linux-kernel - Re: [patch,v2] bdi: add a user-tunable cpu

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Message-ID: <x494nk1pi7h.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 04 Dec 2012 17:26:26 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm\@kvack.org" <linux-mm@...ck.org>,
	Zach Brown <zab@...hat.com>, tj@...nel.org,
	Peter Zijlstra <pzijlstr@...hat.com>, Ingo <mingo@...hat.com>
Subject: Re: [patch,v2] bdi: add a user-tunable cpu_list for the bdi flusher threads

Jens Axboe <jaxboe@...ionio.com> writes:

>>>> @@ -437,6 +488,14 @@ static int bdi_forker_thread(void *ptr)
>>>>  				spin_lock_bh(&bdi->wb_lock);
>>>>  				bdi->wb.task = task;
>>>>  				spin_unlock_bh(&bdi->wb_lock);
>>>> +				mutex_lock(&bdi->flusher_cpumask_mutex);
>>>> +				ret = set_cpus_allowed_ptr(task,
>>>> +							bdi->flusher_cpumask);
>>>> +				mutex_unlock(&bdi->flusher_cpumask_mutex);
>>>
>>> It'd be very useful if we had a kthread_create_cpu_on_cpumask() instead
>>> of a _node() variant, since the latter could easily be implemented on
>>> top of the former. But not really a show stopper for the patch...
>> 
>> Hmm, if it isn't too scary, I might give this a try.
>
> Should not be, pretty much just removing the node part of the create
> struct passed in and making it a cpumask. And for the on_node() case,
> cpumask_of_ndoe() will do the trick.

I think it's a bit more involved than that.  If you look at
kthread_create_on_node, the node portion only applies to where the
memory comes from, it says nothing of scheduling.  To whit:

                /*                                                              
                 * root may have changed our (kthreadd's) priority or CPU mask.
                 * The kernel thread should not inherit these properties.       
                 */
                sched_setscheduler_nocheck(create.result, SCHED_NORMAL, &param);
                set_cpus_allowed_ptr(create.result, cpu_all_mask);

So, if I were to make the change you suggested, I would be modifying the
existing behaviour.  The way things stand, I think
kthread_create_on_node violates the principal of least surprise.  ;-)  I
would prefer a variant that affected scheduling behaviour as well as
memory placement.  Tejun, Peter, Ingo, what are your opinions?

Cheers,
Jeff
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