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

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.


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