Re: [patch,v2] bdi: add a user-tunable cpu_list for the bdi flusherthreads

From: Jens Axboe
Date: Wed Dec 05 2012 - 02:43:46 EST


On 2012-12-04 23:26, Jeff Moyer wrote:
> Jens Axboe <jaxboe@xxxxxxxxxxxx> 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?

Huh you are right, I completely missed that set_cpus_allowed_ptr() uses
cpu_all_mask and not mask_of_node(node). Doesn't make a lot of sense to
me... And yes, in any case, it definitely is a bad API, not very
logical.

--
Jens Axboe

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