linux-kernel - Re: [PATCH 2/4] sched: implement __set_cpus

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Date:	Mon, 31 May 2010 10:01:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 2/4] sched: implement __set_cpus_allowed()

On Thu, 2010-05-13 at 12:48 +0200, Tejun Heo wrote:
> Concurrency managed workqueue needs to be able to migrate tasks to a
> cpu which is online but !active for the following two purposes.
> 
> p1. To guarantee forward progress during cpu down sequence.  Each
>     workqueue which could be depended upon during memory allocation
>     has an emergency worker task which is summoned when a pending work
>     on such workqueue can't be serviced immediately.  cpu hotplug
>     callbacks expect workqueues to work during cpu down sequence
>     (usually so that they can flush them), so, to guarantee forward
>     progress, it should be possible to summon emergency workers to
>     !active but online cpus.

If we do the thing suggested in the previous patch, that is move
clearing active and rebuilding the sched domains until right after
DOWN_PREPARE, this goes away, right?

> p2. To migrate back unbound workers when a cpu comes back online.
>     When a cpu goes down, existing workers are unbound from the cpu
>     and allowed to run on other cpus if there still are pending or
>     running works.  If the cpu comes back online while those workers
>     are still around, those workers are migrated back and re-bound to
>     the cpu.  This isn't strictly required for correctness as long as
>     those unbound workers don't execute works which are newly
>     scheduled after the cpu comes back online; however, migrating back
>     the workers has the advantage of making the behavior more
>     consistent thus avoiding surprises which are difficult to expect
>     and reproduce, and being actually cleaner and easier to implement.

I still don't like this much, if you mark these tasks to simply die when
the queue is exhausted, and flush the queue explicitly on
CPU_UP_PREPARE, you should never need to do this.


After which I think you don't need this patch anymore..
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