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SubjectRe: [PATCH 2/2] kfree_rcu() should use kfree_bulk() interface

On 04/04/2018 12:16 AM, Rao Shoaib wrote:
> On 04/03/2018 07:23 PM, Matthew Wilcox wrote:
>> On Tue, Apr 03, 2018 at 05:55:55PM -0700, Rao Shoaib wrote:
>>> On 04/03/2018 01:58 PM, Matthew Wilcox wrote:
>>>> I think you might be better off with an IDR.  The IDR can always
>>>> contain one entry, so there's no need for this 'rbf_list_head' or
>>>> __rcu_bulk_schedule_list.  The IDR contains its first 64 entries in
>>>> an array (if that array can be allocated), so it's compatible with the
>>>> kfree_bulk() interface.
>>> I have just familiarized myself with what IDR is by reading your
>>> article. If
>>> I am incorrect please correct me.
>>> The list and head you have pointed are only used  if the container
>>> can not
>>> be allocated. That could happen with IDR as well. Note that the
>>> containers
>>> are allocated at boot time and are re-used.
>> No, it can't happen with the IDR.  The IDR can always contain one entry
>> without allocating anything.  If you fail to allocate the second entry,
>> just free the first entry.
>>> IDR seems to have some overhead, such as I have to specifically add the
>>> pointer and free the ID, plus radix tree maintenance.
>> ... what?  Adding a pointer is simply idr_alloc(), and you get back an
>> integer telling you which index it has.  Your data structure has its
>> own set of overhead.
> The only overhead is a pointer that points to the head and an int to
> keep count. If I use idr, I would have to allocate an struct idr which
> is much larger. idr_alloc()/idr_destroy() operations are much more
> costly than updating two pointers. As the pointers are stored in
> slots/nodes corresponding to the id, I would  have to retrieve the
> pointers by calling idr_remove() to pass them to be freed, the
> slots/nodes would constantly be allocated and freed.
> IDR is a very useful interface for allocating/managing ID's but I
> really do not see the justification for using it over here, perhaps
> you can elaborate more on the benefits and also on how I can just pass
> the array to be freed.
> Shoaib
I may have mis-understood your comment. You are probably suggesting that
I use IDR instead of allocating following containers.

+ struct rcu_bulk_free_container *rbf_container;
+ struct rcu_bulk_free_container *rbf_cached_container;

IDR uses radix_tree_node which allocates following two arrays. since I
do not need any ID's why not just use the radix_tree_node directly, but
I do not need a radix tree either, so why not just use an array. That is
what I am doing.

void __rcu      *slots[RADIX_TREE_MAP_SIZE];
unsigned long   tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS]; ==> Not

As far as allocation failure is concerned, the allocation has to be done
at run time. If the allocation of a container can fail, so can the
allocation of radix_tree_node as it also requires memory.

I really do not see any advantages of using IDR. The structure I have is
much simpler and does exactly what I need.


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