On Wed, Apr 14, 2021 at 05:56:53PM +0200, Vlastimil Babka wrote: > On 4/14/21 5:18 PM, Mel Gorman wrote: > > On Wed, Apr 14, 2021 at 02:56:45PM +0200, Vlastimil Babka wrote: > >> So it seems that this intermediate assignment to zone counters (using > >> atomic_long_set() even) is unnecessary and this could mimic > >> sum_vm_events() that > >> just does the summation on a local array? > >> > > > > The atomic is unnecessary for sure but using a local array is > > problematic because of your next point. > > IIUC vm_events seems to do fine without a centralized array and handling CPU > hot > remove at the sime time ... >
The vm_events are more global in nature. They are not reported to userspace on a per-zone (/proc/zoneinfo) basis or per-node (/sys/devices/system/node/node*/numastat) basis so they are not equivalent. > >> And probably a bit more serious is that vm_events have > >> vm_events_fold_cpu() to > >> deal with a cpu going away, but after your patch the stats counted on a > >> cpu just > >> disapepar from the sums as it goes offline as there's no such thing for > >> the numa > >> counters. > >> > > > > That is a problem I missed. Even if zonestats was preserved on > > hot-remove, fold_vm_zone_numa_events would not be reading the CPU so > > hotplug events jump all over the place. > > > > So some periodic folding is necessary. I would still prefer not to do it > > by time but it could be done only on overflow or when a file like > > /proc/vmstat is read. I'll think about it a bit more and see what I come > > up with. > > ... because vm_events_fold_cpu() seems to simply move the stats from the CPU > being offlined to the current one. So the same approach should be enough for > NUMA stats? > Yes, or at least very similar. -- Mel Gorman SUSE Labs
