Re: [PATCH 00/14] replace call_rcu by kfree_rcu for simple kmem_cache_free callback

[Vlastimil Babka <vbabka@suse.cz>]

On 6/18/24 6:48 PM, Paul E. McKenney wrote:
> On Tue, Jun 18, 2024 at 11:31:00AM +0200, Uladzislau Rezki wrote:
>> > On 6/17/24 8:42 PM, Uladzislau Rezki wrote:
>> > >> +
>> > >> +	s = container_of(work, struct kmem_cache, async_destroy_work);
>> > >> +
>> > >> +	// XXX use the real kmem_cache_free_barrier() or similar thing here
>> > > It implies that we need to introduce kfree_rcu_barrier(), a new API, which i
>> > > wanted to avoid initially.
>> > 
>> > I wanted to avoid new API or flags for kfree_rcu() users and this would
>> > be achieved. The barrier is used internally so I don't consider that an
>> > API to avoid. How difficult is the implementation is another question,
>> > depending on how the current batching works. Once (if) we have sheaves
>> > proven to work and move kfree_rcu() fully into SLUB, the barrier might
>> > also look different and hopefully easier. So maybe it's not worth to
>> > invest too much into that barrier and just go for the potentially
>> > longer, but easier to implement?
>> > 
>> Right. I agree here. If the cache is not empty, OK, we just defer the
>> work, even we can use a big 21 seconds delay, after that we just "warn"
>> if it is still not empty and leave it as it is, i.e. emit a warning and
>> we are done.
>> 
>> Destroying the cache is not something that must happen right away. 
> 
> OK, I have to ask...
> 
> Suppose that the cache is created and destroyed by a module and
> init/cleanup time, respectively.  Suppose that this module is rmmod'ed
> then very quickly insmod'ed.
> 
> Do we need to fail the insmod if the kmem_cache has not yet been fully
> cleaned up?

We don't have any such link between kmem_cache and module to detect that, so
we would have to start tracking that. Probably not worth the trouble.

>  If not, do we have two versions of the same kmem_cache in
> /proc during the overlap time?

Hm could happen in /proc/slabinfo but without being harmful other than
perhaps confusing someone. We could filter out the caches being destroyed
trivially.

Sysfs and debugfs might be more problematic as I suppose directory names
would clash. I'll have to check... might be even happening now when we do
detect leaked objects and just leave the cache around... thanks for the
question.

> 							Thanx, Paul
> 
>> > > Since you do it asynchronous can we just repeat
>> > > and wait until it a cache is furry freed?
>> > 
>> > The problem is we want to detect the cases when it's not fully freed
>> > because there was an actual read. So at some point we'd need to stop the
>> > repeats because we know there can no longer be any kfree_rcu()'s in
>> > flight since the kmem_cache_destroy() was called.
>> > 
>> Agree. As noted above, we can go with 21 seconds(as an example) interval
>> and just perform destroy(without repeating).
>> 
>> --
>> Uladzislau Rezki