linux-kernel - Re: [RFC PATCH] perf: Add PERF_RECORD_SWITCH to indicate context switches

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Message-ID: <20150611163430.GC25760@tassilo.jf.intel.com>
Date:	Thu, 11 Jun 2015 09:34:30 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Adrian Hunter <adrian.hunter@...el.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Jiri Olsa <jolsa@...hat.com>,
	Stephane Eranian <eranian@...gle.com>,
	mathieu.poirier@...aro.org, Pawel Moll <pawel.moll@....com>
Subject: Re: [RFC PATCH] perf: Add PERF_RECORD_SWITCH to indicate context
 switches

On Thu, Jun 11, 2015 at 04:15:48PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 09, 2015 at 05:21:10PM +0300, Adrian Hunter wrote:
> > Tracepoints are no good at all for non-privileged users
> > because they need either CAP_SYS_ADMIN or
> > /proc/sys/kernel/perf_event_paranoid <= -1.
> > 
> > On the other hand, kernel software events need either
> > CAP_SYS_ADMIN or /proc/sys/kernel/perf_event_paranoid <= 1.
> 
> So while I think it makes sense to allow some tracepoint outside of that
> priv level, IOW have a per tracepoint priv level filter thingy, I don't
> think sched_switch() is one of those because it explicitly exposes
> timing information on other tasks.

It's trivial for a running program to measure when it gets context switched
by looking at timing.  I don't think this event provides anything new over that.

-Andi
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