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From:   Anup Patel <anup@brainfault.org>
Date:   Sun, 26 Apr 2020 19:08:03 +0530
Message-ID: <CAAhSdy2f2-SQP6TdgxA0HM2ft3eBJd6kEkB--RH=2gUuLktXLQ@mail.gmail.com>
Subject: Re: [PATCH] irqchip/sifive-plic: allow many cores to handle IRQs
To:     Zong Li <zong.li@sifive.com>
Cc:     Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        David Abdurachmanov <david.abdurachmanov@sifive.com>,
        Marc Zyngier <maz@kernel.org>
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+Mark Z

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.

Regards,
Anup