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

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Message-ID: <20150611141548.GW19282@twins.programming.kicks-ass.net>
Date:	Thu, 11 Jun 2015 16:15:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Andi Kleen <ak@...ux.intel.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 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.

> This new PERF_RECORD_SWITCH event does not have those problems
> and it also has a couple of other small advantages. It is
> easier to use because it is an auxiliary event (like mmap,
> comm and task events) which can be enabled by setting a single
> bit. It is smaller than sched:sched_switch and easier to parse.

Right, so the one wee problem I have is that this only provides sched_in
data, I imagine people might be interested in sched_out as well.

Typically the switch even provides prev and next and thereby is
complete, but since we're limiting it to the one specific task, we'll
not have the sched_out data.

> @@ -812,6 +813,18 @@ enum perf_event_type {
>  	 */
>  	PERF_RECORD_ITRACE_START		= 12,
>  
> +	/*
> +	 *
> +	 *
> +	 * struct {
> +	 *	struct perf_event_header	header;
> +	 *	u32				pid, tid;
> +	 *	u64				time;

all 3 are already part of sample_id.

> +	 * 	struct sample_id		sample_id;
> +	 * };
> + 	 */
> +	PERF_RECORD_SWITCH			= 13,
--
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