RE: [RFC PATCH 2/2] Revert "sched/topology: Warn when NUMA diameter > 2"

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Valentin Schneider [mailto:valentin.schneider@xxxxxxx]
> Sent: Thursday, February 4, 2021 4:55 AM
> To: linux-kernel@xxxxxxxxxxxxxxx
> Cc: vincent.guittot@xxxxxxxxxx; mgorman@xxxxxxx; mingo@xxxxxxxxxx;
> peterz@xxxxxxxxxxxxx; dietmar.eggemann@xxxxxxx; morten.rasmussen@xxxxxxx;
> linuxarm@xxxxxxxxxxxxx; xuwei (O) <xuwei5@xxxxxxxxxx>; Liguozhu (Kenneth)
> <liguozhu@xxxxxxxxxxxxx>; tiantao (H) <tiantao6@xxxxxxxxxxxxx>; wanghuiqiang
> <wanghuiqiang@xxxxxxxxxx>; Zengtao (B) <prime.zeng@xxxxxxxxxxxxx>; Jonathan
> Cameron <jonathan.cameron@xxxxxxxxxx>; guodong.xu@xxxxxxxxxx; Song Bao Hua
> (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>; Meelis Roos <mroos@xxxxxxxx>
> 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@xxxxxxx>

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