Message-ID: <4C470B46.4040604@kernel.org>
Date: Wed, 21 Jul 2010 16:59:18 +0200
From: Tejun Heo <tj@kernel.org>
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To: David Howells <dhowells@redhat.com>
CC: Arjan van de Ven <arjan@linux.intel.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        torvalds@linux-foundation.org, mingo@elte.hu,
        linux-kernel@vger.kernel.org, jeff@garzik.org,
        akpm@linux-foundation.org, rusty@rustcorp.com.au,
        cl@linux-foundation.org, oleg@redhat.com, axboe@kernel.dk,
        dwalker@codeaurora.org, stefanr@s5r6.in-berlin.de, florian@mickler.org,
        andi@firstfloor.org, mst@redhat.com, randy.dunlap@oracle.com,
        Arjan van de Ven <arjan@infradead.org>
Subject: Re: [PATCHSET] workqueue: implement and use WQ_UNBOUND
References: <7b6bba36-5330-4e27-b7a9-3a4113b6b379@email.android.com> <21485.1279717725@redhat.com>
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Hello,

On 07/21/2010 03:08 PM, David Howells wrote:
> Tejun Heo <tj@kernel.org> wrote:
> 
>> As all unbound works are served by the same gcwq, non reentrancy is
>> automatically guaranteed.
> 
> That doesn't actually explain _how_ it's non-reentrant.  The gcwq includes a
> collection of threads that can execute from it, right?  If so, what mechanism
> prevents two threads from executing the same work item, if that work item
> isn't bound to a CPU?  I've been trying to figure this out from the code, but
> I don't see it offhand.

Sharing the same gcwq is why workqueues bound to one CPU have
non-reentrancy, so they're using the same mechanism.  If it doesn't
work for unbound workqueues, the normal ones are broken too.  Each
gcwq keeps track of currently running works in a hash table and looks
whether the work in question is already executing before starting
executing it.  It's a bit complex but as a work_struct may be freed
once execution starts, the status needs to be tracked outside.

>>> Btw, how does this fare in an RT system, where work items bound to a CPU
>>> can't get executed because their CPU is busy with an RT thread, even
>>> though there are other, idle CPUs?
>>
>> Sure, there's nothing special about unbound workers.  They're just normal
>> kthreads.
> 
> I should've been clearer: As I understand it, normal (unbound) worker items
> are bound to the CPU on which they were queued, and will be executed there
> only (barring CPU removal).  If that's the case, isn't it possible that work
> items can be prevented from getting execution time by an RT thread that's
> hogging a CPU and won't let go?

Yeah, for bound workqueues, sure.  That's exactly the same as the
original workqueue implementation.  For unbound workqueues, it doesn't
matter.

Thanks.

-- 
tejun
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