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    Subject[PATCH v3] 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 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 <-------------------+

    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>
    Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
    ---
    -v3:
    Mainly updated according to Valentin's comments. While the approach was
    started by me, Valentin contributed the most useful edit and comments.
    Thanks, Valentin!

    * fixed a potential issue that re-used sgc might be located in
    a sched_domain which will be degenrated;
    * code cleanup to make it more readable

    While Valentin started another approach which completely removed overlapped
    sched_group, we both agree that it is better to have a solution which won't
    touch machines without 3-hops issue first:
    https://lore.kernel.org/lkml/jhjpn1a232z.mognet@arm.com/

    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 5d3675c7a76b..ab5ebf17f30a 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;
    --
    2.25.1
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