Re: [PATCH] of: Rework and simplify phandle cache to use a fixed size

[Rob Herring]

On Wed, Dec 11, 2019 at 5:23 PM Rob Herring <robh@xxxxxxxxxx> wrote:
>
> The phandle cache was added to speed up of_find_node_by_phandle() by
> avoiding walking the whole DT to find a matching phandle. The
> implementation has several shortcomings:
>
>   - The cache is designed to work on a linear set of phandle values.
>     This is true for dtc generated DTs, but not for other cases such as
>     Power.
>   - The cache isn't enabled until of_core_init() and a typical system
>     may see hundreds of calls to of_find_node_by_phandle() before that
>     point.
>   - The cache is freed and re-allocated when the number of phandles
>     changes.
>   - It takes a raw spinlock around a memory allocation which breaks on
>     RT.
>
> Change the implementation to a fixed size and use hash_32() as the
> cache index. This greatly simplifies the implementation. It avoids
> the need for any re-alloc of the cache and taking a reference on nodes
> in the cache. We only have a single source of removing cache entries
> which is of_detach_node().
>
> Using hash_32() removes any assumption on phandle values improving
> the hit rate for non-linear phandle values. The effect on linear values
> using hash_32() is about a 10% collision. The chances of thrashing on
> colliding values seems to be low.
>
> To compare performance, I used a RK3399 board which is a pretty typical
> system. I found that just measuring boot time as done previously is
> noisy and may be impacted by other things. Also bringing up secondary
> cores causes some issues with measuring, so I booted with 'nr_cpus=1'.
> With no caching, calls to of_find_node_by_phandle() take about 20124 us
> for 1248 calls. There's an additional 288 calls before time keeping is
> up. Using the average time per hit/miss with the cache, we can calculate
> these calls to take 690 us (277 hit / 11 miss) with a 128 entry cache
> and 13319 us with no cache or an uninitialized cache.
>
> Comparing the 3 implementations the time spent in
> of_find_node_by_phandle() is:
>
> no cache:        20124 us (+ 13319 us)
> 128 entry cache:  5134 us (+ 690 us)
> current cache:     819 us (+ 13319 us)
>
> We could move the allocation of the cache earlier to improve the
> current cache, but that just further complicates the situation as it
> needs to be after slab is up, so we can't do it when unflattening (which
> uses memblock).
>
> Reported-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Cc: Segher Boessenkool <segher@xxxxxxxxxxxxxxxxxxx>
> Cc: Frank Rowand <frowand.list@xxxxxxxxx>
> Signed-off-by: Rob Herring <robh@xxxxxxxxxx>
> ---
>  drivers/of/base.c       | 133 ++++++++--------------------------------
>  drivers/of/dynamic.c    |   2 +-
>  drivers/of/of_private.h |   4 +-
>  drivers/of/overlay.c    |  10 ---
>  4 files changed, 28 insertions(+), 121 deletions(-)

[...]

> -       if (phandle_cache) {
> -               if (phandle_cache[masked_handle] &&
> -                   handle == phandle_cache[masked_handle]->phandle)
> -                       np = phandle_cache[masked_handle];
> -               if (np && of_node_check_flag(np, OF_DETACHED)) {
> -                       WARN_ON(1); /* did not uncache np on node removal */
> -                       of_node_put(np);
> -                       phandle_cache[masked_handle] = NULL;
> -                       np = NULL;
> -               }
> +       if (phandle_cache[handle_hash] &&
> +           handle == phandle_cache[handle_hash]->phandle)
> +               np = phandle_cache[handle_hash];
> +       if (np && of_node_check_flag(np, OF_DETACHED)) {
> +               WARN_ON(1); /* did not uncache np on node removal */

BTW, I don't think this check is even valid. If we failed to detach
and remove the node from the cache, then we could be accessing np
after freeing it.

Rob