Date: Thu, 17 Jun 2010 16:15:39 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Tejun Heo <tj@kernel.org>
Cc: Daniel Walker <dwalker@codeaurora.org>, mingo@elte.hu, awalls@radix.net,
       linux-kernel@vger.kernel.org, jeff@garzik.org, rusty@rustcorp.com.au,
       cl@linux-foundation.org, dhowells@redhat.com, arjan@linux.intel.com,
       johannes@sipsolutions.net, oleg@redhat.com, axboe@kernel.dk
Subject: Re: Overview of concurrency managed workqueue
Message-Id: <20100617161539.d4ea62c0.akpm@linux-foundation.org>
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On Wed, 16 Jun 2010 18:55:05 +0200
Tejun Heo <tj@kernel.org> wrote:

> It was about using wq for cpu intensive / RT stuff.  Linus said,
> 
>   So stop arguing about irrelevancies. Nobody uses workqueues for RT
>   or for CPU-intensive crap. It's not what they were designed for, or
>   used for.

kernel/padata.c uses workqueues for cpu-intensive work, as I understand
it.

I share Daniel's concerns here.  Being able to set a worker thread's
priority or policy isn't a crazy thing.  Also one might want to specify
that a work item be executed on one of a node's CPUs, or within a
cpuset's CPUs, maybe other stuff.  I have vague feelings that there's
already code in the kernel somewhere which does some of these things.

(Please remind me what your patches did about create_rt_workqueue and
stop_machine?)

(Please note that drivers/media/video/ivtv/ivtv-irq.c is currently
running sched_setscheduler() against a workqueue thread of its own
creation, so we have precedent).

If someone wants realtime service for a work item then at present, the
way to do that is to create your own kernel threads, set their policy
and start feeding them work items.  That sounds like a sensible
requirement and implementation to me.  But how does it translate into
the new implementation?

The priority/policy logically attaches to the work itself, not to the
thread which serves it.  So one would want to be able to provide that
info at queue_work()-time.  Could the workqueue core then find a thread,
set its policy/priority, schedule it and then let the CPU scheduler do
its usual thing with it?

That doesn't sound too bad?  Add policy/priority/etc fields to the
work_struct?
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