linux-kernel - Re: [PATCH 1/3] perf: add context field to perf

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Date:	Tue, 5 Jul 2011 15:30:26 +0100
From:	Will Deacon <will.deacon@....com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Avi Kivity <avi@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"acme@...stprotocols.net" <acme@...stprotocols.net>,
	Jason Wessel <jason.wessel@...driver.com>
Subject: Re: [PATCH 1/3] perf: add context field to perf_event

Hi Frederic,

On Mon, Jul 04, 2011 at 02:58:24PM +0100, Frederic Weisbecker wrote:
> On Wed, Jun 29, 2011 at 05:27:25PM +0100, Will Deacon wrote:
> > Hi Frederic,
> > 
> > Thanks for including me on CC.
> > 
> > On Wed, Jun 29, 2011 at 05:08:45PM +0100, Frederic Weisbecker wrote:
> > > On Wed, Jun 29, 2011 at 06:42:35PM +0300, Avi Kivity wrote:
> > > > The perf_event overflow handler does not receive any caller-derived
> > > > argument, so many callers need to resort to looking up the perf_event
> > > > in their local data structure.  This is ugly and doesn't scale if a
> > > > single callback services many perf_events.
> > > >
> > > > Fix by adding a context parameter to perf_event_create_kernel_counter()
> > > > (and derived hardware breakpoints APIs) and storing it in the perf_event.
> > > > The field can be accessed from the callback as event->overflow_handler_context.
> > > > All callers are updated.
> > > >
> > > > Signed-off-by: Avi Kivity <avi@...hat.com>
> > > 
> > > I believe it can micro-optimize ptrace through register_user_hw_breakpoint() because
> > > we could store the index of the breakpoint that way, instead of iterating through 4 slots.
> > > 
> > > Perhaps it can help in arm too, adding Will in Cc.
> > 
> > Yes, we could store the breakpoint index in there and it would save us
> > walking over the breakpoints when one fires. Not sure this helps us for
> > anything else though. My main gripe with the ptrace interface to
> > hw_breakpoints is that we have to convert all the breakpoint information
> > from ARM_BREAKPOINT_* to HW_BREAKPOINT_* and then convert it all back again
> > in the hw_breakpoint code. Yuck!
> 
> Agreed, I don't like that either.
> 
> Would you like to improve that? We probably need to be able to pass some arch data
> through the whole call of breakpoint creation, including perf_event_create_kernel_counter().

Sure, I'll make some time to look at this and try and get an RFC out in the
next few weeks.

> There can be a transition step where we can either take generic attr or arch datas, until
> every archs are converted. So that you can handle the arm part and other arch developers
> can relay.

Yup.

> 
> Another thing I would like to do in the even longer term is to not use perf anymore
> for ptrace breakpoints, because that involves a heavy dependency and few people are
> happy with that. Instead we should just have a generic hook into the sched_switch()
> and handle pure ptrace breakpoints there. The central breakpoint API would still be
> there to reserve/schedule breakpoint resources between ptrace and perf.
> 
> But beeing able to create ptrace breakpoints without converting to generic perf attr
> is a necessary first step in order to achieve this.

Agreed, but I'll bear that in mind so I don't make it any more difficult
than it already is!

Cheers,

Will
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