[PATCH v4 1/2] sched/numa: Skip VMA scanning on memory pinned to one NUMA node via cpuset.mems

[Libo Chen <libo.chen@oracle.com>]

When the memory of the current task is pinned to one NUMA node by cgroup,
there is no point in continuing the rest of VMA scanning and hinting page
faults as they will just be overhead. With this change, there will be no
more unnecessary PTE updates or page faults in this scenario.

We have seen up to a 6x improvement on a typical java workload running on
VMs with memory and CPU pinned to one NUMA node via cpuset in a two-socket
AARCH64 system. With the same pinning, on a 18-cores-per-socket Intel
platform, we have seen 20% improvment in a microbench that creates a
30-vCPU selftest KVM guest with 4GB memory, where each vCPU reads 4KB
pages in a fixed number of loops.

Signed-off-by: Libo Chen <libo.chen@oracle.com>
Tested-by: Chen Yu <yu.c.chen@intel.com>
---
 kernel/sched/fair.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e43993a4e580..c9903b1b3948 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3329,6 +3329,13 @@ static void task_numa_work(struct callback_head *work)
 	if (p->flags & PF_EXITING)
 		return;
 
+	/*
+	 * Memory is pinned to only one NUMA node via cpuset.mems, naturally
+	 * no page can be migrated.
+	 */
+	if (cpusets_enabled() && nodes_weight(cpuset_current_mems_allowed) == 1)
+		return;
+
 	if (!mm->numa_next_scan) {
 		mm->numa_next_scan = now +
 			msecs_to_jiffies(sysctl_numa_balancing_scan_delay);
-- 
2.43.5