linux-kernel - Re: workqueue thing

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Message-ID: <4B2F7879.2080901@kernel.org>
Date:	Mon, 21 Dec 2009 22:30:33 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	torvalds@...ux-foundation.org, awalls@...ix.net,
	linux-kernel@...r.kernel.org, jeff@...zik.org, mingo@...e.hu,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net, andi@...stfloor.org
Subject: Re: workqueue thing

Hello, Peter.

On 12/21/2009 06:22 PM, Peter Zijlstra wrote:
> On Mon, 2009-12-21 at 12:04 +0900, Tejun Heo wrote:
>> When IO goes wrong, in extreme
>> cases, it can easily take over thirty secs to recover and that's
>> required by the hardware specifications, so anything which ends up
>> waiting on IO can take a pretty long time.  The only piece of code
>> which is necessary to support that is the code necessary to migrate
>> back tasks to CPUs when they come online again.  It's not a lot of
>> ugly code. 
> 
> Why does it need to get migrated back, there are no affinity promises if
> you allow hotplug to continue, so it might as well complete and continue
> on the other cpu.
> 
> And yes, it is a lot of very ugly code.

Migrating to online but !active CPU is necessary to call rescuers
during CPU_DOWN_PREPARE which is necessary to guarantee forward
progress during cpu down operation.  Given that, the only extra code
which is necessary purely for migrating back when a CPU comes back
online is a few tens of lines of code which handles TRUSTEE_RELEASE
case.  That's not a lot.  If we do it differently (ie. let unbound
workers not process new works, just drain and let them die), it will
take more code.

I think you're primarily concerned with the scheduler modifications
and think that the choose-between-two-masks on migration is ugly.  I
agree it's not the prettiest thing in this world but then again it's
not a lot of code.  The reason why it looks ugly is because the way
migration is implemented and parameter is passed in.  API-wise, I
think making kthread_bind() synchronized against cpu onliness should
be pretty clean.

Thanks.

-- 
tejun
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