Date: Thu, 10 Aug 2017 18:41:49 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Cc: linux-kernel@vger.kernel.org, mingo@redhat.com, bp@suse.de
Subject: Re: [RFC PATCH] sched/topology: Introduce NUMA identity node sched
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On Thu, Aug 10, 2017 at 10:20:52AM -0500, Suravee Suthikulpanit wrote:
> On AMD Family17h-based (EPYC) system, a 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 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)

Right, so traditionally we'd have the DIE level do that, but because
x86_has_numa_in_package we don't generate that, right?

>   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 NUMA node.
> With this hierachy 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 NUMA node. This is not the case for
> the current scheduler.
> 
> Case2:
> When running 'taskset -c 0-7 <a_program_with_8_independent_threads>',
> a properly balanced system should schedule 8 threads on 8 cpus
> (e.g. T0 of C0-C7).  However, current scheduler would schedule
> some threads on the same cpu, while others are idle.

Sure.. could you amend with a few actual performance numbers?

> Introducing NUMA identity node sched domain, which is based on how
> SRAT/SLIT table define a NUMA node. This results in the following
> hierachy of sched domains on the same system described above.
> 
>   domain0 SMT       (span a core)
>   domain1 MC        (span a last-level-cache)
>   domain2 NUMA_IDEN (span a NUMA node)

Hate that name though..

>   domain3 NUMA      (span a socket: 4 nodes)
>   domain4 NUMA      (span a system: 8 nodes)
> 
> This fixes the improper load balancing cases mentioned above.
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
> ---
>  kernel/sched/topology.c | 24 +++++++++++++++++++++---
>  1 file changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 79895ae..c57df98 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 +1448,24 @@ void sched_init_numa(void)
>  		tl[i] = sched_domain_topology[i];
>  
>  	/*
> +	 * Ignore the NUMA identity level if it has the same cpumask
> +	 * as previous level. This is the case for:
> +	 *   - System with last-level-cache (MC) sched domain span a NUMA node.
> +	 *   - System with 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(NUMA_IDEN)

Shall we make that:

			SD_INIT_NAME(NODE),

instead?

> +		};

This misses a set of '{}'. While C doesn't require it, out coding style
warrants blocks around any multi-line statement.

So what you've forgotten to mention is that for those systems where the
LLC == NODE this now superfluous level gets removed by the degenerate
code. Have you verified that does the right thing?

> +
> +	/*
>  	 * .. 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,
> -- 
> 2.7.4
>