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

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Date:   Wed, 9 Nov 2016 10:59:27 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     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>, adrian.hunter@...el.com
Subject: Re: [PATCH] perf tools pt: Remove obsolete paragraph in intel-pt.c

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.