linux-kernel - Re: [PATCH v1] perf cs-etm: Pass -1 as pid value for machine__set_current

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Message-ID: <20211118171412.GB2530497@p14s>
Date:   Thu, 18 Nov 2021 10:14:12 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, coresight@...ts.linaro.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] perf cs-etm: Pass -1 as pid value for
 machine__set_current_tid()

Good morning Leo,

On Sat, Nov 13, 2021 at 10:35:40PM +0800, Leo Yan wrote:
> Currently, cs-etm passes the tid value for both tid and pid parameters
> when calling machine__set_current_tid(), this can lead to confusion for
> thread handling.  E.g. we arbitrarily pass the same value for pid and
> tid, perf tool will be misled to consider it is a main thread (see
> thread__main_thread()).
> 
> On the other hand, Perf tool only can retrieve tid from Arm CoreSight
> context packet, and we have no chance to know pid (it maps to kernel's
> task_struct::tgid) from hardware tracing data.  For this reason, this
> patch passes -1 as pid for function machine__set_current_tid().
> 
> Signed-off-by: Leo Yan <leo.yan@...aro.org>
> ---
>  tools/perf/util/cs-etm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index f323adb1af85..eed1a5930072 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -1118,7 +1118,7 @@ int cs_etm__etmq_set_tid(struct cs_etm_queue *etmq,
>  	if (cs_etm__get_cpu(trace_chan_id, &cpu) < 0)
>  		return err;
>  
> -	err = machine__set_current_tid(etm->machine, cpu, tid, tid);
> +	err = machine__set_current_tid(etm->machine, cpu, -1, tid);

I remember wondering about what to do with the pid parameter when I wrote this
patch... 

Do you have a before-and-after snapshot you can add to the changelog?  I also
think it will require a "Fixes" tag.  In your next revision please CC James
since you guys are working in that area nowadays.

Thanks,
Mathieu

>  	if (err)
>  		return err;
>  
> -- 
> 2.25.1
>