On Tue, Jun 18, 2024 at 07:21:42PM +0200, Vlastimil Babka wrote: > 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. Fair enough! > > 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. Or mark them in /proc/slabinfo? Yet another column, yay!!! Or script breakage from flagging the name somehow, for example, trailing "/" character. > 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. "It is a service that I provide." ;-) But yes, we might be living with it already and there might already be ways people deal with it. 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 >
