linux-kernel - Re: [PATCH 0/3] drm: commit

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Message-ID: <20200921142831.GA4268@mtj.duckdns.org>
Date:   Mon, 21 Sep 2020 10:28:31 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org,
        Rob Clark <robdclark@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-arm-msm@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>, timmurray@...gle.com
Subject: Re: [PATCH 0/3] drm: commit_work scheduling

Hello,

On Mon, Sep 21, 2020 at 11:21:54AM +0200, Daniel Vetter wrote:
> The part I don't like about this is that it all feels rather hacked
> together, and if we add more stuff (or there's some different thing in the
> system that also needs rt scheduling) then it doesn't compose.
> 
> So question to rt/worker folks: What's the best way to let userspace set
> the scheduling mode and priorities of things the kernel does on its
> behalf? Surely we're not the first ones where if userspace runs with some
> rt priority it'll starve out the kernel workers that it needs. Hardcoding
> something behind a subsystem ioctl (which just means every time userspace
> changes what it does, we need a new such flag or mode) can't be the right
> thing.

Maybe not first but there haven't been many. The main benefit of workqueue
is that the users get to pool the worker threads automatically. I don't
think the existing workqueue design is something suitable for actual RT use
cases. Furthermore, there are inherent conflicts between sharing resources
and RT as this this patchset is already showing w/ needing per-crtc worker
thread. Maybe we can further abstract it if there are more use cases but for
now kthread_worker based implementation sounds about right to me.

Thanks.

-- 
tejun