linux-kernel - Re: [PATCH v4 3/4] perf: xgene: Add APM X-Gene SoC Performance Monitoring Unit driver

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Message-ID: <20160628165941.GS31744@leverpostej>
Date:	Tue, 28 Jun 2016 17:59:41 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Tai Tri Nguyen <ttnguyen@....com>
Cc:	Marc Zyngier <marc.zyngier@....com>,
	Will Deacon <will.deacon@....com>, catalin.marinas@....com,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	patches <patches@....com>
Subject: Re: [PATCH v4 3/4] perf: xgene: Add APM X-Gene SoC Performance
 Monitoring Unit driver

On Tue, Jun 28, 2016 at 09:39:36AM -0700, Tai Tri Nguyen wrote:
> Hi Mark,
> 
> On Tue, Jun 28, 2016 at 7:14 AM, Mark Rutland <mark.rutland@....com> wrote:
> > On Tue, Jun 28, 2016 at 02:21:38PM +0100, Marc Zyngier wrote:
> >> On 28/06/16 12:13, Mark Rutland wrote:
> >> > Marc, is there a sensible way to prevent irq balancers from changing the
> >> > affinity of an IRQ, e.g. a kernel-side pinning mechanism, or some way we
> >> > can be notified and reject changes?
> >>
> >> You can get notified (see irq_set_affinity_notifier), but there no way
> >> to veto the change.
> >
> > :(
> >
> >> What should probably be done is to set the affinity hint
> >> (irq_set_affinity_hint), and use the notifier to migrate the context
> >> if possible. Note that you'll be called in process context, which will
> >> race against interrupts being delivered on the new CPU.
> >
> > I'll have to go digging into what exactly perf_pmu_migrate_context
> > requires. Given the race, I'm not sure if that's going to work. It's
> > certainly not going to be self contained.
> >
> > That also won't work for CPU PMUs, where it makes no sense to migrate
> > context or IRQs.  For those we appear to already be using have
> > IRQF_NOBALANCING, which sounds like exactly what we want.
> >
> > That appears to influence irq_can_set_affinity(), which the procfs
> > helpers check.
> >
> > Tai, can you try requesting the IRQ with the IRQF_NOBALANCING flag?
> 
> This seems to work.
> I also tried to change smp_affinity through procfs and it returns write error.
> The interrupt seems to be excluded from irq balancing.
> Should I make the change?

Yes please. I believe you also need IRQF_NO_THREAD per the CPU PMU
drivers, so please add both flags. I'll do the same for the CCN and CCI
PMU drivers.

Thanks,
Mark.