Re: [RFC][PATCH v3 7/10] workqueue: add WQ_IDLEPRI

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 26 May 2011 19:30:18 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> On Thu, 26 May 2011 11:38:08 +0200
> Tejun Heo <tj@kernel.org> wrote:
> 
> > Hello, KAMEZAWA.
> > 
> > On Thu, May 26, 2011 at 02:30:24PM +0900, KAMEZAWA Hiroyuki wrote:
> > > When this idea came to me, I wonder which is better to maintain
> > > memcg's thread pool or add support in workqueue for generic use. In
> > > genral, I feel enhancing genric one is better...so, wrote this one.
> > 
> > Sure, if it's something which can be useful for other users, it makes
> > sense to make it generic.
> > 
> Thank you for review.
> 
> 
> > > Index: memcg_async/include/linux/workqueue.h
> > > ===================================================================
> > > --- memcg_async.orig/include/linux/workqueue.h
> > > +++ memcg_async/include/linux/workqueue.h
> > > @@ -56,7 +56,8 @@ enum {
> > >  
> > >  	/* special cpu IDs */
> > >  	WORK_CPU_UNBOUND	= NR_CPUS,
> > > -	WORK_CPU_NONE		= NR_CPUS + 1,
> > > +	WORK_CPU_IDLEPRI	= NR_CPUS + 1,
> > > +	WORK_CPU_NONE		= NR_CPUS + 2,
> > >  	WORK_CPU_LAST		= WORK_CPU_NONE,
> > 
> > Hmmm... so, you're defining another fake CPU a la unbound CPU.  I'm
> > not sure whether it's really necessary to create its own worker pool
> > tho.  The reason why SCHED_OTHER is necessary is because it may
> > consume large amount of CPU cycles.  Workqueue already has UNBOUND -
> > for an unbound one, workqueue code simply acts as generic worker pool
> > provider and everything other than work item dispatching and worker
> > management are deferred to scheduler and the workqueue user.
> > 
> yes.
> 
> > Is there any reason memcg can't just use UNBOUND workqueue and set
> > scheduling priority when the work item starts and restore it when it's
> > done? 
> 
> I thought of that. But I didn't do that because I wasn't sure how others
> will think about changing exisitng workqueue priority...and I was curious
> to know how workqueue works.
> 
> > If it's gonna be using UNBOUND at all, I don't think changing
> > scheduling policy would be a noticeable overhead and I find having
> > separate worker pools depending on scheduling priority somewhat silly.
> > 
> ok.
> 
> > We can add a mechanism to manage work item scheduler priority to
> > workqueue if necessary tho, I think.  But that would be per-workqueue
> > attribute which is applied during execution, not something per-gcwq.
> > 
> 
> In the next version, I'll try some like..
> ==
> 	process_one_work(...) {
> 		.....
> 		spin_unlock_irq(&gcwq->lock);
> 		.....
> 		if (cwq->wq->flags & WQ_IDLEPRI) {
> 			set_scheduler(...SCHED_IDLE...)
> 			cond_resched();
> 			scheduler_switched = true;
> 		}
> 		f(work) 
> 		if (scheduler_switched)
> 			set_scheduler(...SCHED_OTHER...)
> 		spin_lock_irq(&gcwq->lock);
> 	}
> ==
> Patch size will be much smaller. (Should I do this in memcg's code ??)
> 

BTW, my concern is that if f(work) is enough short,effect of SCHED_IDLE will never
be found because SCHED_OTHER -> SCHED_IDLE -> SCHED_OTHER switch is very fast.
Changed "weight" of CFQ never affects the next calculation of vruntime..of the
thread and the work will show the same behavior with SCHED_OTHER.

I'm sorry if I misunderstand CFQ and setscheduler().

Thanks,
-Kame




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