Re: [PATCH 04/10 V4] workqueue: add manage_workers_slowpath()

[Lai Jiangshan <laijs@cn.fujitsu.com>]

On 09/05/2012 09:12 AM, Tejun Heo wrote:
> 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.
> 

This approach is a little like my unsent approach3.(I will explain in other mail)
This approach is most complicated and changing more code if it is implemented.

First we should rebind/unbind separated by pool. because,
	if we queue the rebind-work to high-pri pool, we will break normal-pool
	vice versa

and this forces us move DISASSOCIATED to pool-flags.
and this forces us add more code in cpu-notify

second, reuse POOL_MANAGE_WORKERS, or add new one.

third, need to restruct of rebind/unbind and change a lot in worker_thread.

my partial/unsent approach3 has almost the same problem.
(different, my approach3 don't use work item, it is checked and called from
the "recheck" label of worker_thread. it is called with WORKER_PREP bit set
and it uses "mutex_trylock" to grab lock like manage_workers())

how much code will be changed for only unbind part of this approach:


 kernel/workqueue.c |  103 ++++++++++++++++++++++++++++++++++++++--------------
 1 files changed, 76 insertions(+), 27 deletions(-)


Thanks
Lai