Date:   Tue, 25 Oct 2016 11:09:18 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Cheng Chao <cs.os.kernel@...il.com>
Cc:     tglx@...utronix.de, jason@...edaemon.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] irqchip/gic: Enable gic_set_affinity set more than one
 cpu

On 15/10/16 08:23, Cheng Chao wrote:
> On 10/15/2016 01:33 AM, Marc Zyngier wrote:
>>> on 10/13/2016 11:31 PM, Marc Zyngier wrote:
>>>> On Thu, 13 Oct 2016 18:57:14 +0800
>>>> Cheng Chao <cs.os.kernel@...il.com> wrote:
>>>>
>>>>> GIC can distribute an interrupt to more than one cpu,
>>>>> but now, gic_set_affinity sets only one cpu to handle interrupt.
>>>>
>>>> What makes you think this is a good idea? What purpose does it serves?
>>>> I can only see drawbacks to this: You're waking up more than one CPU,
>>>> wasting power, adding jitter and clobbering the cache.
>>>>
>>>> I assume you see a benefit to that approach, so can you please spell it
>>>> out?
>>>>
>>>
>>> Ok, You are right, but the performance is another point that we should consider.
>>>
>>> We use E1 device to transmit/receive video stream. we find that E1's interrupt is
>>> only on the one cpu that cause this cpu usage is almost 100%,
>>> but other cpus is much lower load, so the performance is not good.
>>> the cpu is 4-core.
>>
>> It looks to me like you're barking up the wrong tree. We have
>> NAPI-enabled network drivers for this exact reason, and adding more
>> interrupts to an already overloaded system doesn't strike me as going in
>> the right direction. May I suggest that you look at integrating NAPI
>> into your E1 driver?
>>
> 
> great, NAPI maybe is a good option, I can try to use NAPI. thank you.
> 
> In other hand, gic_set_affinity sets only one cpu to handle interrupt,
> that really makes me a little confused, why does GIC's driver not like 
> the others(MPIC, APIC etc) to support many cpus to handle interrupt?
> 
> It seems that the GIC's driver constrain too much.

There is several drawbacks to this:
- Cache impacts and power efficiency, as already mentioned
- Not virtualizable (you cannot efficiently implement this in a 
  hypervisor that emulates a GICv2 distributor)
- Doesn't scale (you cannot go beyond 8 CPUs)

I strongly suggest you give NAPI a go, and only then consider
delivering interrupts to multiple CPUs, because multiple CPU
delivery is not future proof.

> I think it is more reasonable to let user decide what to do.
> 
> If I care about the power etc, then I only echo single cpu to
> /proc/irq/xx/smp_affinity, but if I expect more than one cpu to handle 
> one special interrupt, I can echo 'what I expect cpus' to
> /proc/irq/xx/smp_affinity.

If that's what you really want, a better patch may be something like this:

diff --git a/drivers/irqchip/irq-gic.c b/drivers/irqchip/irq-gic.c
index d6c404b..b301d72 100644
--- a/drivers/irqchip/irq-gic.c
+++ b/drivers/irqchip/irq-gic.c
@@ -326,20 +326,25 @@ static int gic_set_affinity(struct irq_data *d, const struct cpumask *mask_val,
 {
 	void __iomem *reg = gic_dist_base(d) + GIC_DIST_TARGET + (gic_irq(d) & ~3);
 	unsigned int cpu, shift = (gic_irq(d) % 4) * 8;
-	u32 val, mask, bit;
-	unsigned long flags;
+	u32 val, mask, bit = 0;
+	unsigned long flags, aff = 0;
 
-	if (!force)
-		cpu = cpumask_any_and(mask_val, cpu_online_mask);
-	else
-		cpu = cpumask_first(mask_val);
+	for_each_cpu(cpu, mask_val) {
+		if (force) {
+			aff = 1 << cpu;
+			break;
+		}
+
+		aff |= cpu_online(cpu) << cpu;
+	}
 
-	if (cpu >= NR_GIC_CPU_IF || cpu >= nr_cpu_ids)
+	if (!aff)
 		return -EINVAL;
 
 	gic_lock_irqsave(flags);
 	mask = 0xff << shift;
-	bit = gic_cpu_map[cpu] << shift;
+	for_each_set_bit(cpu, &aff, nr_cpu_ids)
+		bit |= gic_cpu_map[cpu] << shift;
 	val = readl_relaxed(reg) & ~mask;
 	writel_relaxed(val | bit, reg);
 	gic_unlock_irqrestore(flags);

Thanks,

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

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