Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20221205084127.535-1-xieyongji@bytedance.com> <20221205085846.741-1-xieyongji@bytedance.com>
 <CACGkMEuhYO3neFmxwiBp8C0QTaa+Mb13kken+RZ9QuruMct6tA@mail.gmail.com>
 <CACycT3vu0_xCG7SvdP-zkZkuOGdudx2apOwi3CZ4MOFSe-XAFg@mail.gmail.com> <CACGkMEs9nD--8y-dKsv23OK6G7LAC5dLDRRT8sKA5dUrAo0QbQ@mail.gmail.com>
In-Reply-To: <CACGkMEs9nD--8y-dKsv23OK6G7LAC5dLDRRT8sKA5dUrAo0QbQ@mail.gmail.com>
From:   Yongji Xie <xieyongji@bytedance.com>
Date:   Tue, 20 Dec 2022 16:21:57 +0800
Message-ID: <CACycT3uv0KD679uVPCVWj=vW1EZOTKc2qbp8N-cc5Q6i1=R6vQ@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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Dec 20, 2022 at 2:32 PM Jason Wang <jasowang@redhat.com> wrote:
>
> On Mon, Dec 19, 2022 at 12:56 PM Yongji Xie <xieyongji@bytedance.com> wrote:
> >
> > 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.
>
> 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