Re: [PATCH] perf_events: improve x86 event scheduling (v5)

From: Peter Zijlstra
Date: Mon Jan 18 2010 - 15:01:29 EST

On Mon, 2010-01-18 at 18:29 +0100, Frederic Weisbecker wrote:
> On Mon, Jan 18, 2010 at 06:13:26PM +0100, Peter Zijlstra wrote:
> > On Mon, 2010-01-18 at 17:51 +0100, Frederic Weisbecker wrote:
> >
> > > Right hw_perf_enable/disable have no action on breakpoint events.
> > > These were somehow considered as software events until now.
> > >
> > > That raises the question: why perf_disable() only takes care
> > > of hardware events? Very few software events can trigger
> > > between perf_disable() and perf_enable() sections though.
> > >
> > > May be I should handle breakpoints there.
> >
> > OK, so maybe I'm not understanding the breakpoint stuff correctly, why
> > is it modeled as a software pmu? It has resource constraints like a
> > hardware pmu.
> It doesn't use the software pmu, it uses its own. But what kind
> of properties can it share with other hardware events?
> It has constraints that only need to be checked when we register
> the event. It has also constraint on enable time but nothing
> tricky that requires an overwritten group scheduling.

the only group scheduling bit is hw_perf_group_sched_in(), and I guess
you can get away without hw_perf_disable() because it doesn't generate
nmis, although I'd have to audit the code to verify a properly placed
breakpoint won't trip things up, since the core code basically assumes
counters won't trigger within perf_disable/enable sections.

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