Messages in this thread | | | From | Joel Fernandes <> | Date | Thu, 9 Nov 2023 07:31:36 -0500 | Subject | Re: [PATCH 3/3] sched: Update ->next_balance correctly during newidle balance |
| |
On Thu, Nov 9, 2023 at 5:02 AM Joel Fernandes <joel@joelfernandes.org> wrote: [...] > > > things worse for power on ARM where you have uclamp stuff happening in the > > > load balance paths which is quite heavy when I last traced that.. > > > > > > Further, we have observed in our tracing on real device that the update of > > > rq->next_balance from the newidle path is itself buggy... we observed that > > > because newidle balance may not update rq->last_balance, it is possible that > > > rq->next_balance when updated by update_next_balance() will be updated to a > > > value that is in the past and it will be stuck there for a long time! Perhaps > > > we should investigate more and fix that bug separately. Vineeth could provide > > > more details on the "getting stuck in the past" behavior as well. > > > > sd->last_balance reflects last time an idle/busy load_balance happened > > (newly idle is out of the scope for the points that I mentioned > > previously). So if no load balance happens for a while, the > > rq->next_balance can be in the past but I don't see a problem here. It > > just means that a load balance hasn't happened for a while. It can > > even move backward if it has been set when busy but the cpu is now > > idle > > Sure, but I think it should at least set it by get_sd_balance_interval() into > the future. Like so (untested)? Let me know what you think and thanks!
Btw, I also drew a graph showing the issue without patch: https://i.imgur.com/RgTr45l.png
Each "x" mark is run_rebalance_domains() running on a CPU. As can be seen, there were some 10 occurrences in a span of 15ms in one instance.
Thanks,
- Joel
> ---8<----------------------- > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index a3318aeff9e8..0d6667d31c51 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -11314,6 +11314,30 @@ get_sd_balance_interval(struct sched_domain *sd, int cpu_busy) > return interval; > } > > +/* > + * Update the next balance from newidle balance. > + * The update of next_balance from newidle balance tries to make sure that > + * we don't trigger periodic balance too far in the future on a now-idle > + * system. This is just like update_next_balance except that since > + * sd->last_balance may not have been updated for a while, we're careful to > + * not set next_balance in the past. > + */ > +static inline void > +update_next_balance_newidle(struct sched_domain *sd, unsigned long *next_balance) > +{ > + unsigned long interval, next; > + > + /* used by new idle balance, so cpu_busy = 0 */ > + interval = get_sd_balance_interval(sd, 0); > + next = sd->last_balance + interval; > + > + next = max(next, jiffies + interval); > + > + if (time_after(*next_balance, next)) { > + *next_balance = next; > + } > +} > + > static inline void > update_next_balance(struct sched_domain *sd, unsigned long *next_balance) > { > @@ -12107,7 +12131,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf) > (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) { > > if (sd) > - update_next_balance(sd, &next_balance); > + update_next_balance_newidle(sd, &next_balance); > rcu_read_unlock(); > > goto out; > @@ -12124,7 +12148,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf) > int continue_balancing = 1; > u64 domain_cost; > > - update_next_balance(sd, &next_balance); > + update_next_balance_newidle(sd, &next_balance); > > if (this_rq->avg_idle < curr_cost + sd->max_newidle_lb_cost) > break;
| |