linux-kernel - Re: [PATCH RFC v2 0/4] perf: IRQ-bound performance events

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Date:	Tue, 14 Jan 2014 18:09:29 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <ak@...ux.jf.intel.com>
Subject: Re: [PATCH RFC v2 0/4] perf: IRQ-bound performance events

On Tue, Jan 14, 2014 at 05:07:52PM +0100, Alexander Gordeev wrote:
> On Mon, Jan 13, 2014 at 04:50:37PM +0100, Frederic Weisbecker wrote:
> > On Sat, Jan 04, 2014 at 07:22:32PM +0100, Alexander Gordeev wrote:
> > > Hello,
> > > 
> > > This is version 2 of RFC "perf: IRQ-bound performance events". That is an
> > > introduction of IRQ-bound performance events - ones that only count in a
> > > context of a hardware interrupt handler. Ingo suggested to extend this
> > > functionality to softirq and threaded handlers as well:
> > 
> > Hi Alexander,
> > 
> > I still strongly think we should use toggle events to achieve that:
> > https://lkml.org/lkml/2013/9/25/227
> 
> Hi Frederic,
> 
> The toggle events would not allow counting per-action in hardware interrupt
> context. The design I propose provisions any possible combination of actions/
> IRQs.

I think we could define one event per handler by using tracepoint filters.

> 
> I.e. if we had few drivers on IRQn and few drivers on IRQm we could assign
> an event to let's say ISR0, ISR2 on IRQn and ISR1 on IRQm.

Yeah that should be possible with tracepoints as well.

> Moreover, given that hardware context handlers are running with local
> interrupts disabled and therefore an IRQ-bound event could be enabled/
> disabled only from a single handler at a time - we just need to allocate
> a single hardware counter for any possible combination.

Hmm I don't get what you mean here. Why tracepoint defined event don't work in this scenario?

> 
> I think it would be ideal if the two approaches could be converged somehow,
> but I just do not know how at the moment. I believe the next step is to
> measure the overhead Andi mentioned. That well might be a showstopper for
> either or both approaches.
> 
> By contrast with the hardware context, the toggle events seem to able
> monitoring softirq in its current form.
> 
> As of the threaded context handlers, I have not come up with the idea how to
> make it yet, but it does not seem the toggle events are able eigher.

A per task event should do the trick for threaded irqs profiling.

Thanks.
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