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

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Date:   Thu, 19 Dec 2019 09:31:54 -0600
From:   Frank Rowand <frowand.list@...il.com>
To:     Rob Herring <robh@...nel.org>, devicetree@...r.kernel.org
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Frank Rowand <frowand.list@...il.com>
Subject: Re: [PATCH] of: Rework and simplify phandle cache to use a fixed size

On 12/11/19 5:48 PM, Rob Herring wrote:
> On Wed, Dec 11, 2019 at 5:23 PM Rob Herring <robh@...nel.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@...utronix.de>
>> Cc: Michael Ellerman <mpe@...erman.id.au>
>> Cc: Segher Boessenkool <segher@...nel.crashing.org>
>> Cc: Frank Rowand <frowand.list@...il.com>
>> Signed-off-by: Rob Herring <robh@...nel.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