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.