Re: NULL pointer dereference in pick_next_task_fair

[Peter Zijlstra]

On Fri, Nov 08, 2019 at 01:00:34PM +0100, Peter Zijlstra wrote:
> > That would remove one call site to newidle_balance() too, which I think
> > is good. Hackbench probably won't like that, though.
> 
> Yeah, that fast path really is important. I've got a few patches pending
> there, fixing a few things and that gets me 2% extra on a sched-yield
> benchmark.

That is, the fast path also allows a cpu-cgroup optimization that wins
something in the order of 3% for cgroup workloads.

The cgroup optimization is basically that when we schedule from
fair->fair, we can avoid having to put/set the whole cgroup hierarchy
but only have to update the part that changed.

Couple that with the set_next_buddy() from dequeue_task_fair(), which
results in the next task being more likely to be from the same cgroup,
and you've got a win.