sched/fair: Fix zero_vruntime tracking fix

In the Linux kernel, the following vulnerability has been resolved:

sched/fair: Fix zero_vruntime tracking fix

John reported that stress-ng-yield could make his machine unhappy and

managed to bisect it to commit b3d99f43c72b ("sched/fair: Fix

zero_vruntime tracking").

The combination of yield and that commit was specific enough to

hypothesize the following scenario:

Suppose we have 2 runnable tasks, both doing yield. Then one will be

eligible and one will not be, because the average position must be in

between these two entities.

Therefore, the runnable task will be eligible, and be promoted a full

slice (all the tasks do is yield after all). This causes it to jump over

the other task and now the other task is eligible and current is no

longer. So we schedule.

Since we are runnable, there is no {de,en}queue. All we have is the

__{en,de}queue_entity() from {put_prev,set_next}_task(). But per the

fingered commit, those two no longer move zero_vruntime.

All that moves zero_vruntime are tick and full {de,en}queue.

This means, that if the two tasks playing leapfrog can reach the

critical speed to reach the overflow point inside one tick's worth of

time, we're up a creek.

Additionally, when multiple cgroups are involved, there is no guarantee

the tick will in fact hit every cgroup in a timely manner. Statistically

speaking it will, but that same statistics does not rule out the

possibility of one cgroup not getting a tick for a significant amount of

time -- however unlikely.

Therefore, just like with the yield() case, force an update at the end

of every slice. This ensures the update is never more than a single

slice behind and the whole thing is within 2 lag bounds as per the

comment on entity_key().