linux-kernel - Re: [PATCH v2 01/11] genirq/affinity:: Export irq_create_affinity

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Message-ID: <CACycT3uY1dfP=5d1go+POh7-J1tUMW+9RXi92KtcFJYMzq-bOQ@mail.gmail.com>
Date:   Mon, 13 Feb 2023 22:50:18 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, Jason Wang <jasowang@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 01/11] genirq/affinity:: Export irq_create_affinity_masks()

On Mon, Feb 13, 2023 at 8:00 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Jan 30, 2023 at 07:53:55PM +0800, Yongji Xie wrote:
> > On Fri, Jan 27, 2023 at 4:22 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > > On Mon, Dec 19, 2022 at 05:36:02PM +0800, Yongji Xie wrote:
> > > > On Mon, Dec 19, 2022 at 3:33 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > > >
> > > > > On Mon, Dec 05, 2022 at 04:41:17PM +0800, Xie Yongji wrote:
> > > > > > Export irq_create_affinity_masks() so that some modules
> > > > > > can make use of it to implement interrupt affinity
> > > > > > spreading mechanism.
> > > > > >
> > > > > > Signed-off-by: Xie Yongji <xieyongji@...edance.com>
> > > > >
> > > > > So this got nacked, what's the plan now?
> > > > >
> > > >
> > > > I‘d like to check with Christoph again first.
> > > >
> > > > Hi Christoph,
> > > >
> > > > Jason will post some patches to get rid of the DMA API for vDPA
> > > > simulators. And the irq affinity algorithm is independent of the IRQ
> > > > subsystem IIUC. So could you allow this patch so that we can reuse the
> > > > algorithm to select the best CPU (per-cpu affinity if possible, or at
> > > > least per-node) to run the virtqueue's irq callback.
> > > >
> > > > Thanks,
> > > > Yongji
> > >
> > > I think you need to explain why you are building low level
> > > affinity masks.
> >
> > In VDUSE case, we use workqueue to run the virtqueue's irq callback.
> > Now I want to queue the irq callback kwork to one specific CPU to get
> > per-cpu affinity if possible, or at least per-node. So I need to use
> > this function to build the low level affinity masks for each
> > virtqueue.
> >
> > > what's the plan now?
> > >
> >
> > If there is no objection, I'll post a new version.
> >
> > Thanks,
> > Yongji
>
> I doubt you made a convicing case here - I think Christoph was saying if
> it is not an irq it should not use an irq affinity API.
> So a new API possibly sharing implementation with irq affinity
> is called for then? Maybe.
>

I'm not sure I get your point on sharing implementation.

I can try to split irq_create_affinity_masks() into a common part and
an irq specific part, and move the common part to a common dir such as
/lib and export it. Then we can use the common part to build a new API
for usage.

But I'm afraid that there will be no difference between the new API
and the irq affinity API except for the name since the new API is
still used for irq affinity management. That means we may still need
the irq specific part in the new API. For example, the virtio-blk
driver doesn't know whether the virtio device is a software-defined
vDPA device or a PCI device, so it will pass a structure irq_affinity
to those APIs, then both the new API and irq affinity API still need
to handle it.

Thanks,
Yongji