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SubjectRE: [RFC PATCH 2/2] Revert "sched/topology: Warn when NUMA diameter > 2"
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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 2/2] Revert "sched/topology: Warn when NUMA diameter > 2"
>
> The scheduler topology code can now figure out what to do with such
> topologies.
>
> This reverts commit b5b217346de85ed1b03fdecd5c5076b34fbb2f0b.
>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>

Yes, this is fine. I actually have seen some other problems we need
to consider.

The current code is probably well consolidated for machines with
2 hops or less. Thus, even after we fix the 3-hops span issue, I
can still see some other issue.

For example, if we change the sd flags and remove the SD_BALANCE
flags for the last hops in sd_init(), we are able to see large
score increase in unixbench.

if (sched_domains_numa_distance[tl->numa_level] > node_reclaim_distance ||
is_3rd_hops_domain(...)) {
sd->flags &= ~(SD_BALANCE_EXEC |
SD_BALANCE_FORK |
SD_WAKE_AFFINE);
}

So guess something needs to be tuned for machines with 3 hops or more.

But we need a kernel which has the fix of 3-hops issue before we can
do more work.

> ---
> kernel/sched/topology.c | 33 ---------------------------------
> 1 file changed, 33 deletions(-)
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index a8f69f234258..0fa41aab74e0 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -688,7 +688,6 @@ cpu_attach_domain(struct sched_domain *sd, struct
> root_domain *rd, int cpu)
> {
> struct rq *rq = cpu_rq(cpu);
> struct sched_domain *tmp;
> - int numa_distance = 0;
>
> /* Remove the sched domains which do not contribute to scheduling. */
> for (tmp = sd; tmp; ) {
> @@ -720,38 +719,6 @@ cpu_attach_domain(struct sched_domain *sd, struct
> root_domain *rd, int cpu)
> sd->child = NULL;
> }
>
> - 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);
> --
> 2.27.0

Thanks
Barry

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