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SubjectRe: [PATCH] of: Rework and simplify phandle cache to use a fixed size
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On 12/11/19 5:48 PM, Rob Herring wrote:
> On Wed, Dec 11, 2019 at 5:23 PM Rob Herring <robh@kernel.org> 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@linutronix.de>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Segher Boessenkool <segher@kernel.crashing.org>
>> Cc: Frank Rowand <frowand.list@gmail.com>
>> Signed-off-by: Rob Herring <robh@kernel.org>
>> ---
>> 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
>

I added the OF_DETACHED checks out of extreme paranoia. They can be
removed.

-Frank

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