linux-kernel - Re: Virtio-scsi multiqueue irq affinity

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Message-ID: <20190325095011.GA23225@ming.t460p>
Date:   Mon, 25 Mar 2019 17:50:13 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Xu <peterx@...hat.com>, Christoph Hellwig <hch@....de>,
        Jason Wang <jasowang@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>, minlei@...hat.com
Subject: Re: Virtio-scsi multiqueue irq affinity

On Mon, Mar 25, 2019 at 09:53:28AM +0100, Thomas Gleixner wrote:
> Ming,
> 
> On Mon, 25 Mar 2019, Ming Lei wrote:
> > On Mon, Mar 25, 2019 at 01:02:13PM +0800, Peter Xu wrote:
> > > One thing I can think of is the real-time scenario where "isolcpus="
> > > is provided, then logically we should not allow any isolated CPUs to
> > > be bound to any of the multi-queue IRQs.  Though Ming Lei and I had a
> > 
> > So far, this behaviour is made by user-space.
> > 
> > >From my understanding, IRQ subsystem doesn't handle "isolcpus=", even
> > though the Kconfig help doesn't mention irq affinity affect:
> > 
> >           Make sure that CPUs running critical tasks are not disturbed by
> >           any source of "noise" such as unbound workqueues, timers, kthreads...
> >           Unbound jobs get offloaded to housekeeping CPUs. This is driven by
> >           the "isolcpus=" boot parameter.
> 
> isolcpus has no effect on the interupts. That's what 'irqaffinity=' is for.

Indeed.

irq_default_affinity is built from 'irqaffinity=', however, we don't
consider irq_default_affinity for managed IRQ affinity.

Looks Peter wants to exclude some CPUs from the spread on managed IRQ.

Thanks,
Ming