sched/numa: skip VMA scanning on memory pinned to one NUMA node via cpuset.mems

Patch series "sched/numa: Skip VMA scanning on memory pinned to one NUMA
node via cpuset.mems", v5.


This patch (of 2):

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.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Libo Chen <[email protected]>
Tested-by: Chen Yu <[email protected]>
Tested-by: Srikanth Aithal <[email protected]>
Tested-by: Venkat Rao Bagalkote <[email protected]>
Cc: "Chen, Tim C" <[email protected]>
Cc: Chris Hyser <[email protected]>
Cc: Daniel Jordan <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: K Prateek Nayak <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Madadi Vineeth Reddy <[email protected]>
Cc: Mel Gorman <mgorman <[email protected]>
Cc: Michal Koutný <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Raghavendra K T <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Vincent Guittot <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Libo Chen

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);