Re: [PATCH 0/4] slab, rcu: move and consolidate TINY_RCU kvfree_rcu() to SLAB

[Vlastimil Babka <vbabka@suse.cz>]

On 2/3/25 20:07, Paul E. McKenney wrote:
> On Mon, Feb 03, 2025 at 10:28:46AM +0100, Vlastimil Babka wrote:
>> Following the move of the TREE_RCU batching kvfree_rcu() implementation
>> to slab, we still have the simple non-batching implementation in tiny
>> RCU, and RCU implementation specific ifdefs in slab code.
>> 
>> Finish the move and integration into slab. Allow using the simple
>> call_rcu() based implementation also with tree RCU when SLUB_TINY is
>> enabled, as its goal is also to limit memory footprint with less concern
>> for top performance.
>> 
>> In order to avoid RCU having to recognize the fake callback function
>> pointers (__is_kvfree_rcu_offset()) when handling call_rcu(), implement
>> a callback that can calculate the object's address from the embedded
>> rcu_head pointer without knowing the specific offset (previously SLOB
>> would not have made it possible, but it's gone now).
>> 
>> After this series, AFAIK only the following kvfree_rcu specific code
>> remains in RCU:
>> 
>> - a call to kfree_rcu_scheduler_running() from rcu_set_runtime_mode()
>> 
>>   - probably necessary and a generic registration interface would be
>>     unnecessary bloat?
> 
> I am OK with this calling directly.
> 
> Alternatively, you could create a core_initcall() that did nothing
> but invoke kfree_rcu_scheduler_running(), and then drop that call from
> rcu_set_runtime_mode().  Up to you guys!

I was afraid there's some ordering necessary with the rest of what
rcu_set_runtime_mode() does, but perhaps there's actually not.

>> - declarations of kfree_rcu() API in include/linux/rcupdate.h
>> 
>>   - could be moved to slab.h after checking for/fixing up potential
>>     missing includes
> 
> It would be good to move them, but no real hurry from this end.
> 
>> git tree:
>> https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=b4/slub-tiny-kfree_rcu
>> 
>> Changes since RFC [1]:
>> - Rebased to v6.14-rc1
>> - Fixed build errors reported by bots.
>> - Changed where kvfree_call_rcu() is moved to in patch 1 to avoid another
>>   move in patch 4.
>> - R-b's by Ulad and Joel
>> - Fix a memory leak in kvfree_rcu_list() thanks to Ulad
>> - Various comments' improvements and fixes (Joel, Ulad)
>> - Rename config to CONFIG_KFREE_RCU_BATCHED (Ulad)
>> 
>> Due to changes, didn't keep Paul's Tested-by (but thanks!)
> 
> I re-ran the tests, so feel free to add it back in.  ;-)

Thanks, will do :)