linux-kernel - Re: [PATCH 2/7] ARM: perf_event: Support percpu irqs for the CPU PMU

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Date:	Mon, 13 Jan 2014 11:52:13 +0000
From:	Will Deacon <will.deacon@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/7] ARM: perf_event: Support percpu irqs for the CPU PMU

On Fri, Jan 10, 2014 at 07:36:57PM +0000, Stephen Boyd wrote:
> On 01/10, Will Deacon wrote:
> > On Thu, Jan 09, 2014 at 07:17:29PM +0000, Stephen Boyd wrote:
> > 
> > > We can avoid the hacky cast of the per-cpu dev token by using the
> > > cpu_pmu pointer directly, but we'll still need to pass something to the
> > > percpu interrupt handler otherwise the genirq layer doesn't allow us to
> > > request the PPI. I can pass hw_events I guess. Is that what you're
> > > thinking? Or were you thinking that we could just use
> > > cpu_pmu->handle_irq as the handler argument in request_percpu_irq()? I
> > > can't figure out how that is supposed to work.
> > 
> > Actually, I was thinking you could remove cpu_pmu_dispatch_irq completely
> > and just pass the actual handler straight through to request_percpu_irq. On
> > arm64 we pass the hw_events as the pcpu token, so I'd be inclined to do the
> > same here unless there's a good reason not to.
> > 
> 
> Passing the hw_events as the pcpu token here is kind of hacky.
> The reason is because the token is dereferenced into cpu_pmu in
> armv7pmu_handle_irq() like so:
> 
> 	struct arm_pmu *cpu_pmu = (struct arm_pmu *)dev;
> 
> It would be great if we could pass cpu_pmu directly to the
> request call like so:
> 
> 	request_percpu_irq(irq, cpu_pmu->handle_irq, "arm-pmu", &cpu_pmu);
> 
> but no. request_percpu_irq() wants a percpu pointer so this won't
> work. If cpu_pmu was declared as DEFINE_PER_CPU, this would work
> out just fine.

That feels really broken though, since we rely on the cpu_pmu being a
container for the struct pmu that was registered with perf core.

> Should the cpu_pmu become a per-cpu variable? That sounds rather
> invasive.

I also don't think that's the right solution, based on the above. It's
actually pretty hard to work out what's the right thing to do here...

We *could* have a per-cpu pointer to the cpu_pmu_pointer, but then we'd
need to update the IRQ handlers, including things like the CCI PMU which
really doesn't care about per-cpu stuff. So after all this, the shim we have
around the IRQ handler for the U8500 SPI workarounds might be the right
thing after all -- it allows us to consolidate the conversion of a pcpu
pointer into the relevant instance (actually any instance, since they'd all
point at the same thing) for the current CPU.

What do you think to having that shim throw away the second level pcpu
pointer in the case of a PPI? (probably means we need to revisit that
renaming again).

Will
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