[patch 0/5] optionally sync per-CPU vmstats counter on return to userspace

[Marcelo Tosatti]

The logic to disable vmstat worker thread, when entering
nohz full, does not cover all scenarios. For example, it is possible
for the following to happen:

1) enter nohz_full, which calls refresh_cpu_vm_stats, syncing the stats.
2) app runs mlock, which increases counters for mlock'ed pages.
3) start -RT loop

Since refresh_cpu_vm_stats from nohz_full logic can happen _before_
the mlock, vmstat shepherd can restart vmstat worker thread on
the CPU in question.
 
To fix this, optionally sync the vmstat counters when returning
from userspace, controllable by a new "vmstat_sync" isolcpus
flags (default off).

See individual patches for details.