Messages in this thread | | | Date | Thu, 4 Feb 2021 15:05:36 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH 1/8] sched/fair: Clean up active balance nr_balance_failed trickery |
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On 02/03/21 18:42, Valentin Schneider wrote: > >> @@ -9805,9 +9810,6 @@ static int load_balance(int this_cpu, struct rq *this_rq, > >> active_load_balance_cpu_stop, busiest, > >> &busiest->active_balance_work); > >> } > >> - > >> - /* We've kicked active balancing, force task migration. */ > >> - sd->nr_balance_failed = sd->cache_nice_tries+1; > > > > This has an impact on future calls to need_active_balance() too, no? We enter > > this path because need_active_balance() returned true; one of the conditions it > > checks for is > > > > return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2); > > > > So since we used to reset nr_balanced_failed to cache_nice_tries+1, the above > > condition would be false in the next call or two IIUC. But since we remove > > that, we could end up here again soon. > > > > Was this intentional? > > > > Partially, I'd say :-) > > If you look at active_load_balance_cpu_stop(), it does > > sd->nr_balance_failed = 0; > > when it successfully pulls a task. So we get a reset of the failed counter > on pull, which I've preserved. As for interactions with later > need_active_balance(), the commit that introduced the current counter write > (which is over 15 years old!): > > 3950745131e2 ("[PATCH] sched: fix SMT scheduling problems") > > only states the task_hot() issue; thus I'm doubtful whether said > interaction was intentional.
The '+1' was added in that comment. 'Original' code was just resetting the nr_balance_failed cache_nice_tries, so that we don't do another one too soon I think.
With this change, no active balance is allowed until later. Which makes sense. I can't see why we would have allowed another kick sooner tbh. But as you say, this is ancient piece of logic.
I agree I can't see a reason to worry about this (potential) change of behavior.
Thanks
-- Qais Yousef
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