linux-kernel - Re: [PATCH v2] of: cache phandle nodes to reduce cost of of_find_node_by

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Date:   Mon, 12 Feb 2018 09:58:37 +0100
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     <frowand.list@...il.com>, Rob Herring <robh+dt@...nel.org>,
        <cpandya@...eaurora.org>
CC:     <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] of: cache phandle nodes to reduce cost of
 of_find_node_by_phandle()

On 2018-02-12 07:27, frowand.list@...il.com wrote:
> From: Frank Rowand <frank.rowand@...y.com>
> 
> Create a cache of the nodes that contain a phandle property.  Use this
> cache to find the node for a given phandle value instead of scanning
> the devicetree to find the node.  If the phandle value is not found
> in the cache, of_find_node_by_phandle() will fall back to the tree
> scan algorithm.
> 
> The cache is initialized in of_core_init().
> 
> The cache is freed via a late_initcall_sync() if modules are not
> enabled.

Maybe a few words about the memory consumption of this solution versus
the other proposed ones. Other nits below.

> +static void of_populate_phandle_cache(void)
> +{
> +	unsigned long flags;
> +	phandle max_phandle;
> +	u32 nodes = 0;
> +	struct device_node *np;
> +
> +	if (phandle_cache)
> +		return;

What's the point of that check? And shouldn't it be done inside the
spinlock if at all?

> +	max_phandle = live_tree_max_phandle();
> +
> +	raw_spin_lock_irqsave(&devtree_lock, flags);
> +
> +	for_each_of_allnodes(np)
> +		nodes++;

Why not save a walk over all nodes and a spin_lock/unlock pair by
combining the node count with the max_phandle computation? But you've
just moved the existing live_tree_max_phandle, so probably better as a
followup patch.

> +	/* sanity cap for malformed tree */
> +	if (max_phandle > nodes)
> +		max_phandle = nodes;
> +
> +	phandle_cache = kzalloc((max_phandle + 1) * sizeof(*phandle_cache),
> +				GFP_ATOMIC);

Maybe kcalloc. Sure, you've capped max_phandle so there's no real risk
of overflow.

> +	for_each_of_allnodes(np)
> +		if (np->phandle != OF_PHANDLE_ILLEGAL  &&
> +		    np->phandle <= max_phandle &&
> +		    np->phandle)

I'd reverse the order of these conditions so that for all the nodes with
no phandle we only do the np->phandle check. Also, extra whitespace
before &&.

> +			phandle_cache[np->phandle] = np;
> +
> +	max_phandle_cache = max_phandle;
> +
> +	raw_spin_unlock_irqrestore(&devtree_lock, flags);
> +}
> +

Rasmus