linux-kernel - Re: [PATCH] perf tools pt: Remove obsolete paragraph in intel-pt.c

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Date:   Wed, 9 Nov 2016 16:01:12 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Cc:     Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] perf tools pt: Remove obsolete paragraph in intel-pt.c

On 09/11/16 15:59, Arnaldo Carvalho de Melo wrote:
> Em Wed, Nov 09, 2016 at 10:14:26AM -0300, Arnaldo Carvalho de Melo escreveu:
>> Em Tue, Nov 08, 2016 at 04:11:00PM -0800, Andi Kleen escreveu:
>>> From: Andi Kleen <ak@...ux.intel.com>
>>>
>>> Since the unprivileged sched switch event was added in perf,
>>> PT doesn't need need perf_event_paranoid=-1 anymore for
>>> per cpu decoding. So remove the obsolete paragraph in
>>> the documentation.
>>
>> Thanks for pointing that out, I'll do something slightly different tho,
>> pointing out that from kernel X.Y.Z, when the unprivileged
>> PERF_RECORD_SWITCH metadata event was introduced, this is no longer an
>> issue, having to be considered only on older kernels.
> 
> It ended up as:
> 
> diff --git a/tools/perf/Documentation/intel-pt.txt b/tools/perf/Documentation/intel-pt.txt
> index c6c8318e38a2..4d12db118476 100644
> --- a/tools/perf/Documentation/intel-pt.txt
> +++ b/tools/perf/Documentation/intel-pt.txt
> @@ -546,6 +546,18 @@ mode by using the --per-thread option.
>  Privileged vs non-privileged users
>  ----------------------------------
>  
> +The v4.2 kernel introduced support for a context switch metadata event,
> +PERF_RECORD_SWITCH, which allows unprivileged users to see when their processes
> +are scheduled out and in, just not by whom, which is left for the
> +PERF_RECORD_SWITCH_CPU_WIDE, that is only accessible in system wide context,
> +which in turn requires CAP_SYS_ADMIN.
> +
> +Please see the 45ac1403f564 ("perf: Add PERF_RECORD_SWITCH to indicate context
> +switches") commit, that introduces these metadata events for further info.
> +
> +When working with kernels < v4.2, the following considerations must be taken,
> +as the sched:sched_switch tracepoints will be used to receive such information:
> +
>  Unless /proc/sys/kernel/perf_event_paranoid is set to -1, unprivileged users
>  have memory limits imposed upon them.  That affects what buffer sizes they can
>  have as outlined above.

Maybe put that last paragraph about memory limits above the new text.