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SubjectRe: [PATCH v3] sched/topology: Introduce NUMA identity node sched domain
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Hi,

Are there any other concerns with this patch?

Thanks,
Suravee

On 9/7/17 00:20, Suravee Suthikulpanit wrote:
> On AMD Family17h-based (EPYC) system, a logical NUMA node can contain
> upto 8 cores (16 threads) with the following topology.
>
> ----------------------------
> C0 | T0 T1 | || | T0 T1 | C4
> --------| || |--------
> C1 | T0 T1 | L3 || L3 | T0 T1 | C5
> --------| || |--------
> C2 | T0 T1 | #0 || #1 | T0 T1 | C6
> --------| || |--------
> C3 | T0 T1 | || | T0 T1 | C7
> ----------------------------
>
> Here, there are 2 last-level (L3) caches per logical NUMA node.
> A socket can contain upto 4 NUMA nodes, and a system can support
> upto 2 sockets. With full system configuration, current scheduler
> creates 4 sched domains:
>
> domain0 SMT (span a core)
> domain1 MC (span a last-level-cache)
> domain2 NUMA (span a socket: 4 nodes)
> domain3 NUMA (span a system: 8 nodes)
>
> Note that there is no domain to represent cpus spaning a logical
> NUMA node. With this hierarchy of sched domains, the scheduler does
> not balance properly in the following cases:
>
> Case1:
> When running 8 tasks, a properly balanced system should
> schedule a task per logical NUMA node. This is not the case for
> the current scheduler.
>
> Case2:
> In some cases, threads are scheduled on the same cpu, while other
> cpus are idle. This results in run-to-run inconsistency. For example:
>
> taskset -c 0-7 sysbench --num-threads=8 --test=cpu \
> --cpu-max-prime=100000 run
>
> Total execution time ranges from 25.1s to 33.5s depending on threads
> placement, where 25.1s is when all 8 threads are balanced properly
> on 8 cpus.
>
> Introducing NUMA identity node sched domain, which is based on how
> SRAT/SLIT table define a logical NUMA node. This results in the following
> hierarchy of sched domains on the same system described above.
>
> domain0 SMT (span a core)
> domain1 MC (span a last-level-cache)
> domain2 NODE (span a logical NUMA node)
> domain3 NUMA (span a socket: 4 nodes)
> domain4 NUMA (span a system: 8 nodes)
>
> This fixes the improper load balancing cases mentioned above.
>
> Note that in case cpumask of the last-level-cache and NODE domains
> are the same (e.g. on AMD family10h/15h servers), the NODE domain
> will be excluded. Therefore, this change will not affect those systems.
>
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
> ---
> kernel/sched/topology.c | 26 +++++++++++++++++++++++---
> 1 file changed, 23 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 79895ae..98a8bbc 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1335,6 +1335,10 @@ void sched_init_numa(void)
> if (!sched_domains_numa_distance)
> return;
>
> + /* Includes NUMA identity node at level 0. */
> + sched_domains_numa_distance[level++] = curr_distance;
> + sched_domains_numa_levels = level;
> +
> /*
> * O(nr_nodes^2) deduplicating selection sort -- in order to find the
> * unique distances in the node_distance() table.
> @@ -1382,8 +1386,7 @@ void sched_init_numa(void)
> return;
>
> /*
> - * 'level' contains the number of unique distances, excluding the
> - * identity distance node_distance(i,i).
> + * 'level' contains the number of unique distances
> *
> * The sched_domains_numa_distance[] array includes the actual distance
> * numbers.
> @@ -1445,9 +1448,26 @@ void sched_init_numa(void)
> tl[i] = sched_domain_topology[i];
>
> /*
> + * Do not setup NUMA node level if it has the same cpumask
> + * as sched domain at previous level. This is the case for
> + * system with:
> + * LLC == NODE : LLC (MC) sched domain span a NUMA node.
> + * DIE == NODE : DIE sched domain span a NUMA node.
> + *
> + * Assume all NUMA nodes are identical, so only check node 0.
> + */
> + if (!cpumask_equal(sched_domains_numa_masks[0][0], tl[i-1].mask(0))) {
> + tl[i++] = (struct sched_domain_topology_level){
> + .mask = sd_numa_mask,
> + .numa_level = 0,
> + SD_INIT_NAME(NODE)
> + };
> + }
> +
> + /*
> * .. and append 'j' levels of NUMA goodness.
> */
> - for (j = 0; j < level; i++, j++) {
> + for (j = 1; j < level; i++, j++) {
> tl[i] = (struct sched_domain_topology_level){
> .mask = sd_numa_mask,
> .sd_flags = cpu_numa_flags,
>

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