Re: [PATCH 3/3] sched: Update ->next_balance correctly during newidle balance

[Vincent Guittot <vincent.guittot@linaro.org>]

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;