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

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Date:   Wed, 9 Nov 2016 11:39:41 -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:
> >> 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.

Ok, as it is not affected by the new text, thanks.

- Arnaldo