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

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Feb 04, 2025 at 11:02:47AM +0100, Vlastimil Babka wrote:
> 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.

I *think* that they are independent, but I agree that it would be good
to carefully double-check.

The basis for my belief is that it is OK to invoke call_rcu() long
before rcu_init() is invoked, let alone the scheduler being ready.

							Thanx, Paul

> >> - 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 :)
> 
>