Messages in this thread | | | Date | Mon, 27 Apr 2020 09:27:24 +0100 | From | Marc Zyngier <> | Subject | Re: [PATCH] irqchip/sifive-plic: allow many cores to handle IRQs |
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Hi Anup,
On 2020-04-26 14:38, Anup Patel wrote: > +Mark Z
s/k/c/ ;-)
> > On Sun, Apr 26, 2020 at 6:49 PM Zong Li <zong.li@sifive.com> wrote: >> >> On Sun, Apr 26, 2020 at 8:47 PM Anup Patel <anup@brainfault.org> >> wrote: >> > >> > On Sun, Apr 26, 2020 at 4:37 PM Zong Li <zong.li@sifive.com> wrote: >> > > >> > > Currently, driver forces the IRQs to be handled by only one core. This >> > > patch provides the way to enable others cores to handle IRQs if needed, >> > > so users could decide how many cores they wanted on default by boot >> > > argument. >> > > >> > > Use 'irqaffinity' boot argument to determine affinity. If there is no >> > > irqaffinity in dts or kernel configuration, use irq default affinity, >> > > so all harts would try to claim IRQ. >> > > >> > > For example, add irqaffinity=0 in chosen node to set irq affinity to >> > > hart 0. It also supports more than one harts to handle irq, such as set >> > > irqaffinity=0,3,4. >> > > >> > > You can change IRQ affinity from user-space using procfs. For example, >> > > you can make CPU0 and CPU2 serve IRQ together by the following command: >> > > >> > > echo 4 > /proc/irq/<x>/smp_affinity >> > > >> > > Signed-off-by: Zong Li <zong.li@sifive.com> >> > > --- >> > > drivers/irqchip/irq-sifive-plic.c | 21 +++++++-------------- >> > > 1 file changed, 7 insertions(+), 14 deletions(-) >> > > >> > > diff --git a/drivers/irqchip/irq-sifive-plic.c b/drivers/irqchip/irq-sifive-plic.c >> > > index d0a71febdadc..bc1440d54185 100644 >> > > --- a/drivers/irqchip/irq-sifive-plic.c >> > > +++ b/drivers/irqchip/irq-sifive-plic.c >> > > @@ -111,15 +111,12 @@ static inline void plic_irq_toggle(const struct cpumask *mask, >> > > static void plic_irq_unmask(struct irq_data *d) >> > > { >> > > struct cpumask amask; >> > > - unsigned int cpu; >> > > struct plic_priv *priv = irq_get_chip_data(d->irq); >> > > >> > > cpumask_and(&amask, &priv->lmask, cpu_online_mask); >> > > - cpu = cpumask_any_and(irq_data_get_affinity_mask(d), >> > > - &amask); >> > > - if (WARN_ON_ONCE(cpu >= nr_cpu_ids)) >> > > - return; >> > > - plic_irq_toggle(cpumask_of(cpu), d, 1); >> > > + cpumask_and(&amask, &amask, irq_data_get_affinity_mask(d)); >> > > + >> > > + plic_irq_toggle(&amask, d, 1); >> > > } >> > > >> > > static void plic_irq_mask(struct irq_data *d) >> > > @@ -133,24 +130,20 @@ static void plic_irq_mask(struct irq_data *d) >> > > static int plic_set_affinity(struct irq_data *d, >> > > const struct cpumask *mask_val, bool force) >> > > { >> > > - unsigned int cpu; >> > > struct cpumask amask; >> > > struct plic_priv *priv = irq_get_chip_data(d->irq); >> > > >> > > cpumask_and(&amask, &priv->lmask, mask_val); >> > > >> > > if (force) >> > > - cpu = cpumask_first(&amask); >> > > + cpumask_copy(&amask, mask_val); >> > > else >> > > - cpu = cpumask_any_and(&amask, cpu_online_mask); >> > > - >> > > - if (cpu >= nr_cpu_ids) >> > > - return -EINVAL; >> > > + cpumask_and(&amask, &amask, cpu_online_mask); >> > > >> > > plic_irq_toggle(&priv->lmask, d, 0); >> > > - plic_irq_toggle(cpumask_of(cpu), d, 1); >> > > + plic_irq_toggle(&amask, d, 1); >> > > >> > > - irq_data_update_effective_affinity(d, cpumask_of(cpu)); >> > > + irq_data_update_effective_affinity(d, &amask); >> > > >> > > return IRQ_SET_MASK_OK_DONE; >> > > } >> > > -- >> > > 2.26.1 >> > > >> > >> > I strongly oppose (NACK) this patch due to performance reasons. >> > >> > In PLIC, if we enable an IRQ X for N CPUs then when IRQ X occurs: >> > 1) All N CPUs will take interrupt >> > 2) All N CPUs will try to read PLIC CLAIM register >> > 3) Only one of the CPUs will see IRQ X using the CLAIM register >> > but other N - 1 CPUs will see no interrupt and return back to what >> > they were doing. In other words, N - 1 CPUs will just waste CPU >> > every time IRQ X occurs. >> > >> > Example1, one Application doing heavy network traffic will >> > degrade performance of other applications because with every >> > network RX/TX interrupt N-1 CPUs will waste CPU trying to >> > process network interrupt. >> > >> > Example1, one Application doing heavy MMC/SD traffic will >> > degrade performance of other applications because with every >> > SPI read/write interrupt N-1 CPUs will waste CPU trying to >> > process it. >> > >> > In fact, the current PLIC approach is actually a performance >> > optimization. This implementation also works fine with in-kernel >> > load-balancer and user space load balancer. >> > >> >> Yes, it's exactly, I know what you pointed out. But the idea of this >> patch is just providing a way that users could enable other cores if >> they wanted, it could still enable only one core by this change. The >> purpose here is thinking of flexible use, rather than limitation. >> Maybe it would be a happy medium that we make the default case enable >> only one core? It is a good open discussion. > > Making the default case as enable only one core is just a work-around. > > As-per my understanding, if we set affinity mask of N CPUs for IRQ X > then it does not mean all N CPUs should receive IRQ X rather it means > that exactly one of the N CPUs will receive IRQ X and the IRQ receiving > CPU will be fixed (reflected by effective affinity returned by the > driver). > > If we ignore above semantics and still provide a mechanism to target > IRQ X to N CPUs then most likely someone will try and run into > performance issues. > > Please don't go this path. The performance impact in case of Guest/VM > is even worst because PLIC is trap-n-emulated by hypervisors as MMIO > device.
Just to add my view on this, as we get the same request on the ARM side once every other year:
- the "broadcast" aspect of the interrupt signalling is usually a waste of CPU cycles on all CPUs but the one that has actually takes it.
- the fact that you have to use MMIO to interact with the interrupt controller potentially makes this even worse if you don't have some per-CPU datapath that will handle this really fast.
- not hitting the same CPU at all times for a given interrupt means that the caches are usually cold for this interrupt, meaning the gain in latency is pretty dubious.
There is a very small (but also very vocal) fringe of users that insist that they cannot live without this broadcast aspect, because it allows them to shave a couple of cycles off a trivial interrupt handler on systems that are otherwise mostly idle.
My answer to this is that Linux is a general purpose OS, and not a glorified interrupt handler. We aim to have balanced response times, and give *userspace* control over where the interrupt happens in the long run.
HTH,
M. -- Jazz is not dead. It just smells funny...
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