Messages in this thread Patch in this message | | | Date | Tue, 14 Nov 2023 16:43:12 +0100 | From | Vincent Guittot <> | Subject | Re: [PATCH 3/3] sched: Update ->next_balance correctly during newidle balance |
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Le jeudi 09 nov. 2023 à 10:02:54 (+0000), Joel Fernandes a écrit : > Hi Vincent, > > Sorry for late reply, I was in Tokyo all these days and was waiting to get to > writing a proper reply. See my replies below: > > On Thu, Oct 26, 2023 at 04:23:35PM +0200, Vincent Guittot wrote: > > On Sun, 22 Oct 2023 at 02:28, Joel Fernandes <joel@joelfernandes.org> wrote: > > > > > > On Fri, Oct 20, 2023 at 03:40:14PM +0200, Vincent Guittot wrote: > > > > On Fri, 20 Oct 2023 at 03:40, Joel Fernandes (Google) > > > > <joel@joelfernandes.org> wrote: > > > > > > > > > > From: "Vineeth Pillai (Google)" <vineeth@bitbyteword.org> > > > > > > > > > > When newidle balancing triggers, we see that it constantly clobbers > > > > > rq->next_balance even when there is no newidle balance happening due to > > > > > the cost estimates. Due to this, we see that periodic load balance > > > > > (rebalance_domains) may trigger way more often when the CPU is going in > > > > > and out of idle at a high rate but is no really idle. Repeatedly > > > > > triggering load balance there is a bad idea as it is a heavy operation. > > > > > It also causes increases in softirq. > > > > > > > > we have 2 balance intervals: > > > > - one when idle based on the sd->balance_interval = sd_weight > > > > - one when busy which increases the period by multiplying it with > > > > busy_factor = 16 > > > > > > On my production system I see load balance triggering every 4 jiffies! In a > > > > Which kind of system do you have? sd->balance_interval is in ms > > Yes, sorry I meant it triggers every jiffies which is extreme sometimes. It > is an ADL SoC (12th gen Intel, 4 P cores 8 E cores) get_sd_balance_interval() > returns 4 jiffies there. On my Qemu system, I see 8 jiffies.
Do you have details about the sched_domain hierarchy ? That could be part of your problem (see below)
> > [...] > > > > > Another issue is ->last_balance is not updated after newidle balance > > > > > causing mistakes in the ->next_balance calculations. > > > > > > > > newly idle load balance is not equal to idle load balance. It's a > > > > light load balance trying to pull one task and you can't really > > > > consider it to the normal load balance > > > > > > True. However the point is that it is coupled with the other load balance > > > mechanism and the two are not independent. As you can see below, modifying > > > rq->next_balance in newidle also causes the periodic balance to happen more > > > aggressively as well if there is a high transition from busy to idle and > > > viceversa. > > > > As mentioned, rq->next_balance is updated whenever cpu enters idle > > (i.e. in newidle_balance() but it's not related with doing a newly > > idle load balance. > > Yes, I understand that. But my point was that the update of rq->next_balance > from the newidle path is itself buggy and interferes with the load balance > happening from the tick (trigger_load_balance -> run_rebalance_domains).
Newidle path is not buggy. It only uses sd->last_balance + interval to estimate the next balance which is the correct thing to do. Your problem comes from the update of sd->last_balance which never happens and remains in the past whereas you call run_rebalance_domains() which should run load_balance for all domains with a sd->last_balance + interval in the past. Your problem most probably comes from the should_we_balance which always or "almost always" returns false in your use case for some sched_domain and prevents to updat sd->last_balance. Could you try the patch below ? It should fix your problem of trying to rebalance every tick whereas rebalance_domain is called. At least this should show if it's your problem but I'm not sure it's the right things to do all the time ...
--- kernel/sched/fair.c | 18 ++++++------------ 1 file changed, 6 insertions(+), 12 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 3745ca289240..9ea1f42e5362 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -11568,17 +11568,6 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle) need_decay = update_newidle_cost(sd, 0); max_cost += sd->max_newidle_lb_cost;
- /* - * Stop the load balance at this level. There is another - * CPU in our sched group which is doing load balancing more - * actively. - */ - if (!continue_balancing) { - if (need_decay) - continue; - break; - } - interval = get_sd_balance_interval(sd, busy);
need_serialize = sd->flags & SD_SERIALIZE; @@ -11588,7 +11577,12 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle) }
if (time_after_eq(jiffies, sd->last_balance + interval)) { - if (load_balance(cpu, rq, sd, idle, &continue_balancing)) { + /* + * Stop the load balance at this level. There is another + * CPU in our sched group which is doing load balancing more + * actively. + */ + if (continue_balancing && load_balance(cpu, rq, sd, idle, &continue_balancing)) { /* * The LBF_DST_PINNED logic could have changed * env->dst_cpu, so we can't know our idle -- 2.34.1 > > > But your problem is more related with the fact that > > nohz.needs_update is set when stopping cpu timer in order to update > > nohz.next_balance which is then used to kick a "real" idle load > > balance > > Well, independent of nohz idle balance, I think we need to fix this issue as > mentioned above. This effect the periodic one as mentioned in the commit log. > > See here another trace I collected this time dumping the 'interval'. There is > a tug of war happening between the newidle balance and the periodic balance. > > The periodic one sets rq->next_balance for cpu 0 to 760,143 and then the > newidle comes in pulls it back a 118 jiffies to 760,024. This is actually in > the past because jiffies is currently 760,045 !! > > This triggers the periodic balance againwhich sets rq->next_balance back to > 760,143. > > Rinse and repeat. End result is you have periodic balance every jiffies. With > this patch the issue goes away but we could fix it differently as you > mentioned, we need to pull newidle balance back but perhaps not so > aggressively. How about something like the untested diff I enclosed at the > end of this email? > > <idle>-0 [000] 13.081781: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,045) > cyclictest-120 [000] 13.081806: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8) > cyclictest-120 [000] 13.081807: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,045) > cyclictest-120 [000] 13.082130: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8) > cyclictest-120 [000] 13.082338: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8) > cyclictest-120 [000] 13.082636: update_next_balance: sd->next_balance: 761,045 -> 760,024 (jiffies=760,045, interval=8) > <idle>-0 [000] 13.082823: trigger_load_balance: time_after_eq(jiffies=760,046, rq->next_balance=760,024) = 1 > <idle>-0 [000] 13.082823: softirq_raise: vec=7 [action=SCHED] > <idle>-0 [000] 13.082871: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,046) > trace-cmd-114 [000] 13.082876: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8) > trace-cmd-114 [000] 13.082876: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,046) > cyclictest-120 [000] 13.083333: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8) > cyclictest-120 [000] 13.083633: update_next_balance: sd->next_balance: 761,046 -> 760,024 (jiffies=760,046, interval=8) > <idle>-0 [000] 13.083656: trigger_load_balance: time_after_eq(jiffies=760,047, rq->next_balance=760,024) = 1 > <idle>-0 [000] 13.083656: softirq_raise: vec=7 [action=SCHED] > <idle>-0 [000] 13.083702: rebalance_domains: rq[cpu=0]->next_balance: 760,024 -> 760,143 (jiffies=760,047) > cyclictest-120 [000] 13.083729: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8) > cyclictest-120 [000] 13.083730: newidle_balance: this_rq[cpu=0]->next_balance: 760,143 -> 760,024 (jiffies=760,047) > cyclictest-120 [000] 13.083960: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8) > cyclictest-120 [000] 13.084069: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8) > cyclictest-120 [000] 13.084423: update_next_balance: sd->next_balance: 761,047 -> 760,024 (jiffies=760,047, interval=8) > cyclictest-120 [000] 13.084633: trigger_load_balance: time_after_eq(jiffies=760,048, rq->next_balance=760,024) = 1 > cyclictest-120 [000] 13.084633: softirq_raise: vec=7 [action=SCHED] > cyclictest-120 [000] 13.084678: rebalance_domains: rq[cpu=0]->next_balance: > > > > > > Fix by updating last_balance when a newidle load balance actually > > > > > happens and then updating next_balance. This is also how it is done in > > > > > other load balance paths. > > > > > > > > > > Testing shows a significant drop in softirqs when running: > > > > > cyclictest -i 100 -d 100 --latency=1000 -D 5 -t -m -q > > > > > > > > > > Goes from ~6k to ~800. > > > > > > > > Even if your figures look interesting, your patch adds regression in > > > > the load balance and the fairness. > > > > > > Yes I see that now. However it does illustrate the problem IMO. > > > > > > > We can probably do improve the current behavior for decreasing number > > > > of ILB but your proposal is not the right solution IMO > > > > > > One of the problems is if you have task goes idle a lot, then the > > > newidle_balance mechanism triggers the periodic balance every jiffie (once > > > per millisecond on HZ=1000). > > > > every msec seems quite a lot. > > Yeah! > > > > > > > Following are some traces I collected. > > > > > > cyclictest-123 [003] 522.650574 newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,157) > > > <idle>-0 [003] 522.651443 trigger_load_balance: time_after_eq(jiffies=221,158, rq->next_balance=221,145) = 1 > > > <idle>-0 [003] 522.651461 rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,158) > > > cyclictest-123 [003] 522.651494 newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,158) > > > <idle>-0 [003] 522.652522 trigger_load_balance: time_after_eq(jiffies=221,159, rq->next_balance=221,145) = 1 > > > <idle>-0 [003] 522.652560 rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,159) > > > cyclictest-124 [003] 522.652586 newidle_balance: this_rq[cpu=3]->next_balance: 221,264 -> 221,145 (jiffies=221,159) > > > <idle>-0 [003] 522.654492 trigger_load_balance: time_after_eq(jiffies=221,161, rq->next_balance=221,145) = 1 > > > <idle>-0 [003] 522.654534 rebalance_domains: rq[cpu=3]->next_balance: 221,145 -> 221,264 (jiffies=221,161) > > > > Ok, so IIUC your trace above, this happens because the tick is not > > stop after entering idle so it continues to fire and triggers a load > > balance without checking if there is a need like what is done for nohz > > mode > > The tick is normally not stopped if the CPU is awakened too soon by a timer. > That's pretty normal AFAIK. As you can see in the traces above, cyclictest > keeps waking up. > > > > 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! > > ---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;
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