linux-kernel - Re: [PATCH v2 06/11] vduse: Support automatic irq callback affinity

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Message-ID: <CACycT3uv0KD679uVPCVWj=vW1EZOTKc2qbp8N-cc5Q6i1=R6vQ@mail.gmail.com>
Date:   Tue, 20 Dec 2022 16:21:57 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Hellwig <hch@....de>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/11] vduse: Support automatic irq callback affinity

On Tue, Dec 20, 2022 at 2:32 PM Jason Wang <jasowang@...hat.com> wrote:
>
> On Mon, Dec 19, 2022 at 12:56 PM Yongji Xie <xieyongji@...edance.com> wrote:
> >
> > On Fri, Dec 16, 2022 at 1:30 PM Jason Wang <jasowang@...hat.com> wrote:
> > >
> > > On Mon, Dec 5, 2022 at 4:59 PM Xie Yongji <xieyongji@...edance.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.
>
> If I was not wrong, when using MSI, most arch allows not only the
> cpuid as the destination but policy like rr and low priority first.
>

I see. I think we can remove the irq effective affinity and just use
the irq affinity. If the irq affinity mask contains more than one CPU,
we can use round-robin to spread IRQ between CPUs. But the sysfs
interface for irq affinity should be writable so that someone wants to
stop round-robin and pick one CPU to run irq callback.

Thanks,
Yongji