> > Thanks for the explain. However, such case can happen with THP page > as well as slab. In case of THP page, nr_scanned is 1 but nr_reclaimed > could be 512 so I think vmpressure should have a logic to prevent undeflow > regardless of slab shrinking. > I see. Going to send a vmpressure fix. But, wouldn't the THP case result in incorrect vmpressure reporting even if we fix the vmpressure underflow problem ? >> >> > >> >> unsigned arithmetic results in the pressure value to be >> >> huge, thus resulting in a critical event being sent to >> >> root cgroup. Fix this by not passing the reclaimed slab >> >> count to vmpressure, with the assumption that vmpressure >> >> should show the actual pressure on LRU which is now >> >> diluted by adding reclaimed slab without a corresponding >> >> scanned value. >> > >> > I can't guess justfication of your assumption from the description. >> > Why do we consider only LRU pages for vmpressure? Could you elaborate >> > a bit? >> > >> When we encountered the false events from vmpressure, thought the problem >> could be that slab scanned is not included in sc->nr_scanned, like it is done >> for reclaimed. But later thought vmpressure works only on the scanned and >> reclaimed from LRU. I can explain what I understand, let me know if this is >> incorrect. >> vmpressure is an index which tells the pressure on LRU, and thus an >> indicator of thrashing. In shrink_node when we come out of the inner do-while >> loop after shrinking the lruvec, the scanned and reclaimed corresponds to the >> pressure felt on the LRUs which in turn indicates the pressure on VM. The >> moment we add the slab reclaimed pages to the reclaimed, we dilute the >> actual pressure felt on LRUs. When slab scanned/reclaimed is not included >> in the vmpressure, the values will indicate the actual pressure and if there >> were a lot of slab reclaimed pages it will result in lesser pressure >> on LRUs in the next run which will again be indicated by vmpressure. i.e. the > > I think there is no intention to exclude slab by design of vmpressure. > Beause slab is memory consumption so freeing of slab pages really helps > the memory pressure. Also, there might be slab-intensive workload rather > than LRU. It would be great if vmpressure works well with that case. > But the problem with involving slab for vmpressure is it's not fair with > LRU pages. LRU pages are 1:1 cost model for scan:free but slab shriking > depends the each slab's object population. It means it's impossible to > get stable cost model with current slab shrinkg model, unfortunately. > So I don't obejct this patch although I want to see slab shrink model's > change which is heavy-handed work. > Looking at the code, the slab reclaimed pages started getting passed to vmpressure after the commit ("mm: vmscan: invoke slab shrinkers from shrink_zone()"). But as you said, this may be helpful for slab intensive workloads. But in its current form I think it results in incorrect vmpressure reporting because of not accounting the slab scanned pages. Resending the patch with a modified commit msg since the underflow issue is fixed separately. Thanks Minchan. Vinayak -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>