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

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Date:	Tue, 28 Jun 2016 09:39:36 -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 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:
>> > On Mon, Jun 27, 2016 at 10:54:07AM -0700, Tai Tri Nguyen wrote:
>> >> On Mon, Jun 27, 2016 at 9:00 AM, Mark Rutland <mark.rutland@....com> wrote:
>> >>> On Sat, Jun 25, 2016 at 10:54:20AM -0700, Tai Tri Nguyen wrote:
>> >>>> On Thu, Jun 23, 2016 at 7:32 AM, Mark Rutland <mark.rutland@....com> wrote:
>> >>>>> On Wed, Jun 22, 2016 at 11:06:58AM -0700, Tai Nguyen wrote:
>> >>>>>> +static irqreturn_t xgene_pmu_isr(int irq, void *dev_id)
>> >>>>>> +{
>> >>>>>> +     struct xgene_pmu_dev_ctx *ctx, *temp_ctx;
>> >>>>>> +     struct xgene_pmu *xgene_pmu = dev_id;
>> >>>>>> +     u32 val;
>> >>>>>> +
>> >>>>>> +     xgene_pmu_mask_int(xgene_pmu);
>> >>>>>
>> >>>>> Why do you need to mask the IRQ? This handler is called in hard IRQ
>> >>>>> context.
>> >>>>
>> >>>> Right. Let me change to use raw_spin_lock_irqsave here.
>> >>>
>> >>> Interesting; I see we do that in the CCI PMU driver. What are we trying
>> >>> to protect?
>> >>>
>> >>> We don't do that in the CPU PMU drivers, and I'm missng something here.
>> >>> Hopefully I'm just being thick...
>> >>
>> >> For me, we can't guarantee that the interrupt doesn't happen on the other CPUs.
>> >> The irqbalancer may change the SMP affinity.
>> >
>> > The perf core requires things to occur on the same CPU for correct
>> > synchronisation.
>> >
>> > If an IRQ balancer can change the IRQ affinity behind our back, we have
>> > much bigger problems that affect other uncore PMU drivers.
>> >
>> > 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?

Thanks,
-- 
Tai