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sched/topology: Improve load balancing on AMD EPYC systems

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SD_BALANCE_{FORK,EXEC} and SD_WAKE_AFFINE are stripped in sd_init()
for any sched domains with a NUMA distance greater than 2 hops
(RECLAIM_DISTANCE). The idea being that it's expensive to balance
across domains that far apart.

However, as is rather unfortunately explained in:

  commit 32e45ff43eaf ("mm: increase RECLAIM_DISTANCE to 30")

the value for RECLAIM_DISTANCE is based on node distance tables from
2011-era hardware.

Current AMD EPYC machines have the following NUMA node distances:

 node distances:
 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

where 2 hops is 32.

The result is that the scheduler fails to load balance properly across
NUMA nodes on different sockets -- 2 hops apart.

For example, pinning 16 busy threads to NUMA nodes 0 (CPUs 0-7) and 4
(CPUs 32-39) like so,

  $ numactl -C 0-7,32-39 ./spinner 16

causes all threads to fork and remain on node 0 until the active
balancer kicks in after a few seconds and forcibly moves some threads
to node 4.

Override node_reclaim_distance for AMD Zen.

Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Suravee.Suthikulpanit@amd.com
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thomas.Lendacky@amd.com
Cc: Tony Luck <tony.luck@intel.com>
Link: https://lkml.kernel.org/r/20190808195301.13222-3-matt@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Matt Fleming 2019-08-08 20:53:01 +01:00 committed by Ingo Molnar
parent a2cbfd4655
commit a55c7454a8
5 changed files with 23 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
arch/x86/kernel/cpu/amd.c
View File

@ -8,6 +8,7 @@
#include <linux/sched.h>
#include <linux/sched/clock.h>
#include <linux/random.h>
#include <linux/topology.h>
#include <asm/processor.h>
#include <asm/apic.h>
#include <asm/cacheinfo.h>
@ -824,6 +825,10 @@ static void init_amd_zn(struct cpuinfo_x86 *c)
{
set_cpu_cap(c, X86_FEATURE_ZEN);
#ifdef CONFIG_NUMA
node_reclaim_distance = 32;
#endif
/*
* Fix erratum 1076: CPB feature bit not being set in CPUID.
* Always set it, except when running under a hypervisor.

14
include/linux/topology.h
View File

@ -59,6 +59,20 @@ int arch_update_cpu_topology(void);
*/
#define RECLAIM_DISTANCE 30
#endif
/*
* The following tunable allows platforms to override the default node
* reclaim distance (RECLAIM_DISTANCE) if remote memory accesses are
* sufficiently fast that the default value actually hurts
* performance.
*
* AMD EPYC machines use this because even though the 2-hop distance
* is 32 (3.2x slower than a local memory access) performance actually
* *improves* if allowed to reclaim memory and load balance tasks
* between NUMA nodes 2-hops apart.
*/
extern int __read_mostly node_reclaim_distance;
#ifndef PENALTY_FOR_NODE_WITH_CPUS
#define PENALTY_FOR_NODE_WITH_CPUS (1)
#endif

3
kernel/sched/topology.c
View File

@ -1284,6 +1284,7 @@ static int sched_domains_curr_level;
int sched_max_numa_distance;
static int *sched_domains_numa_distance;
static struct cpumask ***sched_domains_numa_masks;
int __read_mostly node_reclaim_distance = RECLAIM_DISTANCE;
#endif
/*
@ -1402,7 +1403,7 @@ sd_init(struct sched_domain_topology_level *tl,
sd->flags &= ~SD_PREFER_SIBLING;
sd->flags |= SD_SERIALIZE;
if (sched_domains_numa_distance[tl->numa_level] > RECLAIM_DISTANCE) {
if (sched_domains_numa_distance[tl->numa_level] > node_reclaim_distance) {
sd->flags &= ~(SD_BALANCE_EXEC |
SD_BALANCE_FORK |
SD_WAKE_AFFINE);

2
mm/khugepaged.c
View File

@ -710,7 +710,7 @@ static bool khugepaged_scan_abort(int nid)
for (i = 0; i < MAX_NUMNODES; i++) {
if (!khugepaged_node_load[i])
continue;
if (node_distance(nid, i) > RECLAIM_DISTANCE)
if (node_distance(nid, i) > node_reclaim_distance)
return true;
}
return false;

2
mm/page_alloc.c
View File

@ -3522,7 +3522,7 @@ bool zone_watermark_ok_safe(struct zone *z, unsigned int order,
static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
{
return node_distance(zone_to_nid(local_zone), zone_to_nid(zone)) <=
RECLAIM_DISTANCE;
node_reclaim_distance;
}
#else /* CONFIG_NUMA */
static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)