Re: [PATCH] sched/fair: Fix nohz.next_balance update

[Peng Liu]

On Wed, May 06, 2020 at 05:02:56PM +0100, Valentin Schneider wrote:
> 
> On 06/05/20 14:45, Vincent Guittot wrote:
> >> But then we may skip an update if we goto abort, no? Imagine we have just
> >> NOHZ_STATS_KICK, so we don't call any rebalance_domains(), and then as we
> >> go through the last NOHZ CPU in the loop we hit need_resched(). We would
> >> end in the abort part without any update to nohz.next_balance, despite
> >> having accumulated relevant data in the local next_balance variable.
> >
> > Yes but on the other end, the last CPU has not been able to run the
> > rebalance_domain so we must not move  nohz.next_balance otherwise it
> > will have to wait for at least another full period
> > In fact, I think that we have a problem with current implementation
> > because if we abort because  local cpu because busy we might end up
> > skipping idle load balance for a lot of idle CPUs
> >
> > As an example, imagine that we have 10 idle CPUs with the same
> > rq->next_balance which equal nohz.next_balance.  _nohz_idle_balance
> > starts on CPU0, it processes idle lb for CPU1 but then has to abort
> > because of need_resched. If we update nohz.next_balance like
> > currently, the next idle load balance  will happen after a full
> > balance interval whereas we still have 8 CPUs waiting for running an
> > idle load balance.
> >
> > My proposal also fixes this problem
> >
> 
> That's a very good point; so with NOHZ_BALANCE_KICK we can reduce
> nohz.next_balance via rebalance_domains(), and otherwise we would only
> increase it if we go through a complete for_each_cpu() loop in
> _nohz_idle_balance().
> 
> That said, if for some reason we keep bailing out of the loop, we won't
> push nohz.next_balance forward and thus may repeatedly nohz-balance only
> the first few CPUs in the NOHZ mask. I think that can happen if we have
> say 2 tasks pinned to a single rq, in that case nohz_balancer_kick() will
> kick a NOHZ balance whenever now >= nohz.next_balance.

If we face the risk of "repeatly nohz-balance only the first few CPUs",
Maybe we could remember the interrupted CPU and start nohz-balance from
it next time. Just replace the loop in _nohz_idle_balance() like:

for_each_cpu_wrap(cpu, nohz.idle_cpus_mask, nohz.anchor) {
	...
	if (need_resched()) {
		...
		nohz.anchor = cpu;
		...
	}
	...
}

This can mitigate the problem, but this can't help the extreme situation
as @Vincent put, it always failed in the same CPU.