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

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Message-ID: <20121204023405.GE32450@dastard>
Date:	Tue, 4 Dec 2012 13:34:05 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jens Axboe <jaxboe@...ionio.com>, linux-kernel@...r.kernel.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 Mon, Dec 03, 2012 at 01:53:39PM -0500, 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).

The default seems reasonable to me.

> Comments, as always, are appreciated.
.....

> +static ssize_t cpu_list_store(struct device *dev,
> +		struct device_attribute *attr, const char *buf, size_t count)
> +{
> +	struct backing_dev_info *bdi = dev_get_drvdata(dev);
> +	struct bdi_writeback *wb = &bdi->wb;
> +	cpumask_var_t newmask;
> +	ssize_t ret;
> +	struct task_struct *task;
> +
> +	if (!alloc_cpumask_var(&newmask, GFP_KERNEL))
> +		return -ENOMEM;
> +
> +	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);
> +		if (task) {
> +			ret = set_cpus_allowed_ptr(task, newmask);
> +			put_task_struct(task);
> +		}

Why is this set here outside the bdi->flusher_cpumask_mutex?

Also, I'd prefer it named "..._lock" as that is the normal
convention for such variables. You can tell the type of lock from
the declaration or the use...

....

> @@ -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);

As it is set under the lock here....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
--
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