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

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Date:   Mon, 1 Nov 2021 23:11:01 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     German Gomez <german.gomez@....com>
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 German,

On Fri, Oct 29, 2021 at 11:51:16AM +0100, German Gomez wrote:
> Hi Leo,
> 
> The current plan is to define a global flag in the `struct arm_spe` to
> select the method of pid tracing (context pkt, or switch events):
> 
>     struct arm_spe {
>        /* ... */
>        u8        use_ctx_pkt_for_pid;
>     }
> 
> The method could be determined by peeking at the top element of the
> `struct auxtrace_heap` at the beginning of the perf-report. If ctx
> packets have been collected, the first one should have a context_id !=
> -1. We could then tweak this part of Namhyung patch slightly:

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.

> 
>     if (!spe->use_ctx_pkt_for_pid &&
>         (event->header.type == PERF_RECORD_SWITCH_CPU_WIDE ||
>          event->header.type == PERF_RECORD_SWITCH))
>             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.

Thanks,
Leo