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Re: Overview of concurrency managed workqueue

From:  Andrew Morton <akpm-AT-linux-foundation.org>
To:  Tejun Heo <tj-AT-kernel.org>
Subject:  Re: Overview of concurrency managed workqueue
Date:  Thu, 17 Jun 2010 16:15:39 -0700
Cc:  Daniel Walker <dwalker-AT-codeaurora.org>, mingo-AT-elte.hu, awalls-AT-radix.net, linux-kernel-AT-vger.kernel.org, jeff-AT-garzik.org, rusty-AT-rustcorp.com.au, cl-AT-linux-foundation.org, dhowells-AT-redhat.com, arjan-AT-linux.intel.com, johannes-AT-sipsolutions.net, oleg-AT-redhat.com, axboe-AT-kernel.dk
Archive-link:  Article, Thread

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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