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

[Rao Shoaib]

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 __rcuÂÂÂÂÂ *slots[RADIX_TREE_MAP_SIZE];
unsigned longÂÂ tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS]; ==> Not 
needed

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.

Shoaib