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From:   Yongji Xie <xieyongji@bytedance.com>
Date:   Mon, 19 Dec 2022 12:56:10 +0800
Message-ID: <CACycT3vu0_xCG7SvdP-zkZkuOGdudx2apOwi3CZ4MOFSe-XAFg@mail.gmail.com>
Subject: Re: [PATCH v2 06/11] vduse: Support automatic irq callback affinity
To:     Jason Wang <jasowang@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Christoph Hellwig <hch@lst.de>,
        virtualization <virtualization@lists.linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Fri, Dec 16, 2022 at 1:30 PM Jason Wang <jasowang@redhat.com> wrote:
>
> On Mon, Dec 5, 2022 at 4:59 PM Xie Yongji <xieyongji@bytedance.com> wrote:
> >
> > This brings current interrupt affinity spreading mechanism
> > to vduse device. We will make use of irq_create_affinity_masks()
> > to create an irq callback affinity mask for each virtqueue of
> > vduse device. Then we will choose the CPU which has the lowest
> > number of interrupt allocated in the affinity mask to run the
> > irq callback.
>
> This seems a balance mechanism but it might not be the semantic of the
> affinity or any reason we need to do this? I guess we should use at
> least round-robin in this case.
>

Here we try to follow the pci interrupt management mechanism. In VM
cases, the interrupt should always be triggered to one specific CPU
rather than to each CPU in turn.

> >
> > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > ---
> >  drivers/vdpa/vdpa_user/vduse_dev.c | 50 ++++++++++++++++++++++++++++++
> >  1 file changed, 50 insertions(+)
> >
> > diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> > index d126f3e32a20..90c2896039d9 100644
> > --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> > +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> > @@ -23,6 +23,7 @@
> >  #include <linux/nospec.h>
> >  #include <linux/vmalloc.h>
> >  #include <linux/sched/mm.h>
> > +#include <linux/interrupt.h>
> >  #include <uapi/linux/vduse.h>
> >  #include <uapi/linux/vdpa.h>
> >  #include <uapi/linux/virtio_config.h>
> > @@ -58,6 +59,8 @@ struct vduse_virtqueue {
> >         struct work_struct inject;
> >         struct work_struct kick;
> >         int irq_effective_cpu;
> > +       struct cpumask irq_affinity;
> > +       spinlock_t irq_affinity_lock;
>
> Ok, I'd suggest to squash this into patch 5 to make it more easier to
> be reviewed.
>

OK.

> >  };
> >
> >  struct vduse_dev;
> > @@ -123,6 +126,7 @@ struct vduse_control {
> >
> >  static DEFINE_MUTEX(vduse_lock);
> >  static DEFINE_IDR(vduse_idr);
> > +static DEFINE_PER_CPU(unsigned long, vduse_allocated_irq);
> >
> >  static dev_t vduse_major;
> >  static struct class *vduse_class;
> > @@ -710,6 +714,49 @@ static u32 vduse_vdpa_get_generation(struct vdpa_device *vdpa)
> >         return dev->generation;
> >  }
> >
> > +static void vduse_vq_update_effective_cpu(struct vduse_virtqueue *vq)
> > +{
> > +       unsigned int cpu, best_cpu;
> > +       unsigned long allocated, allocated_min = UINT_MAX;
> > +
> > +       spin_lock(&vq->irq_affinity_lock);
> > +
> > +       best_cpu = vq->irq_effective_cpu;
> > +       if (best_cpu != -1)
> > +               per_cpu(vduse_allocated_irq, best_cpu) -= 1;
> > +
> > +       for_each_cpu(cpu, &vq->irq_affinity) {
> > +               allocated = per_cpu(vduse_allocated_irq, cpu);
> > +               if (!cpu_online(cpu) || allocated >= allocated_min)
> > +                       continue;
> > +
> > +               best_cpu = cpu;
> > +               allocated_min = allocated;
> > +       }
> > +       vq->irq_effective_cpu = best_cpu;
> > +       per_cpu(vduse_allocated_irq, best_cpu) += 1;
> > +
> > +       spin_unlock(&vq->irq_affinity_lock);
> > +}
> > +
> > +static void vduse_vdpa_set_irq_affinity(struct vdpa_device *vdpa,
> > +                                       struct irq_affinity *desc)
> > +{
> > +       struct vduse_dev *dev = vdpa_to_vduse(vdpa);
> > +       struct irq_affinity_desc *affd = NULL;
> > +       int i;
> > +
> > +       affd = irq_create_affinity_masks(dev->vq_num, desc);
> > +       if (!affd)
>
> Let's add a comment on the vdpa config ops to say set_irq_affinity()
> is best effort.
>

OK.

Thanks,
Yongji