linux-kernel - Re: [PATCH 4/7] sched: implement force_cpus

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Date:	Tue, 08 Dec 2009 10:02:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	tglx@...utronix.de, mingo@...e.hu, avi@...hat.com, efault@....de,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org,
	Gautham R Shenoy <ego@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 4/7] sched: implement force_cpus_allowed()

On Tue, 2009-12-08 at 17:41 +0900, Tejun Heo wrote:
> Hello,
> 
> On 12/07/2009 08:07 PM, Tejun Heo wrote:
> > On 12/07/2009 07:54 PM, Peter Zijlstra wrote:
> >> So we seem to do cleanup_workqueue_thread() from CPU_POST_DEAD, but at
> >> that time any thread that might still be around will most certainly not
> >> be running on the offlined cpu anymore.
> >>
> >> If you really want to ensure you remain on the cpu, you have to complete
> >> from CPU_DOWN_PREPARE.
> >>
> >> We're not running things from offline CPUs.
> > 
> > Oh, no, we're not doing that.  We can't do that.  What we're doing is
> > to continue to process works which were queued on the now offline cpu
> > unless it has been flushed/cancled from one of the cpu down
> > notifications and the reason why we need to be able to fork after
> > active is clear is to guarantee those flush/cancels don't deadlock.
> 
> Does my explanation justify the patch?

So its only needed in order to flush a workqueue from CPU_DOWN_PREPARE?
And all you need it to place a new kthread on a !active cpu?

Or is this in order to allow migrate_live_tasks() to move the worker
threads away from the dead cpu?

I'm really not thrilled by the whole fork-fest workqueue design.



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