linux-kernel - Re: [RFC] perf arm-spe: Track task context switch for cpu-mode events

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Message-ID: <70967300-4854-30d5-7ab4-5e2601052036@arm.com>
Date:   Mon, 1 Nov 2021 15:36:52 +0000
From:   German Gomez <german.gomez@....com>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Namhyung Kim <namhyung@...nel.org>,
        James Clark <james.clark@....com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Ian Rogers <irogers@...gle.com>,
        Stephane Eranian <eranian@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [RFC] perf arm-spe: Track task context switch for cpu-mode events

Hi Leo,

On 01/11/2021 15:11, Leo Yan wrote:
> Hi German,
>
> On Fri, Oct 29, 2021 at 11:51:16AM +0100, German Gomez wrote:
> [...]
> Have one concern: if cannot find the context packet, will the decoder
> drop the SPE packets until it find the first context packet?  If this
> is the case, I am concern the decoder will run out for all packets
> and doesn't generate any samples if the SPE trace data doesn't contain
> any context packet.

Not really. It will only peek at the first decoded packet without
dropping it. I couldn't think of a corner case where the decoder might
miss a context packet for the first records (I also haven't seen any -1
values so far).

>> ï¿½ï¿½ï¿½ if (!spe->use_ctx_pkt_for_pid &&
>> ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ (event->header.type == PERF_RECORD_SWITCH_CPU_WIDE ||
>> ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ event->header.type == PERF_RECORD_SWITCH))
>> ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ err = arm_spe_context_switch(spe, event, sample);
>>
>> Then we could apply patch [1] which wasn't fully merged in the end,
>> including similar `if (spe->use_ctx_pkt_for_pid)` to collect the pid/tid
>> from the context packets.
>>
>> What do you think?
> Except the above concern, the solution looks good to me.

I realized I cannot use the heap for it will not work in timeless
decoding. We can still use the queues though. By the way, is this return
statement in the arm_spe__setup_queue() function misplaced?

    if (spe->timeless_decoding)
            return 0;

Judging by the long comment in the arm_spe_run_decoder() function, it
seems like it should be placed somewhere below the call to "ret =
arm_spe_decode(...)", otherwise arm_spe_run_decoder() will begin with an
uninitialized record?

Thanks,
German

>
> Thanks,
> Leo