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 screwed avg_load and group_type in the sched_group spanning out of the sched_domain, and it also makes select_task_rq_fair() pick an idle CPU outside 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 as the new added sched_group. At the same, we re-use the lower level sgc directly. +------+ +------+ +-------+ +------+ | 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 <-------------------+

While the lower level sgc is re-used, this patch only changes the remote sched_groups for those sched_domains playing grandchild trick, therefore, sgc->next_update is still safe since it's only touched by CPUs that have the group span as local group. And sgc->imbalance is also safe because sd_parent remains the same in load_balance and LB only tries other CPUs from the local group. Moreover, since local groups are not touched, they are still getting roughly equal size in a TL. And should_we_balance() only matters with local groups, so the pull probability of those groups are still roughly equal.

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: [ 1.486271] CPU0 attaching sched-domain(s): [ 1.486820] domain-0: span=0-1 level=MC [ 1.500924] groups: 0:{ span=0 cap=980 }, 1:{ span=1 cap=994 } [ 1.515717] domain-1: span=0-3 level=NUMA [ 1.515903] groups: 0:{ span=0-1 cap=1974 }, 2:{ span=2-3 cap=1989 } [ 1.516989] domain-2: span=0-5 level=NUMA [ 1.517124] groups: 0:{ span=0-3 cap=3963 }, 4:{ span=4-5 cap=1949 } [ 1.517369] domain-3: span=0-7 level=NUMA [ 1.517423] groups: 0:{ span=0-5 mask=0-1 cap=5912 }, 6:{ span=4-7 mask=6-7 cap=4054 } [ 1.520027] CPU1 attaching sched-domain(s): [ 1.520097] domain-0: span=0-1 level=MC [ 1.520184] groups: 1:{ span=1 cap=994 }, 0:{ span=0 cap=980 } [ 1.520429] domain-1: span=0-3 level=NUMA [ 1.520487] groups: 0:{ span=0-1 cap=1974 }, 2:{ span=2-3 cap=1989 } [ 1.520687] domain-2: span=0-5 level=NUMA [ 1.520744] groups: 0:{ span=0-3 cap=3963 }, 4:{ span=4-5 cap=1949 } [ 1.520948] domain-3: span=0-7 level=NUMA [ 1.521038] groups: 0:{ span=0-5 mask=0-1 cap=5912 }, 6:{ span=4-7 mask=6-7 cap=4054 } [ 1.522068] CPU2 attaching sched-domain(s): [ 1.522348] domain-0: span=2-3 level=MC [ 1.522606] groups: 2:{ span=2 cap=1003 }, 3:{ span=3 cap=986 } [ 1.522832] domain-1: span=0-3 level=NUMA [ 1.522885] groups: 2:{ span=2-3 cap=1989 }, 0:{ span=0-1 cap=1974 } [ 1.523043] domain-2: span=0-5 level=NUMA [ 1.523092] groups: 2:{ span=0-3 mask=2-3 cap=4037 }, 4:{ span=4-5 cap=1949 } [ 1.523302] domain-3: span=0-7 level=NUMA [ 1.523352] groups: 2:{ span=0-5 mask=2-3 cap=5986 }, 6:{ span=0-1,4-7 mask=6-7 cap=6102 } [ 1.523748] CPU3 attaching sched-domain(s): [ 1.523774] domain-0: span=2-3 level=MC [ 1.523825] groups: 3:{ span=3 cap=986 }, 2:{ span=2 cap=1003 } [ 1.524009] domain-1: span=0-3 level=NUMA [ 1.524086] groups: 2:{ span=2-3 cap=1989 }, 0:{ span=0-1 cap=1974 } [ 1.524281] domain-2: span=0-5 level=NUMA [ 1.524331] groups: 2:{ span=0-3 mask=2-3 cap=4037 }, 4:{ span=4-5 cap=1949 } [ 1.524534] domain-3: span=0-7 level=NUMA [ 1.524586] groups: 2:{ span=0-5 mask=2-3 cap=5986 }, 6:{ span=0-1,4-7 mask=6-7 cap=6102 } [ 1.524847] CPU4 attaching sched-domain(s): [ 1.524873] domain-0: span=4-5 level=MC [ 1.524954] groups: 4:{ span=4 cap=958 }, 5:{ span=5 cap=991 } [ 1.525105] domain-1: span=4-7 level=NUMA [ 1.525153] groups: 4:{ span=4-5 cap=1949 }, 6:{ span=6-7 cap=2006 } [ 1.525368] domain-2: span=0-1,4-7 level=NUMA [ 1.525428] groups: 4:{ span=4-7 cap=3955 }, 0:{ span=0-1 cap=1974 } [ 1.532726] domain-3: span=0-7 level=NUMA [ 1.532811] groups: 4:{ span=0-1,4-7 mask=4-5 cap=6003 }, 2:{ span=0-3 mask=2-3 cap=4037 } [ 1.534125] CPU5 attaching sched-domain(s): [ 1.534159] domain-0: span=4-5 level=MC [ 1.534303] groups: 5:{ span=5 cap=991 }, 4:{ span=4 cap=958 } [ 1.534490] domain-1: span=4-7 level=NUMA [ 1.534572] groups: 4:{ span=4-5 cap=1949 }, 6:{ span=6-7 cap=2006 } [ 1.534734] domain-2: span=0-1,4-7 level=NUMA [ 1.534783] groups: 4:{ span=4-7 cap=3955 }, 0:{ span=0-1 cap=1974 } [ 1.536057] domain-3: span=0-7 level=NUMA [ 1.536430] groups: 4:{ span=0-1,4-7 mask=4-5 cap=6003 }, 2:{ span=0-3 mask=2-3 cap=3896 } [ 1.536815] CPU6 attaching sched-domain(s): [ 1.536846] domain-0: span=6-7 level=MC [ 1.536934] groups: 6:{ span=6 cap=1005 }, 7:{ span=7 cap=1001 } [ 1.537144] domain-1: span=4-7 level=NUMA [ 1.537262] groups: 6:{ span=6-7 cap=2006 }, 4:{ span=4-5 cap=1949 } [ 1.537553] domain-2: span=0-1,4-7 level=NUMA [ 1.537613] groups: 6:{ span=4-7 mask=6-7 cap=4054 }, 0:{ span=0-1 cap=1805 } [ 1.537872] domain-3: span=0-7 level=NUMA [ 1.537998] groups: 6:{ span=0-1,4-7 mask=6-7 cap=6102 }, 2:{ span=0-5 mask=2-3 cap=5845 } [ 1.538448] CPU7 attaching sched-domain(s): [ 1.538505] domain-0: span=6-7 level=MC [ 1.538586] groups: 7:{ span=7 cap=1001 }, 6:{ span=6 cap=1005 } [ 1.538746] domain-1: span=4-7 level=NUMA [ 1.538798] groups: 6:{ span=6-7 cap=2006 }, 4:{ span=4-5 cap=1949 } [ 1.539048] domain-2: span=0-1,4-7 level=NUMA [ 1.539111] groups: 6:{ span=4-7 mask=6-7 cap=4054 }, 0:{ span=0-1 cap=1805 } [ 1.539571] domain-3: span=0-7 level=NUMA [ 1.539610] groups: 6:{ span=0-1,4-7 mask=6-7 cap=6102 }, 2:{ span=0-5 mask=2-3 cap=5845 }

Reported-by: Valentin Schneider valentin.schneider@arm.com Tested-by: Meelis Roos mroos@linux.ee Reviewed-by: Valentin Schneider valentin.schneider@arm.com Signed-off-by: Barry Song song.bao.hua@hisilicon.com --- -v4: no code changed; mainly rewrote changelog: * add Reviewed-by of Valentin; While the grandchild approach was started by me, Valentin contributed the most useful edit; * add description about sgc->imbalance and next_update according to the comment of Vincent; * add description about the equal size of local group to address Peter's comment

kernel/sched/topology.c | 91 +++++++++++++++++++++++++++-------------- 1 file changed, 61 insertions(+), 30 deletions(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c index 09d35044bd88..12f80587e127 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); @@ -982,6 +953,31 @@ static void init_overlap_sched_group(struct sched_domain *sd, sg->sgc->max_capacity = SCHED_CAPACITY_SCALE; }

+static struct sched_domain * +find_descended_sibling(struct sched_domain *sd, struct sched_domain *sibling) +{ + /* + * The proper descendant would be the one whose child won't span out + * of sd + */ + while (sibling->child && + !cpumask_subset(sched_domain_span(sibling->child), + sched_domain_span(sd))) + sibling = sibling->child; + + /* + * As we are referencing sgc across different topology level, we need + * to go down to skip those sched_domains which don't contribute to + * scheduling because they will be degenerated in cpu_attach_domain + */ + while (sibling->child && + cpumask_equal(sched_domain_span(sibling->child), + sched_domain_span(sibling))) + sibling = sibling->child; + + return sibling; +} + static int build_overlap_sched_groups(struct sched_domain *sd, int cpu) { @@ -1015,6 +1011,41 @@ build_overlap_sched_groups(struct sched_domain *sd, int cpu) if (!cpumask_test_cpu(i, sched_domain_span(sibling))) continue;

+ /* + * Usually we build sched_group by sibling's child sched_domain + * But for machines whose NUMA diameter are 3 or above, we move + * to build sched_group by sibling's proper descendant's child + * domain because sibling's child sched_domain will span out of + * the sched_domain being built as below. + * + * 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. + */ + if (sibling->child && + !cpumask_subset(sched_domain_span(sibling->child), span)) + sibling = find_descended_sibling(sd, sibling); + sg = build_group_from_child_sched_domain(sibling, cpu); if (!sg) goto fail; @@ -1022,7 +1053,7 @@ 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(sibling, sg);

if (!first) first = sg;