linux-kernel - Re: perf: is PERF_COUNT_SW_CONTEXT_SWITCHES a kernel or user event?

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Date:	Mon, 27 Jun 2011 12:43:23 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Vince Weaver <vweaver1@...s.utk.edu>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Stephane Eranian <eranian@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: perf: is PERF_COUNT_SW_CONTEXT_SWITCHES a kernel or user event?

On Fri, 2011-06-24 at 17:03 -0400, Vince Weaver wrote:
> Hello
> 
> the commit included in 2.6.34:
>   perf: Use hot regs with software sched switch/migrate events
>   e49a5bd38159dfb1928fd25b173bc9de4bbadb21
> 
> Changes the behavior of the PERF_COUNT_SW_CONTEXT_SWITCHES
> counter.
> 
> Before 2.6.34 all of the PERF_COUNT_SW_CONTEXT_SWITCHES events were 
> counted as happening in userspace (they show up in "perf stat -e cs:u") 
> but after the commit they always happen in kernelspace ("perf stat -e 
> cs:k").
> 
> Was this intended behavior?
> I'm writing a validation test for this and want to make sure I get it
> right.
> 
> This can be confusing if your tool defaults to userspace only counts (PAPI 
> does this).

hurm, difficult case, like the changelog explains the previous behaviour
wasn't ideal either. Seems like we want somewhat of a middle ground
there, but I'm not quite sure how to make that happen.

Let me ponder things for a bit.
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