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

Uladzislau Rezki
2024-06-18T09:31:07Z

> 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.

> > 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