linux-kernel - Re: [PATCH 04/10 V4] workqueue: add manage_workers

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Message-ID: <20120905011235.GC2836@dhcp-172-17-108-109.mtv.corp.google.com>
Date:	Tue, 4 Sep 2012 18:12:35 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/10 V4] workqueue: add manage_workers_slowpath()

Hello, Lai.

On Sun, Sep 02, 2012 at 12:28:22AM +0800, Lai Jiangshan wrote:
> If hotplug code grabbed the manager_mutex and worker_thread try to create
> a worker, the manage_worker() will return false and worker_thread go to
> process work items. Now, on the CPU, all workers are processing work items,
> no idle_worker left/ready for managing. It breaks the concept of workqueue
> and it is bug.
> 
> So when manage_worker() failed to grab the manager_mutex, it should
> try to enter normal process contex and then compete on the manager_mutex
> instead of return false.
> 
> To safely do this, we add manage_workers_slowpath() and the worker
> go to process work items mode to do the managing jobs. thus
> managing jobs are processed via work item and can free to compete
> on manager_mutex.

Ummm.... this seems overly complicated.  How about scheduling rebind
work to a worker and forcing it to break out of the work processing
loop?  I think it can be done fairly easily using POOL_MANAGE_WORKERS
- set it from the rebind function, break out of work processing loop
if it's set, replace need_to_manage_workers() with POOL_MANAGE_WORKERS
test (the function really isn't necessary) and always jump back to
recheck afterwards.  It might need a bit more mangling here and there
but that should be the essence of it.  I'll give a stab at it later
today.

Thanks.

-- 
tejun
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