linux-kernel - Re: [PATCH 2/2] perf: Use hot regs with software sched switch/migrate events

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Message-ID: <20100329225354.GD12254@nowhere>
Date:	Tue, 30 Mar 2010 00:53:56 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH 2/2] perf: Use hot regs with software sched
	switch/migrate events

On Tue, Mar 30, 2010 at 12:43:54AM +0200, Frederic Weisbecker wrote:
> On Mon, Mar 29, 2010 at 08:05:38PM +0200, Peter Zijlstra wrote:
> > On Mon, 2010-03-29 at 19:47 +0200, Frederic Weisbecker wrote:
> > > 
> > > 
> > > I'm going to make a quick fix for perf_fetch_caller_regs() that
> > > passes task_pt_regs if exclude_kernel for perf/urgent,
> > > and I'll do the above cleanups/invasive fixes on perf/core.
> > > 
> > > 
> > ok, sounds sensible, thanks!
> 
> 
> Actually I have doubts about what should be the strict sense
> of exclude_kernel.
> 
> Does that mean we exclude any event that happened in the kernel?
> Or does that mean we exclude the part that happened in the kernel?
> 
> Depending on the case, we do either.
> 
> In perf_swevent_hrtimer(), we simply go back to task_pt_regs()
> if exclude_kernel.
> 
> But in other software events, we don't such fix, we actually
> filter out the event if it is not user_mode().
> 
> So, I'm a bit confused on what to do.
> I'm tempted to adopt the meaning from perf_swevent_hrtimer()
> for software events too, I'm not sure...


I think this is the right thing to do: jump back to user context
instead of filtering out (unless kernel thread).

Otherwise every software events, trace events included, are totally
pointless with exclude_kernel.

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