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

Paul E. McKenney
2024-06-18T16:48:50Z

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

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