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

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Date:   Tue, 13 Feb 2018 08:49:40 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Frank Rowand <frowand.list@...il.com>
Cc:     Chintan Pandya <cpandya@...eaurora.org>,
        devicetree@...r.kernel.org,
        "linux-kernel@...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 Mon, Feb 12, 2018 at 12:27 AM,  <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.

Please add some data here as to what speed up we see.

> The cache is initialized in of_core_init().
>
> The cache is freed via a late_initcall_sync() if modules are not
> enabled.
>
> Signed-off-by: Frank Rowand <frank.rowand@...y.com>
> ---
>
> Changes since v1:
>   - change short description from
>     of: cache phandle nodes to reduce cost of of_find_node_by_phandle()
>   - rebase on v4.16-rc1
>   - reorder new functions in base.c to avoid forward declaration
>   - add locking around kfree(phandle_cache) for memory ordering
>   - add explicit check for non-null of phandle_cache in
>     of_find_node_by_phandle().  There is already a check for !handle,
>     which prevents accessing a null phandle_cache, but that dependency
>     is not obvious, so this check makes it more apparent.
>   - do not free phandle_cache if modules are enabled, so that
>     cached phandles will be available when modules are loaded
>
>  drivers/of/base.c       | 93 ++++++++++++++++++++++++++++++++++++++++++++++---
>  drivers/of/of_private.h |  5 +++
>  drivers/of/resolver.c   | 21 -----------
>  3 files changed, 94 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/of/base.c b/drivers/of/base.c
> index ad28de96e13f..b3597c250837 100644
> --- a/drivers/of/base.c
> +++ b/drivers/of/base.c
> @@ -91,10 +91,88 @@ int __weak of_node_to_nid(struct device_node *np)
>  }
>  #endif
>
> +static struct device_node **phandle_cache;
> +static u32 max_phandle_cache;
> +
> +phandle live_tree_max_phandle(void)
> +{
> +       struct device_node *node;
> +       phandle max_phandle;
> +       unsigned long flags;
> +
> +       raw_spin_lock_irqsave(&devtree_lock, flags);
> +
> +       max_phandle = 0;
> +       for_each_of_allnodes(node) {
> +               if (node->phandle != OF_PHANDLE_ILLEGAL &&
> +                   node->phandle > max_phandle)
> +                       max_phandle = node->phandle;
> +               }
> +
> +       raw_spin_unlock_irqrestore(&devtree_lock, flags);
> +
> +       return max_phandle;
> +}
> +
> +static void of_populate_phandle_cache(void)
> +{
> +       unsigned long flags;
> +       phandle max_phandle;
> +       u32 nodes = 0;
> +       struct device_node *np;
> +
> +       if (phandle_cache)
> +               return;
> +
> +       max_phandle = live_tree_max_phandle();
> +
> +       raw_spin_lock_irqsave(&devtree_lock, flags);
> +
> +       for_each_of_allnodes(np)
> +               nodes++;
> +
> +       /* sanity cap for malformed tree */

Or any tree that doesn't match your assumptions.

> +       if (max_phandle > nodes)
> +               max_phandle = nodes;

I fail to see how setting max_phandle to number of nodes helps you
here. Either just bail out or mask the phandle values so this works
with any contiguous range.

Please capture somewhere what the assumptions are for the cache to
work (i.e. phandles were allocated contiguously from 0).

Rob