On Wed, Jun 19, 2024 at 11:28:13AM +0200, Vlastimil Babka wrote: > On 6/18/24 7:53 PM, Paul E. McKenney wrote: > > 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. > > Yeah I've been resisting such changes to the layout and this wouldn't be > worth it, apart from changing the name itself but not in a dangerous way > like with "/" :) > > >> 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. > > So it seems if the sysfs/debugfs directories already exist, they will > silently not be created. Wonder if we have such cases today already because > caches with same name exist. I think we do with the zsmalloc using 32 caches > with same name that we discussed elsewhere just recently. > > Also indeed if the cache has leaked objects and won't be thus destroyed, > these directories indeed stay around, as well as the slabinfo entry, and can > prevent new ones from being created (slabinfo lines with same name are not > prevented). > > But it wouldn't be great to introduce this possibility to happen for the > temporarily delayed removal due to kfree_rcu() and a module re-insert, since > that's a legitimate case and not buggy state due to leaks. > > The debugfs directory we could remove immediately before handing over to the > scheduled workfn, but if it turns out there was a leak and the workfn leaves > the cache around, debugfs dir will be gone and we can't check the > alloc_traces/free_traces files there (but we have the per-object info > including the traces in the dmesg splat). > > The sysfs directory is currently removed only with the whole cache being > destryed due to sysfs/kobject lifetime model. I'd love to untangle it for > other reasons too, but haven't investigated it yet. But again it might be > useful for sysfs dir to stay around for inspection, as for the debugfs. > > We could rename the sysfs/debugfs directories before queuing the work? Add > some prefix like GOING_AWAY-$name. If leak is detected and cache stays > forever, another rename to LEAKED-$name. (and same for the slabinfo). But > multiple ones with same name might pile up, so try adding a counter then? > Probably messy to implement, but perhaps the most robust in the end? The > automatic counter could also solve the general case of people using same > name for multiple caches. > > Other ideas? > One question. Maybe it is already late but it is better to ask rather than not. What do you think if we have a small discussion about it on the LPC 2024 as a topic? It might be it is already late or a schedule is set by now. Or we fix it by a conference time. Just a thought. -- Uladzislau Rezki
