linux-kernel - Re: Selecting CPUs for queuing work on

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Date:   Fri, 12 Aug 2022 11:43:07 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Felix Kuehling <felix.kuehling@....com>
Cc:     Philip Yang <Philip.Yang@....com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "amd-gfx@...ts.freedesktop.org" <amd-gfx@...ts.freedesktop.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>, Dave Airlie <airlied@...il.com>
Subject: Re: Selecting CPUs for queuing work on

Hello,

On Fri, Aug 12, 2022 at 04:54:04PM -0400, Felix Kuehling wrote:
> In principle, I think IRQ routing to CPUs can change dynamically with
> irqbalance.

I wonder whether this is something which should be exposed to userland
rather than trying to do dynamically in the kernel and let irqbalance or
whatever deal with it. People use irq affinity to steer these handlings to
specfic CPUs and the usual expectation is that the bottom half handling is
gonna take place on the same cpu usually through softirq. It's kinda awkard
to have this secondary assignment happening implicitly.

> What we need is kind of the opposite of WQ_UNBOUND. As I understand it,
> WQ_UNBOUND can schedule anywhere to maximize concurrency. What we need is to
> schedule to very specific, predictable CPUs. We only have one work item per
> GPU that processes all the interrupts in order, so we don't need the
> concurrency of WQ_UNBOUND.

Each WQ_UNBOUND workqueue has a cpumask associated with it and the cpumask
can be changed dynamically, so it can be used for sth like this, but I'm not
yet convinced that's the right thing to do.

Thanks.

-- 
tejun