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SubjectRE: [RFC PATCH 1/2] sched/topology: Get rid of NUMA overlapping groups
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> -----Original Message-----
> From: Valentin Schneider [mailto:valentin.schneider@arm.com]
> Sent: Thursday, February 4, 2021 4:55 AM
> To: linux-kernel@vger.kernel.org
> Cc: vincent.guittot@linaro.org; mgorman@suse.de; mingo@kernel.org;
> peterz@infradead.org; dietmar.eggemann@arm.com; morten.rasmussen@arm.com;
> linuxarm@openeuler.org; xuwei (O) <xuwei5@huawei.com>; Liguozhu (Kenneth)
> <liguozhu@hisilicon.com>; tiantao (H) <tiantao6@hisilicon.com>; wanghuiqiang
> <wanghuiqiang@huawei.com>; Zengtao (B) <prime.zeng@hisilicon.com>; Jonathan
> Cameron <jonathan.cameron@huawei.com>; guodong.xu@linaro.org; Song Bao Hua
> (Barry Song) <song.bao.hua@hisilicon.com>; Meelis Roos <mroos@linux.ee>
> Subject: [RFC PATCH 1/2] sched/topology: Get rid of NUMA overlapping groups
>
> As pointed out in commit
>
> b5b217346de8 ("sched/topology: Warn when NUMA diameter > 2")
>
> overlapping groups result in broken topology data structures whenever the
> underlying system has a NUMA diameter greater than 2. This stems from
> overlapping groups being built from sibling domain's spans, yielding bogus
> transitivity relations the like of:
>
> distance(A, B) <= 30 && distance(B, C) <= 20
> =>
> distance(A, C) <= 30
>
> As discussed with Barry, a feasible approach is to catch bogus overlapping
> groups and fix them after the fact [1].
>
> A more proactive approach would be to prevent aforementioned bogus
> relations from being built altogether, implies departing from the
> "group span is sibling domain child's span" strategy. Said strategy only
> works for diameter <= 2, which fortunately or unfortunately is currently
> the most common case.
>
> The chosen approach is, for NUMA domains:
> a) have the local group be the child domain's span, as before
> b) have all remote groups span only their respective node
>
> This boils down to getting rid of overlapping groups.
>

Hi Valentin,

While I like your approach, this will require more time
to evaluate possible influence as the approach also affects
all machines without 3-hops issue. So x86 platforms need to
be tested and benchmark is required.

What about we firstly finish the review of "grandchild" approach
v2 and have a solution for kunpeng920 and Sun Fire X4600-M2
while not impacting other machines which haven't 3-hops issues
first?

I would appreciate very much if you could comment on v2:
https://lore.kernel.org/lkml/20210203111201.20720-1-song.bao.hua@hisilicon.com/


> Note that b) requires introducing cross sched_domain_topology_level
> references for sched_group_capacity. This is a somewhat prickly matter as
> we need to ensure whichever group we hook into won't see its domain
> degenerated (which was never an issue when such references were bounded
> within a single topology level).
>
> This lifts the NUMA diameter restriction, although yields more groups in
> the NUMA domains. As an example, here is the distance matrix for
> an AMD Epyc:
>
> node 0 1 2 3 4 5 6 7
> 0: 10 16 16 16 32 32 32 32
> 1: 16 10 16 16 32 32 32 32
> 2: 16 16 10 16 32 32 32 32
> 3: 16 16 16 10 32 32 32 32
> 4: 32 32 32 32 10 16 16 16
> 5: 32 32 32 32 16 10 16 16
> 6: 32 32 32 32 16 16 10 16
> 7: 32 32 32 32 16 16 16 10
>
> Emulating this on QEMU yields, before the patch:
> [ 0.386745] CPU0 attaching sched-domain(s):
> [ 0.386969] domain-0: span=0-3 level=NUMA
> [ 0.387708] groups: 0:{ span=0 cap=1008 }, 1:{ span=1 cap=1007 },
> 2:{ span=2 cap=1007 }, 3:{ span=3 cap=998 }
> [ 0.388505] domain-1: span=0-7 level=NUMA
> [ 0.388700] groups: 0:{ span=0-3 cap=4020 }, 4:{ span=4-7 cap=4014 }
> [ 0.389861] CPU1 attaching sched-domain(s):
> [ 0.390020] domain-0: span=0-3 level=NUMA
> [ 0.390200] groups: 1:{ span=1 cap=1007 }, 2:{ span=2 cap=1007 },
> 3:{ span=3 cap=998 }, 0:{ span=0 cap=1008 }
> [ 0.390701] domain-1: span=0-7 level=NUMA
> [ 0.390874] groups: 0:{ span=0-3 cap=4020 }, 4:{ span=4-7 cap=4014 }
> [ 0.391460] CPU2 attaching sched-domain(s):
> [ 0.391664] domain-0: span=0-3 level=NUMA
> [ 0.392750] groups: 2:{ span=2 cap=1007 }, 3:{ span=3 cap=998 }, 0:{ span=0
> cap=1008 }, 1:{ span=1 cap=1007 }
> [ 0.393672] domain-1: span=0-7 level=NUMA
> [ 0.393961] groups: 0:{ span=0-3 cap=4020 }, 4:{ span=4-7 cap=4014 }
> [ 0.394645] CPU3 attaching sched-domain(s):
> [ 0.394792] domain-0: span=0-3 level=NUMA
> [ 0.394961] groups: 3:{ span=3 cap=998 }, 0:{ span=0 cap=1008 }, 1:{ span=1
> cap=1007 }, 2:{ span=2 cap=1007 }
> [ 0.395749] domain-1: span=0-7 level=NUMA
> [ 0.396098] groups: 0:{ span=0-3 cap=4020 }, 4:{ span=4-7 cap=4014 }
> [ 0.396455] CPU4 attaching sched-domain(s):
> [ 0.396603] domain-0: span=4-7 level=NUMA
> [ 0.396771] groups: 4:{ span=4 cap=1001 }, 5:{ span=5 cap=1004 },
> 6:{ span=6 cap=1003 }, 7:{ span=7 cap=1006 }
> [ 0.397274] domain-1: span=0-7 level=NUMA
> [ 0.397454] groups: 4:{ span=4-7 cap=4014 }, 0:{ span=0-3 cap=4020 }
> [ 0.397801] CPU5 attaching sched-domain(s):
> [ 0.397945] domain-0: span=4-7 level=NUMA
> [ 0.398110] groups: 5:{ span=5 cap=1004 }, 6:{ span=6 cap=1003 },
> 7:{ span=7 cap=1006 }, 4:{ span=4 cap=1001 }
> [ 0.398605] domain-1: span=0-7 level=NUMA
> [ 0.398773] groups: 4:{ span=4-7 cap=4014 }, 0:{ span=0-3 cap=4020 }
> [ 0.399109] CPU6 attaching sched-domain(s):
> [ 0.399253] domain-0: span=4-7 level=NUMA
> [ 0.399418] groups: 6:{ span=6 cap=1003 }, 7:{ span=7 cap=1006 },
> 4:{ span=4 cap=1001 }, 5:{ span=5 cap=1004 }
> [ 0.400562] domain-1: span=0-7 level=NUMA
> [ 0.400741] groups: 4:{ span=4-7 cap=4020 }, 0:{ span=0-3 cap=4020 }
> [ 0.401083] CPU7 attaching sched-domain(s):
> [ 0.401231] domain-0: span=4-7 level=NUMA
> [ 0.401395] groups: 7:{ span=7 cap=1006 }, 4:{ span=4 cap=1004 },
> 5:{ span=5 cap=1007 }, 6:{ span=6 cap=1003 }
> [ 0.401906] domain-1: span=0-7 level=NUMA
> [ 0.402076] groups: 4:{ span=4-7 cap=4020 }, 0:{ span=0-3 cap=4020 }
> [ 0.402437] root domain span: 0-7 (max cpu_capacity = 1024)
>
> with the patch:
> [ 0.367436] CPU0 attaching sched-domain(s):
> [ 0.368064] domain-0: span=0-3 level=NUMA
> [ 0.368614] groups: 0:{ span=0 cap=1012 }, 1:{ span=1 cap=1002 },
> 2:{ span=2 cap=1000 }, 3:{ span=3 cap=993 }
> [ 0.369490] domain-1: span=0-7 level=NUMA
> [ 0.369682] groups: 0:{ span=0-3 cap=4007 }, 4:{ span=4 cap=991 },
> 5:{ span=5 cap=1003 }, 6:{ span=6 cap=1003 }, 7:{ span=7 cap=998 }
> [ 0.371132] CPU1 attaching sched-domain(s):
> [ 0.371290] domain-0: span=0-3 level=NUMA
> [ 0.371462] groups: 1:{ span=1 cap=1002 }, 2:{ span=2 cap=1000 },
> 3:{ span=3 cap=993 }, 0:{ span=0 cap=1012 }
> [ 0.372720] domain-1: span=0-7 level=NUMA
> [ 0.372906] groups: 0:{ span=0-3 cap=4007 }, 4:{ span=4 cap=991 },
> 5:{ span=5 cap=1003 }, 6:{ span=6 cap=1003 }, 7:{ span=7 cap=998 }
> [ 0.373678] CPU2 attaching sched-domain(s):
> [ 0.373833] domain-0: span=0-3 level=NUMA
> [ 0.374006] groups: 2:{ span=2 cap=1000 }, 3:{ span=3 cap=993 }, 0:{ span=0
> cap=1012 }, 1:{ span=1 cap=1002 }
> [ 0.374516] domain-1: span=0-7 level=NUMA
> [ 0.374689] groups: 0:{ span=0-3 cap=4007 }, 4:{ span=4 cap=991 },
> 5:{ span=5 cap=1003 }, 6:{ span=6 cap=1003 }, 7:{ span=7 cap=998 }
> [ 0.375337] CPU3 attaching sched-domain(s):
> [ 0.375491] domain-0: span=0-3 level=NUMA
> [ 0.375666] groups: 3:{ span=3 cap=993 }, 0:{ span=0 cap=1012 }, 1:{ span=1
> cap=1002 }, 2:{ span=2 cap=1000 }
> [ 0.376639] domain-1: span=0-7 level=NUMA
> [ 0.376818] groups: 0:{ span=0-3 cap=4007 }, 4:{ span=4 cap=991 },
> 5:{ span=5 cap=1003 }, 6:{ span=6 cap=1003 }, 7:{ span=7 cap=998 }
> [ 0.377465] CPU4 attaching sched-domain(s):
> [ 0.377616] domain-0: span=4-7 level=NUMA
> [ 0.377844] groups: 4:{ span=4 cap=991 }, 5:{ span=5 cap=1003 }, 6:{ span=6
> cap=1003 }, 7:{ span=7 cap=998 }
> [ 0.378445] domain-1: span=0-7 level=NUMA
> [ 0.378622] groups: 4:{ span=4-7 cap=3995 }, 0:{ span=0 cap=1012 },
> 1:{ span=1 cap=1001 }, 2:{ span=2 cap=1000 }, 3:{ span=3 cap=993 }
> [ 0.379296] CPU5 attaching sched-domain(s):
> [ 0.379453] domain-0: span=4-7 level=NUMA
> [ 0.379629] groups: 5:{ span=5 cap=1003 }, 6:{ span=6 cap=1003 },
> 7:{ span=7 cap=998 }, 4:{ span=4 cap=991 }
> [ 0.380499] domain-1: span=0-7 level=NUMA
> [ 0.380800] groups: 4:{ span=4-7 cap=4001 }, 0:{ span=0 cap=1012 },
> 1:{ span=1 cap=1001 }, 2:{ span=2 cap=1000 }, 3:{ span=3 cap=998 }
> [ 0.381475] CPU6 attaching sched-domain(s):
> [ 0.381641] domain-0: span=4-7 level=NUMA
> [ 0.381882] groups: 6:{ span=6 cap=1003 }, 7:{ span=7 cap=998 }, 4:{ span=4
> cap=997 }, 5:{ span=5 cap=1003 }
> [ 0.382419] domain-1: span=0-7 level=NUMA
> [ 0.382594] groups: 4:{ span=4-7 cap=4001 }, 0:{ span=0 cap=1012 },
> 1:{ span=1 cap=1001 }, 2:{ span=2 cap=1004 }, 3:{ span=3 cap=998 }
> [ 0.383253] CPU7 attaching sched-domain(s):
> [ 0.383407] domain-0: span=4-7 level=NUMA
> [ 0.383584] groups: 7:{ span=7 cap=998 }, 4:{ span=4 cap=997 }, 5:{ span=5
> cap=1003 }, 6:{ span=6 cap=1003 }
> [ 0.384089] domain-1: span=0-7 level=NUMA
> [ 0.384516] groups: 4:{ span=4-7 cap=4001 }, 0:{ span=0 cap=1012 },
> 1:{ span=1 cap=1001 }, 2:{ span=2 cap=1004 }, 3:{ span=3 cap=998 }
> [ 0.385503] root domain span: 0-7 (max cpu_capacity = 1024)
>
> IOW, this does more than double the number of groups at higher domains, the
> impact of which has yet to be measured.
>
> XXX: this only changes the group generation; actually removing SD_OVERLAP
> and all the related fluff is yet to be done.
>
> [1]:
> http://lore.kernel.org/r/20210201033830.15040-1-song.bao.hua@hisilicon.com
>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
> kernel/sched/topology.c | 44 ++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 43 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 09d35044bd88..a8f69f234258 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -982,6 +982,41 @@ static void init_overlap_sched_group(struct sched_domain
> *sd,
> sg->sgc->max_capacity = SCHED_CAPACITY_SCALE;
> }
>
> +static struct sched_domain *find_node_domain(struct sched_domain *sd)
> +{
> + struct sched_domain *parent;
> +
> + BUG_ON(!(sd->flags & SD_NUMA));
> +
> + /* Get to the level above NODE */
> + while (sd && sd->child) {
> + parent = sd;
> + sd = sd->child;
> +
> + if (!(sd->flags & SD_NUMA))
> + break;
> + }
> + /*
> + * We're going to create cross topology level sched_group_capacity
> + * references. This can only work if the domains resulting from said
> + * levels won't be degenerated, as we need said sgc to be periodically
> + * updated: it needs to be attached to the local group of a domain
> + * that didn't get degenerated.
> + *
> + * Of course, groups aren't available yet, so we can't call the usual
> + * sd_degenerate(). Checking domain spans is the closest we get.
> + * Start from NODE's parent, and keep going up until we get a domain
> + * we're sure won't be degenerated.
> + */
> + while (sd->parent &&
> + cpumask_equal(sched_domain_span(sd), sched_domain_span(parent))) {
> + sd = parent;
> + parent = sd->parent;
> + }

So this is because the sched_domain which doesn't contribute to scheduler
will be destroyed during cpu_attach_domain() since sd and parent span
the seam mask?

> +
> + return parent;
> +}
> +
> static int
> build_overlap_sched_groups(struct sched_domain *sd, int cpu)
> {
> @@ -1015,6 +1050,13 @@ build_overlap_sched_groups(struct sched_domain *sd, int
> cpu)
> if (!cpumask_test_cpu(i, sched_domain_span(sibling)))
> continue;
>
> + /*
> + * Local group is child domain's span, as is tradition.
> + * Non-local groups will only span remote nodes.
> + */
> + if (first)
> + sibling = find_node_domain(sibling);
> +
> sg = build_group_from_child_sched_domain(sibling, cpu);
> if (!sg)
> goto fail;
> @@ -1022,7 +1064,7 @@ 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;
> --
> 2.27.0

Thanks
Barry

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