linux-kernel - Re: [PATCHSET] mempool, percpu, blkcg: fix percpu stat allocation and remove stats

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Message-ID: <20120307170544.GA30676@google.com>
Date:	Wed, 7 Mar 2012 09:05:44 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, axboe@...nel.dk,
	hughd@...gle.com, avi@...hat.com, nate@...nel.net,
	cl@...ux-foundation.org, linux-kernel@...r.kernel.org,
	dpshah@...gle.com, ctalbott@...gle.com, rni@...gle.com
Subject: Re: [PATCHSET] mempool, percpu, blkcg: fix percpu stat allocation
 and remove stats_lock

On Wed, Mar 07, 2012 at 09:55:56AM -0500, Vivek Goyal wrote:
> +/*
> + * Worker for allocating per cpu stat for blk groups. This is scheduled
> + * once there are some groups on the alloc_list waiting for allocation
> + */
> +static void blkio_stat_alloc_fn(struct work_struct *work)
> +{
> +
> +	struct blkio_group *blkg, *n;
> +	int i;
> +
> +alloc_stats:
> +	spin_lock_irq(&alloc_list_lock);
> +		if (list_empty(&alloc_list)) {
> +			/* No more groups needing per cpu stat allocation */
> +			spin_unlock_irq(&alloc_list_lock);
> +			return;
> +		}
> +	spin_unlock_irq(&alloc_list_lock);

I don't think we really need the above.  Just proceed with allocation.

> +	for (i = 0; i < BLKIO_NR_POLICIES; i++) {
> +		if (pcpu_stats[i] != NULL)
> +			continue;
> +
> +		pcpu_stats[i] = alloc_percpu(struct blkio_group_stats_cpu);
> +		/* Allocatoin failed. Try again after some time. */
		          ^^ typo
> +		if (pcpu_stats[i] == NULL) {
> +			msleep(10);
> +			goto alloc_stats;
> +		}
> +	}

Why not queue_delayed_work(system_nrt_wq, work, msecs_to_jiffies(10))?
Why hog the worker thread?

> +
> +	spin_lock_irq(&blkio_list_lock);
> +	spin_lock(&alloc_list_lock);
> +
> +	list_for_each_entry_safe(blkg, n, &alloc_list, alloc_node) {
> +		for (i = 0; i < BLKIO_NR_POLICIES; i++) {
> +			struct blkio_policy_type *pol = blkio_policy[i];
> +			struct blkg_policy_data *pd;
> +
> +			if (!pol)
> +				continue;
> +
> +			if (!blkg->pd[i])
> +				continue;
> +
> +			pd = blkg->pd[i];
> +			if (pd->stats_cpu)
> +				continue;
> +
> +			pd->stats_cpu = pcpu_stats[i];
> +			pcpu_stats[i] = NULL;

I think this can be slightly more compact.  How about the following?

	struct blkg_policy_data *pd = blkg->pd[i];

	if (blkio_policy[i] && pd && !pd->stats_cpu)
		swap(pd->stats_cpu, pcpu_stats[i]);

> +		}
> +		list_del_init(&blkg->alloc_node);
> +		break;

If we're breaking after the first iteration, why are we using
list_for_each at all?

> +	}

and we can test whether we need to loop here, right?

Thanks.

-- 
tejun
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