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

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Message-ID: <20161109144412.GI12125@kernel.org>
Date:   Wed, 9 Nov 2016 11:44:12 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        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

Em Wed, Nov 09, 2016 at 04:01:12PM +0200, Adrian Hunter escreveu:
> 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:
> > +++ 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.

Ok, adding your Acked-by, it now stands as, please ack:

commit c955ced680270b95a3de6a2434311befaeaea948
Author: Arnaldo Carvalho de Melo <acme@...hat.com>
Date:   Wed Nov 9 11:04:05 2016 -0300

    perf intel-pt: Update documentation about context switch events
    
    Since the unprivileged sched switch event was added in perf, PT doesn't
    need need perf_event_paranoid=-1 anymore for per cpu decoding.
    
    Add a note stating that that is only needed for kernels < 4.2.
    
    Reported-by: Andi Kleen <ak@...ux.intel.com>
    Report-link: http://lkml.kernel.org/r/http://lkml.kernel.org/n/tip-x2ybghpqxxn3zu0m8o7qi42r@git.kernel.org
    Acked-by: Adrian Hunter <adrian.hunter@...el.com>
    Fixes: 45ac1403f564 ("perf: Add PERF_RECORD_SWITCH to indicate context switches")
    Link: http://lkml.kernel.org/n/tip-x2ybghpqxxn3zu0m8o7qi42r@git.kernel.org
    Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>

diff --git a/tools/perf/Documentation/intel-pt.txt b/tools/perf/Documentation/intel-pt.txt
index c6c8318e38a2..b0b3007d3c9c 100644
--- a/tools/perf/Documentation/intel-pt.txt
+++ b/tools/perf/Documentation/intel-pt.txt
@@ -550,6 +550,18 @@ 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.
 
+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 are
 not permitted to use tracepoints which means there is insufficient side-band
 information to decode Intel PT in per-cpu mode, and potentially workload-only
@@ -564,8 +576,11 @@ sched_switch tracepoint
 -----------------------
 
 The sched_switch tracepoint is used to provide side-band data for Intel PT
-decoding.  sched_switch events are automatically added. e.g. the second event
-shown below
+decoding in kernels where the PERF_RECORD_SWITCH metadata event isn't
+available.
+
+The sched_switch events are automatically added. e.g. the second event shown
+below:
 
 	$ perf record -vv -e intel_pt//u uname
 	------------------------------------------------------------