Messages in this thread | | | From | Vincent Guittot <> | Date | Thu, 7 May 2020 14:53:05 +0200 | Subject | Re: [PATCH] sched/fair: Fix nohz.next_balance update |
| |
On Thu, 7 May 2020 at 14:41, Peng Liu <iwtbavbm@gmail.com> wrote: > > 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
If we rerun _nohz_idle_balance before the balance interval, the 1st idle CPUs that has already been balanced will be skipped because there rq->next_balance will be after jiffies and we will start calling rebalance_domains for idle CPUs which have not yet been balance. So I'm not sure that this will help a lot because we have to go through all idle CPU to set the nohz.next_balance at the end
> as @Vincent put, it always failed in the same CPU.
In the case that i described above, the problem comes from the cpu that is selected to run the ilb but if we kick ilb again, it will not be selected again.
| |