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Date: Tue, 6 Sep 2016 15:01:27 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Mike Galbraith <umgwanakikbuti@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
LKML <linux-kernel@...r.kernel.org>,
Rik van Riel <riel@...hat.com>
Subject: Re: [v2 patch v3.18+ regression fix] sched: Further improve spurious
CPU_IDLE active migrations
Le Monday 05 Sep 2016 à 18:26:53 (+0200), Mike Galbraith a écrit :
> Coming back to this, how about this instead, only increase the group
> imbalance threshold when sd_llc_size == 2. Newer L3 equipped
> processors then aren't affected.
>
Not sure that all systems with sd_llc_size == 2 wants this behavior.
Why not adding a sched_feature for changing the 2nd half of the test for some systems ?
something like below
---
kernel/sched/fair.c | 11 ++++++++---
kernel/sched/features.h | 7 +++++++
2 files changed, 15 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4185e0a..65c9363 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7395,9 +7395,14 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
* significant if the diff is greater than 1 otherwise we
* might end up to just move the imbalance on another group
*/
- if ((busiest->group_type != group_overloaded) &&
- (local->idle_cpus <= (busiest->idle_cpus + 1)))
- goto out_balanced;
+ if (busiest->group_type != group_overloaded) {
+ int imbalance = 1;
+ if (!sched_feat(BALANCE_IDLE_CPUS))
+ imbalance = __this_cpu_read(sd_llc_size);
+
+ if (local->idle_cpus <= busiest->idle_cpus + imbalance)
+ goto out_balanced;
+ }
} else {
/*
* In the CPU_NEWLY_IDLE, CPU_NOT_IDLE cases, use
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 69631fa..16c34ec 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -69,3 +69,10 @@ SCHED_FEAT(RT_RUNTIME_SHARE, true)
SCHED_FEAT(LB_MIN, false)
SCHED_FEAT(ATTACH_AGE_LOAD, true)
+/*
+ * Try to balance the number of idle CPUs in each group to minimize contention
+ * on shared ressources. Nevertheless, some older systems without L3 seems to
+ * prefer to share resource for minimizing the migration between groups
+ */
+SCHED_FEAT(BALANCE_IDLE_CPUS, true)
+
--
>
>
> 43f4d666 partially cured uprious migrations, but when there are
> completely idle groups on a lightly loaded processor, and there is
> a buddy pair occupying the busiest group, we will not attempt to
> migrate due to select_idle_sibling() buddy placement, leaving the
> busiest queue with one task. We skip balancing, but increment
> nr_balance_failed until we kick active balancing, and bounce a
> buddy pair endlessly, demolishing throughput.
>
> Increase group imbalance threshold to two when sd_llc_size == 2 to
> allow buddies to share L2 without affecting larger L3 processors.
>
> Regression detected on X5472 box, which has 4 MC groups of 2 cores.
>
> netperf -l 60 -H 127.0.0.1 -t UDP_STREAM -i5,1 -I 95,5
> pre:
> !!! WARNING
> !!! Desired confidence was not achieved within the specified iterations.
> !!! This implies that there was variability in the test environment that
> !!! must be investigated before going further.
> !!! Confidence intervals: Throughput : 66.421%
> !!! Local CPU util : 0.000%
> !!! Remote CPU util : 0.000%
>
> Socket Message Elapsed Messages
> Size Size Time Okay Errors Throughput
> bytes bytes secs # # 10^6bits/sec
>
> 212992 65507 60.00 1779143 0 15539.49
> 212992 60.00 1773551 15490.65
>
> post:
> Socket Message Elapsed Messages
> Size Size Time Okay Errors Throughput
> bytes bytes secs # # 10^6bits/sec
>
> 212992 65507 60.00 3719377 0 32486.01
> 212992 60.00 3717492 32469.54
>
> Signed-off-by: Mike Galbraith <mgalbraith@...e.de>
> Fixes: caeb178c sched/fair: Make update_sd_pick_busiest() return 'true' on a busier sd
> Cc: <stable@...r.kernel.org> # v3.18+
> ---
> kernel/sched/fair.c | 17 ++++++++++++-----
> 1 file changed, 12 insertions(+), 5 deletions(-)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7249,12 +7249,19 @@ static struct sched_group *find_busiest_
> * This cpu is idle. If the busiest group is not overloaded
> * and there is no imbalance between this and busiest group
> * wrt idle cpus, it is balanced. The imbalance becomes
> - * significant if the diff is greater than 1 otherwise we
> - * might end up to just move the imbalance on another group
> + * significant if the diff is greater than 1 for most CPUs,
> + * or 2 for older CPUs having multiple groups of 2 cores
> + * sharing an L2, otherwise we may end up uselessly moving
> + * the imbalance to another group, or starting a tug of war
> + * with idle L2 groups constantly ripping communicating
> + * tasks apart, and no L3 to mitigate the cache miss pain.
> */
> - if ((busiest->group_type != group_overloaded) &&
> - (local->idle_cpus <= (busiest->idle_cpus + 1)))
> - goto out_balanced;
> + if (busiest->group_type != group_overloaded) {
> + int imbalance = __this_cpu_read(sd_llc_size) == 2 ? 2 : 1;
> +
> + if (local->idle_cpus <= busiest->idle_cpus + imbalance)
> + goto out_balanced;
> + }
> } else {
> /*
> * In the CPU_NEWLY_IDLE, CPU_NOT_IDLE cases, use
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