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authorMatt Fleming <matt@codeblueprint.co.uk>2019-08-08 22:53:01 +0300
committerIngo Molnar <mingo@kernel.org>2019-09-03 10:17:37 +0300
commita55c7454a8c887b226a01d7eed088ccb5374d81e (patch)
tree43123b848008c49e81f6511613a1599c92b48fe5 /include/linux/topology.h
parenta2cbfd46559e809c8165773b7fe8afa058b35414 (diff)
downloadlinux-a55c7454a8c887b226a01d7eed088ccb5374d81e.tar.xz
sched/topology: Improve load balancing on AMD EPYC systems
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>
Diffstat (limited to 'include/linux/topology.h')
-rw-r--r--include/linux/topology.h14
1 files changed, 14 insertions, 0 deletions
diff --git a/include/linux/topology.h b/include/linux/topology.h
index 47a3e3c08036..579522ec446c 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -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