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

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Message-ID: <50BE5C99.6070703@fusionio.com>
Date:	Tue, 4 Dec 2012 21:27:05 +0100
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Zach Brown <zab@...hat.com>
Subject: Re: [patch,v2] bdi: add a user-tunable cpu_list for the bdi flusher
 threads

On 2012-12-04 21:23, Jeff Moyer wrote:
> Jens Axboe <jaxboe@...ionio.com> writes:
> 
>> On 2012-12-03 19:53, Jeff Moyer wrote:
>>> Hi,
>>>
>>> In realtime environments, it may be desirable to keep the per-bdi
>>> flusher threads from running on certain cpus.  This patch adds a
>>> cpu_list file to /sys/class/bdi/* to enable this.  The default is to tie
>>> the flusher threads to the same numa node as the backing device (though
>>> I could be convinced to make it a mask of all cpus to avoid a change in
>>> behaviour).
>>
>> Looks sane, and I think defaulting to the home node is a sane default.
>> One comment:
>>
>>> +	ret = cpulist_parse(buf, newmask);
>>> +	if (!ret) {
>>> +		spin_lock(&bdi->wb_lock);
>>> +		task = wb->task;
>>> +		if (task)
>>> +			get_task_struct(task);
>>> +		spin_unlock(&bdi->wb_lock);
>>
>> bdi->wb_lock needs to be bh safe. The above should have caused lockdep
>> warnings for you.
> 
> No lockdep complaints.  I'll double check that's enabled (but I usually
> have it enabled...).
> 
>>> @@ -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.

-- 
Jens Axboe

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