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

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Date:	Tue, 04 Dec 2012 15:23:13 -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>
Subject: Re: [patch,v2] bdi: add a user-tunable cpu_list for the bdi flusher threads

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.

Thanks!
Jeff
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