linux-kernel - Re: [PATCH v2 7/7] Documentation: coresight: Add PID tracing description

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Date:   Fri, 5 Feb 2021 13:42:16 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Mike Leach <mike.leach@...aro.org>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        John Garry <john.garry@...wei.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Daniel Kiss <Daniel.Kiss@....com>,
        Denis Nikitin <denik@...omium.org>,
        Coresight ML <coresight@...ts.linaro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 7/7] Documentation: coresight: Add PID tracing
 description

On Thu, Feb 04, 2021 at 12:14:12PM +0000, Mike Leach wrote:

[...]

> > >>> +To support tracing PID for the kernel runs at different exception levels,
> > >>> +the PMU formats are defined as follow:
> > >>> +
> > >>> +  "contextid1": Available on both EL1 kernel and EL2 kernel.  When the
> > >>> +                kernel is running at EL1, "contextid1" enables the PID
> > >>> +                tracing; when the kernel is running at EL2, this enables
> > >>> +                tracing the PID of guest applications.
> > >>> +
> > >>> +  "contextid2": Only usable when the kernel is running at EL2.  When
> > >>> +                selected, enables PID tracing on EL2 kernel.
> > >>> +
> > >>> +  "contextid":  Will be an alias for the option that enables PID
> > >>> +                tracing.  I.e,
> > >>> +                contextid == contextid1, on EL1 kernel.
> > >>> +                contextid == contextid2, on EL2 kernel.
> > >>> +
> > >>> +The perf tool automatically sets corresponding bit for the "contextid" config,
> > >>> +therefore, the user doesn't have to bother which EL the kernel is running.
> > >>> +
> > >>> +  i.e, perf record -e cs_etm/contextid/u -- uname
> > >>> +    or perf record -e cs_etm//u -- uname
> > >>> +
> > >>> +will always do the "PID" tracing, independent of the kernel EL.
> > >>> +
> > >>
> > >> This is telling me that both cs_etm// and cs_etm/contextid/ have the
> > >> same effect - trace PID. Is this correct?
> > >
> >
> > Just to make this clear, this is not a side effect of the patch.
> 
> Which is fine - but the documentation should accurately reflect what
> is happening on the system.
> This is a new paragraph about the PID tracing or otherwise, Even if
> some of the effects pre-date this patch, they have to be accurately
> communicated.
> I am also reading the new paragraph in the context of the rest of the
> coresight.rst document - which is a user level document explaining the
> basic operation of the coresight system and tools.
> This document mentions no other perf command line parameters relevant
> to coresight other than the @sink option.It actually calls out to the
> OpenCSD docs to provide further information.
> 
> > The perf
> > tool driver automatically adds the "contextid" tracing and timestamp for
> > "system wide" and process bound events, as they traces get mixed into
> > the single sink. So these options are added implicitly by the perf tool
> > to make the decoding easier.
> >
> 
> That's fine - I have no problem with contextID trace enabled by
> default. Context ID is relatively low overhead - and only emitted at
> start of trace  / context changes.
> But the explanation of the parameters currently reads as though they
> always have an effect - and not putting them in there will omit the
> effect - unless you spot the very subtle line at the end.
> 
> The user does not need to know about parameters that have no effect!

Thanks for the suggestion, Mike.

> Perhaps a better approach would be to explain the above - an explicit
> statement that "perf will always enable PID/ contextID tracing at the
> relevant EL - but for EL2 it is possible to make specific adjustments
> using parameters......."

Usually users assume the PMU format has no effect if without set it; but
this is not the case for the config "contextid", this config has been
automatically enabled by perf tool.

Based on your suggesiton, will refine the descrption for two things:
clarify what's the common usage for EL1/EL2, and what's specific for
EL2.

Thanks,
Leo