events.  I don't see a nice way of
> doing that from the perf_event_attr, and that is even without thinking
> about the different flavours of tracers out there, all with their own
> features.

Yes, the sequencers and counters seem tricky. Here's a wild idea: can
the sequencer/counter configuration be expressed as an eBPF program? Or,
can an eBPF program be used to program those?

> I've looked around and the only clean way I found to support this is
> via an ioctl().  That way each tracer can easily identify the sink it
> should be using without smearing the perf_event_attr structure.  I
> would be happy to explore a different avenue should you think of
> something.

Yes, I also have something similar on my todo list and I was previously
thinking along the lines of pipe()/splice(). As in, you take the AUX
event file descriptor and feed it to the sink, at which point the trace
path is configured. I need to dig up the notes that I made back in the
day to continue this conversation in more concrete terms.

Regards,
--
Alex