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

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Date:	Tue, 4 Dec 2012 21:14:00 +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 2012-12-03 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.

> +		if (task) {
> +			ret = set_cpus_allowed_ptr(task, newmask);
> +			put_task_struct(task);
> +		}
> +		if (ret == 0) {
> +			mutex_lock(&bdi->flusher_cpumask_mutex);
> +			cpumask_copy(bdi->flusher_cpumask, newmask);
> +			mutex_unlock(&bdi->flusher_cpumask_mutex);
> +			ret = count;
> +		}
> +	}

> @@ -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...

-- 
Jens Axboe

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