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/20250424024523.2298272-1-libo.chen@oracle.com
Link: https://lkml.kernel.org/r/20250424024523.2298272-2-libo.chen@oracle.com
Signed-off-by: Libo Chen <libo.chen@oracle.com>
Tested-by: Chen Yu <yu.c.chen@intel.com>
Tested-by: Srikanth Aithal <sraithal@amd.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Cc: "Chen, Tim C" <tim.c.chen@intel.com>
Cc: Chris Hyser <chris.hyser@oracle.com>
Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Cc: Mel Gorman <mgorman <mgorman@suse.de>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Raghavendra K T <raghavendra.kt@amd.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0fb9bf995a4795a64204d98cb08c2cd31a0d7667..b3b715e8a7cbc6b8d3ac2bae9ebbef8e1e693e6c 100644 (file)
--- 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);