netdev - Re: [PATCH 04/16] dma-debug: add allocator code

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Date:	Sun, 11 Jan 2009 00:43:52 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Joerg Roedel <joerg.roedel@....com>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	dwmw2@...radead.org, fujita.tomonori@....ntt.co.jp,
	netdev@...r.kernel.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 04/16] dma-debug: add allocator code


* Joerg Roedel <joerg.roedel@....com> wrote:

> +		printk(KERN_ERR "DMA-API: debugging out of memory "
> +				"- disabling\n");

btw., i'd suggest to not break kernel messages mid-string, but do 
something like this instead:

> +		printk(KERN_ERR
		 "DMA-API: debugging out of memory - disabling\n");

Also, i'd use WARN() - it might be useful to see what callsite depleted 
the pool.

> +	entry = list_entry(free_entries.next, struct dma_debug_entry, list);
> +	list_del(&entry->list);
> +	memset(entry, 0, sizeof(*entry));
> +
> +	num_free_entries -= 1;
> +	if (num_free_entries < min_free_entries)
> +		min_free_entries = num_free_entries;

unlikely() i guess.

Regarding the entry pool locking:

> +static void dma_entry_free(struct dma_debug_entry *entry)
> +{
> +	unsigned long flags;
> +
> +	/*
> +	 * add to beginning of the list - this way the entries are
> +	 * more likely cache hot when they are reallocated.
> +	 */
> +	spin_lock_irqsave(&free_entries_lock, flags);
> +	list_add(&entry->list, &free_entries);
> +	num_free_entries += 1;
> +	spin_unlock_irqrestore(&free_entries_lock, flags);

it might make sense to cache entries in the buckets - hence reuse the 
bucket spinlock. This means a somewhat higher effective pool size, but it 
also avoids a global lock.

	Ingo
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