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Subject[PATCH] sched/topology: fix the issue groups don't span domain->span for NUMA diameter > 2
Date
As long as NUMA diameter > 2, building sched_domain by sibling's child
domain will definitely create a sched_domain with sched_group which will
span out of the sched_domain:

+------+ +------+ +-------+ +------+
| node | 12 |node | 20 | node | 12 |node |
| 0 +---------+1 +--------+ 2 +-------+3 |
+------+ +------+ +-------+ +------+

domain0 node0 node1 node2 node3

domain1 node0+1 node0+1 node2+3 node2+3
+
domain2 node0+1+2 |
group: node0+1 |
group:node2+3 <-------------------+

when node2 is added into the domain2 of node0, kernel is using the child
domain of node2's domain2, which is domain1(node2+3). Node 3 is outside
the span of the domain including node0+1+2.

This will make load_balance() run based on the avg_load in the sched_group
spanning out of the sched_domain, and it also makes select_task_rq_fair()
pick an idle CPU out of the sched_domain.

Real servers which suffer from this problem include Kunpeng920 and 8-node
Sun Fire X4600-M2, at least.

Here we move to use the *child* domain of the *child* domain of node2's
domain2 to build the sched_group.

+------+ +------+ +-------+ +------+
| node | 12 |node | 20 | node | 12 |node |
| 0 +---------+1 +--------+ 2 +-------+3 |
+------+ +------+ +-------+ +------+

domain0 node0 node1 +- node2 node3
|
domain1 node0+1 node0+1 | node2+3 node2+3
|
domain2 node0+1+2 |
group: node0+1 |
group:node2 <-------------------+

A tricky thing is that we shouldn't use the sgc of the 1st CPU of node2
for the sched_group generated by grandchild, otherwise, when this cpu
becomes the balance_cpu of another sched_group of cpus other than node0,
our sched_group generated by grandchild will access the same sgc with
the sched_group generated by child of another CPU.

So in init_overlap_sched_group(), sgc's capacity be overwritten:
build_balance_mask(sd, sg, mask);
cpu = cpumask_first_and(sched_group_span(sg), mask);

sg->sgc = *per_cpu_ptr(sdd->sgc, cpu);

And WARN_ON_ONCE(!cpumask_equal(group_balance_mask(sg), mask)) will
also be triggered:
static void init_overlap_sched_group(struct sched_domain *sd,
struct sched_group *sg)
{
if (atomic_inc_return(&sg->sgc->ref) == 1)
cpumask_copy(group_balance_mask(sg), mask);
else
WARN_ON_ONCE(!cpumask_equal(group_balance_mask(sg), mask));
}

So here move to use the sgc of the 2nd cpu. For the corner case, if NUMA
has only one CPU, we will still trigger this WARN_ON_ONCE. But It is
really unlikely to be a real case for one NUMA to have one CPU only.

Tested by the below topology:
qemu-system-aarch64 -M virt -nographic \
-smp cpus=8 \
-numa node,cpus=0-1,nodeid=0 \
-numa node,cpus=2-3,nodeid=1 \
-numa node,cpus=4-5,nodeid=2 \
-numa node,cpus=6-7,nodeid=3 \
-numa dist,src=0,dst=1,val=12 \
-numa dist,src=0,dst=2,val=20 \
-numa dist,src=0,dst=3,val=22 \
-numa dist,src=1,dst=2,val=22 \
-numa dist,src=2,dst=3,val=12 \
-numa dist,src=1,dst=3,val=24 \
-m 4G -cpu cortex-a57 -kernel arch/arm64/boot/Image

w/o patch, we get lots of "groups don't span domain->span":
[ 0.802139] CPU0 attaching sched-domain(s):
[ 0.802193] domain-0: span=0-1 level=MC
[ 0.802443] groups: 0:{ span=0 cap=1013 }, 1:{ span=1 cap=979 }
[ 0.802693] domain-1: span=0-3 level=NUMA
[ 0.802731] groups: 0:{ span=0-1 cap=1992 }, 2:{ span=2-3 cap=1943 }
[ 0.802811] domain-2: span=0-5 level=NUMA
[ 0.802829] groups: 0:{ span=0-3 cap=3935 }, 4:{ span=4-7 cap=3937 }
[ 0.802881] ERROR: groups don't span domain->span
[ 0.803058] domain-3: span=0-7 level=NUMA
[ 0.803080] groups: 0:{ span=0-5 mask=0-1 cap=5843 }, 6:{ span=4-7 mask=6-7 cap=4077 }
[ 0.804055] CPU1 attaching sched-domain(s):
[ 0.804072] domain-0: span=0-1 level=MC
[ 0.804096] groups: 1:{ span=1 cap=979 }, 0:{ span=0 cap=1013 }
[ 0.804152] domain-1: span=0-3 level=NUMA
[ 0.804170] groups: 0:{ span=0-1 cap=1992 }, 2:{ span=2-3 cap=1943 }
[ 0.804219] domain-2: span=0-5 level=NUMA
[ 0.804236] groups: 0:{ span=0-3 cap=3935 }, 4:{ span=4-7 cap=3937 }
[ 0.804302] ERROR: groups don't span domain->span
[ 0.804520] domain-3: span=0-7 level=NUMA
[ 0.804546] groups: 0:{ span=0-5 mask=0-1 cap=5843 }, 6:{ span=4-7 mask=6-7 cap=4077 }
[ 0.804677] CPU2 attaching sched-domain(s):
[ 0.804687] domain-0: span=2-3 level=MC
[ 0.804705] groups: 2:{ span=2 cap=934 }, 3:{ span=3 cap=1009 }
[ 0.804754] domain-1: span=0-3 level=NUMA
[ 0.804772] groups: 2:{ span=2-3 cap=1943 }, 0:{ span=0-1 cap=1992 }
[ 0.804820] domain-2: span=0-5 level=NUMA
[ 0.804836] groups: 2:{ span=0-3 mask=2-3 cap=3991 }, 4:{ span=0-1,4-7 mask=4-5 cap=5985 }
[ 0.804944] ERROR: groups don't span domain->span
[ 0.805108] domain-3: span=0-7 level=NUMA
[ 0.805134] groups: 2:{ span=0-5 mask=2-3 cap=5899 }, 6:{ span=0-1,4-7 mask=6-7 cap=6125 }
[ 0.805223] CPU3 attaching sched-domain(s):
[ 0.805232] domain-0: span=2-3 level=MC
[ 0.805249] groups: 3:{ span=3 cap=1009 }, 2:{ span=2 cap=934 }
[ 0.805319] domain-1: span=0-3 level=NUMA
[ 0.805336] groups: 2:{ span=2-3 cap=1943 }, 0:{ span=0-1 cap=1992 }
[ 0.805383] domain-2: span=0-5 level=NUMA
[ 0.805399] groups: 2:{ span=0-3 mask=2-3 cap=3991 }, 4:{ span=0-1,4-7 mask=4-5 cap=5985 }
[ 0.805458] ERROR: groups don't span domain->span
[ 0.805605] domain-3: span=0-7 level=NUMA
[ 0.805626] groups: 2:{ span=0-5 mask=2-3 cap=5899 }, 6:{ span=0-1,4-7 mask=6-7 cap=6125 }
[ 0.805712] CPU4 attaching sched-domain(s):
[ 0.805721] domain-0: span=4-5 level=MC
[ 0.805738] groups: 4:{ span=4 cap=984 }, 5:{ span=5 cap=924 }
[ 0.805787] domain-1: span=4-7 level=NUMA
[ 0.805803] groups: 4:{ span=4-5 cap=1908 }, 6:{ span=6-7 cap=2029 }
[ 0.805851] domain-2: span=0-1,4-7 level=NUMA
[ 0.805867] groups: 4:{ span=4-7 cap=3937 }, 0:{ span=0-3 cap=3935 }
[ 0.805915] ERROR: groups don't span domain->span
[ 0.806108] domain-3: span=0-7 level=NUMA
[ 0.806130] groups: 4:{ span=0-1,4-7 mask=4-5 cap=5985 }, 2:{ span=0-3 mask=2-3 cap=3991 }
[ 0.806214] CPU5 attaching sched-domain(s):
[ 0.806222] domain-0: span=4-5 level=MC
[ 0.806240] groups: 5:{ span=5 cap=924 }, 4:{ span=4 cap=984 }
[ 0.806841] domain-1: span=4-7 level=NUMA
[ 0.806866] groups: 4:{ span=4-5 cap=1908 }, 6:{ span=6-7 cap=2029 }
[ 0.806934] domain-2: span=0-1,4-7 level=NUMA
[ 0.806953] groups: 4:{ span=4-7 cap=3937 }, 0:{ span=0-3 cap=3935 }
[ 0.807004] ERROR: groups don't span domain->span
[ 0.807312] domain-3: span=0-7 level=NUMA
[ 0.807386] groups: 4:{ span=0-1,4-7 mask=4-5 cap=5985 }, 2:{ span=0-3 mask=2-3 cap=3991 }
[ 0.807686] CPU6 attaching sched-domain(s):
[ 0.807710] domain-0: span=6-7 level=MC
[ 0.807750] groups: 6:{ span=6 cap=1017 }, 7:{ span=7 cap=1012 }
[ 0.807840] domain-1: span=4-7 level=NUMA
[ 0.807870] groups: 6:{ span=6-7 cap=2029 }, 4:{ span=4-5 cap=1908 }
[ 0.807952] domain-2: span=0-1,4-7 level=NUMA
[ 0.807985] groups: 6:{ span=4-7 mask=6-7 cap=4077 }, 0:{ span=0-5 mask=0-1 cap=5843 }
[ 0.808045] ERROR: groups don't span domain->span
[ 0.808257] domain-3: span=0-7 level=NUMA
[ 0.808571] groups: 6:{ span=0-1,4-7 mask=6-7 cap=6125 }, 2:{ span=0-5 mask=2-3 cap=5899 }
[ 0.808848] CPU7 attaching sched-domain(s):
[ 0.808860] domain-0: span=6-7 level=MC
[ 0.808880] groups: 7:{ span=7 cap=1012 }, 6:{ span=6 cap=1017 }
[ 0.808953] domain-1: span=4-7 level=NUMA
[ 0.808974] groups: 6:{ span=6-7 cap=2029 }, 4:{ span=4-5 cap=1908 }
[ 0.809034] domain-2: span=0-1,4-7 level=NUMA
[ 0.809055] groups: 6:{ span=4-7 mask=6-7 cap=4077 }, 0:{ span=0-5 mask=0-1 cap=5843 }
[ 0.809128] ERROR: groups don't span domain->span
[ 0.810361] domain-3: span=0-7 level=NUMA
[ 0.810400] groups: 6:{ span=0-1,4-7 mask=6-7 cap=5961 }, 2:{ span=0-5 mask=2-3 cap=5903 }

w/ patch, we don't get "groups don't span domain->span" any more:
[ 0.868907] CPU0 attaching sched-domain(s):
[ 0.868962] domain-0: span=0-1 level=MC
[ 0.869179] groups: 0:{ span=0 cap=1013 }, 1:{ span=1 cap=983 }
[ 0.869405] domain-1: span=0-3 level=NUMA
[ 0.869438] groups: 0:{ span=0-1 cap=1996 }, 2:{ span=2-3 cap=2006 }
[ 0.869542] domain-2: span=0-5 level=NUMA
[ 0.869559] groups: 0:{ span=0-3 cap=4002 }, 5:{ span=4-5 cap=2048 }
[ 0.869603] domain-3: span=0-7 level=NUMA
[ 0.869618] groups: 0:{ span=0-5 mask=0-1 cap=5980 }, 6:{ span=4-7 mask=6-7 cap=4016 }
[ 0.870303] CPU1 attaching sched-domain(s):
[ 0.870314] domain-0: span=0-1 level=MC
[ 0.870334] groups: 1:{ span=1 cap=983 }, 0:{ span=0 cap=1013 }
[ 0.870381] domain-1: span=0-3 level=NUMA
[ 0.870396] groups: 0:{ span=0-1 cap=1996 }, 2:{ span=2-3 cap=2006 }
[ 0.870440] domain-2: span=0-5 level=NUMA
[ 0.870454] groups: 0:{ span=0-3 cap=4002 }, 5:{ span=4-5 cap=2048 }
[ 0.870507] domain-3: span=0-7 level=NUMA
[ 0.870530] groups: 0:{ span=0-5 mask=0-1 cap=5980 }, 6:{ span=4-7 mask=6-7 cap=4016 }
[ 0.870611] CPU2 attaching sched-domain(s):
[ 0.870619] domain-0: span=2-3 level=MC
[ 0.870634] groups: 2:{ span=2 cap=1007 }, 3:{ span=3 cap=999 }
[ 0.870677] domain-1: span=0-3 level=NUMA
[ 0.870691] groups: 2:{ span=2-3 cap=2006 }, 0:{ span=0-1 cap=1996 }
[ 0.870734] domain-2: span=0-5 level=NUMA
[ 0.870748] groups: 2:{ span=0-3 mask=2-3 cap=4054 }, 5:{ span=4-5 cap=2048 }
[ 0.870795] domain-3: span=0-7 level=NUMA
[ 0.870809] groups: 2:{ span=0-5 mask=2-3 cap=6032 }, 6:{ span=0-1,4-7 mask=6-7 cap=6064 }
[ 0.870913] CPU3 attaching sched-domain(s):
[ 0.870921] domain-0: span=2-3 level=MC
[ 0.870936] groups: 3:{ span=3 cap=999 }, 2:{ span=2 cap=1007 }
[ 0.870979] domain-1: span=0-3 level=NUMA
[ 0.870993] groups: 2:{ span=2-3 cap=2006 }, 0:{ span=0-1 cap=1996 }
[ 0.871035] domain-2: span=0-5 level=NUMA
[ 0.871049] groups: 2:{ span=0-3 mask=2-3 cap=4054 }, 5:{ span=4-5 cap=2048 }
[ 0.871096] domain-3: span=0-7 level=NUMA
[ 0.871110] groups: 2:{ span=0-5 mask=2-3 cap=6032 }, 6:{ span=0-1,4-7 mask=6-7 cap=6064 }
[ 0.871177] CPU4 attaching sched-domain(s):
[ 0.871185] domain-0: span=4-5 level=MC
[ 0.871200] groups: 4:{ span=4 cap=977 }, 5:{ span=5 cap=1001 }
[ 0.871243] domain-1: span=4-7 level=NUMA
[ 0.871257] groups: 4:{ span=4-5 cap=1978 }, 6:{ span=6-7 cap=1968 }
[ 0.871300] domain-2: span=0-1,4-7 level=NUMA
[ 0.871314] groups: 4:{ span=4-7 cap=3946 }, 1:{ span=0-1 cap=2048 }
[ 0.871356] domain-3: span=0-7 level=NUMA
[ 0.871370] groups: 4:{ span=0-1,4-7 mask=4-5 cap=5994 }, 2:{ span=0-3 mask=2-3 cap=4054 }
[ 0.871436] CPU5 attaching sched-domain(s):
[ 0.871443] domain-0: span=4-5 level=MC
[ 0.871457] groups: 5:{ span=5 cap=1001 }, 4:{ span=4 cap=977 }
[ 0.871512] domain-1: span=4-7 level=NUMA
[ 0.871893] groups: 4:{ span=4-5 cap=1978 }, 6:{ span=6-7 cap=1968 }
[ 0.871949] domain-2: span=0-1,4-7 level=NUMA
[ 0.871966] groups: 4:{ span=4-7 cap=3946 }, 1:{ span=0-1 cap=2048 }
[ 0.872010] domain-3: span=0-7 level=NUMA
[ 0.872025] groups: 4:{ span=0-1,4-7 mask=4-5 cap=5994 }, 2:{ span=0-3 mask=2-3 cap=4054 }
[ 0.872115] CPU6 attaching sched-domain(s):
[ 0.872123] domain-0: span=6-7 level=MC
[ 0.872139] groups: 6:{ span=6 cap=993 }, 7:{ span=7 cap=975 }
[ 0.872186] domain-1: span=4-7 level=NUMA
[ 0.872202] groups: 6:{ span=6-7 cap=1968 }, 4:{ span=4-5 cap=1978 }
[ 0.872246] domain-2: span=0-1,4-7 level=NUMA
[ 0.872260] groups: 6:{ span=4-7 mask=6-7 cap=4016 }, 1:{ span=0-1 cap=2048 }
[ 0.872309] domain-3: span=0-7 level=NUMA
[ 0.872323] groups: 6:{ span=0-1,4-7 mask=6-7 cap=6064 }, 2:{ span=0-5 mask=2-3 cap=6032 }
[ 0.872392] CPU7 attaching sched-domain(s):
[ 0.872399] domain-0: span=6-7 level=MC
[ 0.872414] groups: 7:{ span=7 cap=975 }, 6:{ span=6 cap=993 }
[ 0.872458] domain-1: span=4-7 level=NUMA
[ 0.872472] groups: 6:{ span=6-7 cap=1968 }, 4:{ span=4-5 cap=1978 }
[ 0.872662] domain-2: span=0-1,4-7 level=NUMA
[ 0.872685] groups: 6:{ span=4-7 mask=6-7 cap=4016 }, 1:{ span=0-1 cap=2048 }
[ 0.872737] domain-3: span=0-7 level=NUMA
[ 0.872752] groups: 6:{ span=0-1,4-7 mask=6-7 cap=6064 }, 2:{ span=0-5 mask=2-3 cap=6032 }

Reported-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
---
Differences with RFC v2
* added tested-by Meelis Roos for the fixed "8-node Sun Fire X4600-M2"
* removed the hacking code in balance_mask and should_we_balance()
* removed the redundant "from_grandchild" field from sched_group

The patch is based on 5.11-rc6;

kernel/sched/topology.c | 83 ++++++++++++++++++++++++++---------------
1 file changed, 52 insertions(+), 31 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 5d3675c7a76b..100feb2fd8a0 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -723,35 +723,6 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
for (tmp = sd; tmp; tmp = tmp->parent)
numa_distance += !!(tmp->flags & SD_NUMA);

- /*
- * FIXME: Diameter >=3 is misrepresented.
- *
- * Smallest diameter=3 topology is:
- *
- * node 0 1 2 3
- * 0: 10 20 30 40
- * 1: 20 10 20 30
- * 2: 30 20 10 20
- * 3: 40 30 20 10
- *
- * 0 --- 1 --- 2 --- 3
- *
- * NUMA-3 0-3 N/A N/A 0-3
- * groups: {0-2},{1-3} {1-3},{0-2}
- *
- * NUMA-2 0-2 0-3 0-3 1-3
- * groups: {0-1},{1-3} {0-2},{2-3} {1-3},{0-1} {2-3},{0-2}
- *
- * NUMA-1 0-1 0-2 1-3 2-3
- * groups: {0},{1} {1},{2},{0} {2},{3},{1} {3},{2}
- *
- * NUMA-0 0 1 2 3
- *
- * The NUMA-2 groups for nodes 0 and 3 are obviously buggered, as the
- * group span isn't a subset of the domain span.
- */
- WARN_ONCE(numa_distance > 2, "Shortest NUMA path spans too many nodes\n");
-
sched_domain_debug(sd, cpu);

rq_attach_root(rq, rd);
@@ -916,6 +887,11 @@ build_balance_mask(struct sched_domain *sd, struct sched_group *sg, struct cpuma
if (!sibling->child)
continue;

+ while (sibling->child &&
+ !cpumask_subset(sched_domain_span(sibling->child),
+ sched_domain_span(sd)))
+ sibling = sibling->child;
+
/* If we would not end up here, we can't continue from here */
if (!cpumask_equal(sg_span, sched_domain_span(sibling->child)))
continue;
@@ -955,7 +931,8 @@ build_group_from_child_sched_domain(struct sched_domain *sd, int cpu)
}

static void init_overlap_sched_group(struct sched_domain *sd,
- struct sched_group *sg)
+ struct sched_group *sg,
+ int from_grandchild)
{
struct cpumask *mask = sched_domains_tmpmask2;
struct sd_data *sdd = sd->private;
@@ -964,6 +941,12 @@ static void init_overlap_sched_group(struct sched_domain *sd,

build_balance_mask(sd, sg, mask);
cpu = cpumask_first_and(sched_group_span(sg), mask);
+ /*
+ * for the group generated by grandchild, use the sgc of 2nd cpu
+ * because the 1st cpu might be used by another sched_group
+ */
+ if (from_grandchild && cpumask_weight(mask) > 1)
+ cpu = cpumask_next_and(cpu, sched_group_span(sg), mask);

sg->sgc = *per_cpu_ptr(sdd->sgc, cpu);
if (atomic_inc_return(&sg->sgc->ref) == 1)
@@ -996,6 +979,7 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu)

for_each_cpu_wrap(i, span, cpu) {
struct cpumask *sg_span;
+ int from_grandchild = 0;

if (cpumask_test_cpu(i, covered))
continue;
@@ -1015,6 +999,43 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu)
if (!cpumask_test_cpu(i, sched_domain_span(sibling)))
continue;

+ /*
+ * for NUMA diameter >= 3, building sched_domain by sibling's
+ * child's child domain to prevent sched_group from spanning
+ * out of sched_domain
+ * if we don't do this, Diameter >=3 is misrepresented:
+ *
+ * Smallest diameter=3 topology is:
+ *
+ * node 0 1 2 3
+ * 0: 10 20 30 40
+ * 1: 20 10 20 30
+ * 2: 30 20 10 20
+ * 3: 40 30 20 10
+ *
+ * 0 --- 1 --- 2 --- 3
+ *
+ * NUMA-3 0-3 N/A N/A 0-3
+ * groups: {0-2},{1-3} {1-3},{0-2}
+ *
+ * NUMA-2 0-2 0-3 0-3 1-3
+ * groups: {0-1},{1-3} {0-2},{2-3} {1-3},{0-1} {2-3},{0-2}
+ *
+ * NUMA-1 0-1 0-2 1-3 2-3
+ * groups: {0},{1} {1},{2},{0} {2},{3},{1} {3},{2}
+ *
+ * NUMA-0 0 1 2 3
+ *
+ * The NUMA-2 groups for nodes 0 and 3 are obviously buggered, as the
+ * group span isn't a subset of the domain span.
+ */
+ while (sibling->child &&
+ !cpumask_subset(sched_domain_span(sibling->child),
+ span)) {
+ sibling = sibling->child;
+ from_grandchild = 1;
+ }
+
sg = build_group_from_child_sched_domain(sibling, cpu);
if (!sg)
goto fail;
@@ -1022,7 +1043,7 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu)
sg_span = sched_group_span(sg);
cpumask_or(covered, covered, sg_span);

- init_overlap_sched_group(sd, sg);
+ init_overlap_sched_group(sd, sg, from_grandchild);

if (!first)
first = sg;
--
2.25.1
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