Re: [PATCH 4/5] workqueue: don't expose workqueue_attrs to users

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

On 05/11/2015 10:59 PM, Tejun Heo wrote:
> On Mon, May 11, 2015 at 05:35:51PM +0800, Lai Jiangshan wrote:
>> workqueue_attrs is an internal-like structure and is exposed with
>> apply_workqueue_attrs() whose user has to investigate the structure
>> before use.
>>
>> And the apply_workqueue_attrs() API is inconvenient with the structure.
>> The user (although there is no user yet currently) has to assemble
>> several LoC to use:
>> 	attrs = alloc_workqueue_attrs();
>> 	if (!attrs)
>> 		return;
>> 	attrs->nice = ...;
>> 	copy cpumask;
>> 	attrs->no_numa = ...;
>> 	apply_workqueue_attrs();
>> 	free_workqueue_attrs();
>>
>> It is too elaborate. This patch changes apply_workqueue_attrs() API,
>> and one-line-code is enough to be called from user:
>> 	apply_workqueue_attrs(wq, cpumask, nice, numa);
>>
>> This patch also reduces the code of workqueue.c, about -50 lines.
>> wq_sysfs_prep_attrs() is removed, wq_[nice|cpumask|numa]_store()
>> directly access to the ->unbound_attrs with the protection
>> of apply_wqattrs_lock();
>>
>> This patch is also a preparation patch of next patch which
>> remove no_numa out from the structure workqueue_attrs which
>> requires apply_workqueue_attrs() has an argument to pass numa affinity.
> 
> I'm not sure about this.  Yeah, sure, it's a bit more lines of code
> but at the same time this'd allow us to make the public interface
> atomic too.  What we prolly should do is changing the interface so
> that we do
> 
> 	attrs = prepare_workqueue_attrs(gfp_mask);	/* allocate, lock & copy */
> 	/* modify attrs as desired */
> 	commit_workqueue_attrs(attrs);			/* apply, unlock and free */

I think the workqueue.c has too much complicated and rarely used APIs
and exposes too much in this way.  No one can set the nice value
and the cpuallowed of a task atomically.

If the user want atomic-able, Her/he can just disable WQ_SYSFS
on its workqueue and maintain a copy of the cpumask, nice, numa values
under its own lock.

> 
> Thanks.
>