On Wed, Nov 14, 2018 at 03:46:02AM +0100, Frederic Weisbecker wrote: > We need to read the nice value of the task running on any CPU, possibly > remotely, in order to correctly support kcpustat on nohz_full. > Unfortunately we can't just read task_nice(tsk) when tsk runs on another > CPU because its nice value may be concurrently changed. There could be a > risk that a recently modified nice value is thought to apply for a longer > while than is supposed to. > > For example if a task runs at T0 with nice = -10, then its nice value > is changed at T0 + 1 second with nice = 10, a reader at T0 + 1 second > could think that the task had this "nice == 10" value since the beginning > (T0) and spuriously account 1 second nice time on kcpustat instead of 1 > second user time. > > So we need to track the nice value changes under vtime seqcount. Start > with context switches and account the vtime nice-ness on top of it.
Huh, what!? That doesn't make any sense..