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

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Message-ID: <CACgAJHy+dcDeq_Zr+7kCKKCpXK2qeyXjxuAOQjULdooT3G6RHw@mail.gmail.com>
Date:	Tue, 28 Jun 2016 11:05:50 -0700
From:	Tai Tri Nguyen <ttnguyen@....com>
To:	Mark Rutland <mark.rutland@....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

Hi Mark,

On Tue, Jun 28, 2016 at 9:59 AM, Mark Rutland <mark.rutland@....com> wrote:
> 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.

Btw, please skip the v5 I've sent out yesterday. I'll soon send the v6
with this change.

Thanks,
-- 
Tai