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

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Message-ID: <577279E2.6070202@arm.com>
Date:	Tue, 28 Jun 2016 14:21:38 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Mark Rutland <mark.rutland@....com>,
	Tai Tri Nguyen <ttnguyen@....com>
Cc:	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 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.


	M.
-- 
Jazz is not dead. It just smells funny...