sched/numa: add statistics of numa balance task

JIRA: https://issues.redhat.com/browse/RHEL-110301

commit ad6b26b
Author: Chen Yu <yu.c.chen@intel.com>
Date:   Fri May 23 20:51:15 2025 +0800

    sched/numa: add statistics of numa balance task

    On systems with NUMA balancing enabled, it has been found that tracking
    task activities resulting from NUMA balancing is beneficial.  NUMA
    balancing employs two mechanisms for task migration: one is to migrate
    a task to an idle CPU within its preferred node, and the other is to
    swap tasks located on different nodes when they are on each other's
    preferred nodes.

    The kernel already provides NUMA page migration statistics in
    /sys/fs/cgroup/mytest/memory.stat and /proc/{PID}/sched.  However, it
    lacks statistics regarding task migration and swapping.  Therefore,
    relevant counts for task migration and swapping should be added.

    The following two new fields:

    numa_task_migrated
    numa_task_swapped

    will be shown in /sys/fs/cgroup/{GROUP}/memory.stat, /proc/{PID}/sched
    and /proc/vmstat.

    Introducing both per-task and per-memory cgroup (memcg) NUMA balancing
    statistics facilitates a rapid evaluation of the performance and
    resource utilization of the target workload.  For instance, users can
    first identify the container with high NUMA balancing activity and then
    further pinpoint a specific task within that group, and subsequently
    adjust the memory policy for that task.  In short, although it is
    possible to iterate through /proc/$pid/sched to locate the problematic
    task, the introduction of aggregated NUMA balancing activity for tasks
    within each memcg can assist users in identifying the task more
    efficiently through a divide-and-conquer approach.

    As Libo Chen pointed out, the memcg event relies on the text names in
    vmstat_text, and /proc/vmstat generates corresponding items based on
    vmstat_text.  Thus, the relevant task migration and swapping events
    introduced in vmstat_text also need to be populated by
    count_vm_numa_event(), otherwise these values are zero in /proc/vmstat.

    In theory, task migration and swap events are part of the scheduler's
    activities.  The reason for exposing them through the
    memory.stat/vmstat interface is that we already have NUMA balancing
    statistics in memory.stat/vmstat, and these events are closely related
    to each other.  Following Shakeel's suggestion, we describe the
    end-to-end flow/story of all these events occurring on a timeline for
    future reference:

    The goal of NUMA balancing is to co-locate a task and its memory pages
    on the same NUMA node.  There are two strategies: migrate the pages to
    the task's node, or migrate the task to the node where its pages
    reside.

    Suppose a task p1 is running on Node 0, but its pages are located on
    Node 1.  NUMA page fault statistics for p1 reveal its "page footprint"
    across nodes.  If NUMA balancing detects that most of p1's pages are on
    Node 1:

    1.Page Migration Attempt:
    The Numa balance first tries to migrate p1's pages to Node 0.
    The numa_page_migrate counter increments.

    2.Task Migration Strategies:
    After the page migration finishes, Numa balance checks every
    1 second to see if p1 can be migrated to Node 1.

    Case 2.1: Idle CPU Available

      If Node 1 has an idle CPU, p1 is directly scheduled there.  This
      event is logged as numa_task_migrated.

    Case 2.2: No Idle CPU (Task Swap)

      If all CPUs on Node1 are busy, direct migration could cause CPU
      contention or load imbalance.  Instead: The Numa balance selects a
      candidate task p2 on Node 1 that prefers Node 0 (e.g., due to its own
      page footprint).  p1 and p2 are swapped.  This cross-node swap is
      recorded as numa_task_swapped.

    Link: https://lkml.kernel.org/r/d00edb12ba0f0de3c5222f61487e65f2ac58f5b1.1748493462.git.yu.c.chen@intel.com
    Link: https://lkml.kernel.org/r/7ef90a88602ed536be46eba7152ed0d33bad5790.1748002400.git.yu.c.chen@intel.com
    Signed-off-by: Chen Yu <yu.c.chen@intel.com>
    Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
    Tested-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
    Cc: Aubrey Li <aubrey.li@intel.com>
    Cc: Ayush Jain <Ayush.jain3@amd.com>
    Cc: "Chen, Tim C" <tim.c.chen@intel.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Jonathan Corbet <corbet@lwn.net>
    Cc: Libo Chen <libo.chen@oracle.com>
    Cc: Mel Gorman <mgorman <mgorman@suse.de>
    Cc: Michal Hocko <mhocko@kernel.org>
    Cc: Michal Koutný <mkoutny@suse.com>
    Cc: Muchun Song <muchun.song@linux.dev>
    Cc: Roman Gushchin <roman.gushchin@linux.dev>
    Cc: Shakeel Butt <shakeel.butt@linux.dev>
    Cc: Tejun Heo <tj@kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Phil Auld <pauld@redhat.com>

Author: auldp

Documentation/admin-guide/cgroup-v2.rst

@@ -1647,6 +1647,12 @@ The following nested keys are defined.
 	  numa_hint_faults (npn)
 		Number of NUMA hinting faults.
 
+	  numa_task_migrated (npn)
+		Number of task migration by NUMA balancing.
+
+	  numa_task_swapped (npn)
+		Number of task swap by NUMA balancing.
+
 	  pgdemote_kswapd
 		Number of pages demoted by kswapd.
 

include/linux/sched.h

@@ -544,6 +544,10 @@ struct sched_statistics {
 	u64				nr_failed_migrations_running;
 	u64				nr_failed_migrations_hot;
 	u64				nr_forced_migrations;
+#ifdef CONFIG_NUMA_BALANCING
+	u64				numa_task_migrated;
+	u64				numa_task_swapped;
+#endif
 
 	u64				nr_wakeups;
 	u64				nr_wakeups_sync;

include/linux/vm_event_item.h

@@ -64,6 +64,8 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
 		NUMA_HINT_FAULTS,
 		NUMA_HINT_FAULTS_LOCAL,
 		NUMA_PAGE_MIGRATE,
+		NUMA_TASK_MIGRATE,
+		NUMA_TASK_SWAP,
 #endif
 #ifdef CONFIG_MIGRATION
 		PGMIGRATE_SUCCESS, PGMIGRATE_FAIL,

kernel/sched/core.c

@@ -3333,6 +3333,10 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 #ifdef CONFIG_NUMA_BALANCING
 static void __migrate_swap_task(struct task_struct *p, int cpu)
 {
+	__schedstat_inc(p->stats.numa_task_swapped);
+	count_vm_numa_event(NUMA_TASK_SWAP);
+	count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
+
 	if (task_on_rq_queued(p)) {
 		struct rq *src_rq, *dst_rq;
 		struct rq_flags srf, drf;
@@ -7839,8 +7843,9 @@ int migrate_task_to(struct task_struct *p, int target_cpu)
 	if (!cpumask_test_cpu(target_cpu, p->cpus_ptr))
 		return -EINVAL;
 
-	/* TODO: This is not properly updating schedstats */
-
+	__schedstat_inc(p->stats.numa_task_migrated);
+	count_vm_numa_event(NUMA_TASK_MIGRATE);
+	count_memcg_event_mm(p->mm, NUMA_TASK_MIGRATE);
 	trace_sched_move_numa(p, curr_cpu, target_cpu);
 	return stop_one_cpu(curr_cpu, migration_cpu_stop, &arg);
 }

kernel/sched/debug.c

@@ -1189,6 +1189,10 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
 		P_SCHEDSTAT(nr_failed_migrations_running);
 		P_SCHEDSTAT(nr_failed_migrations_hot);
 		P_SCHEDSTAT(nr_forced_migrations);
+#ifdef CONFIG_NUMA_BALANCING
+		P_SCHEDSTAT(numa_task_migrated);
+		P_SCHEDSTAT(numa_task_swapped);
+#endif
 		P_SCHEDSTAT(nr_wakeups);
 		P_SCHEDSTAT(nr_wakeups_sync);
 		P_SCHEDSTAT(nr_wakeups_migrate);

mm/memcontrol.c

@@ -468,6 +468,8 @@ static const unsigned int memcg_vm_event_stat[] = {
 	NUMA_PAGE_MIGRATE,
 	NUMA_PTE_UPDATES,
 	NUMA_HINT_FAULTS,
+	NUMA_TASK_MIGRATE,
+	NUMA_TASK_SWAP,
 #endif
 };
 

mm/vmstat.c

@@ -1340,6 +1340,8 @@ const char * const vmstat_text[] = {
 	"numa_hint_faults",
 	"numa_hint_faults_local",
 	"numa_pages_migrated",
+	"numa_task_migrated",
+	"numa_task_swapped",
 #endif
 #ifdef CONFIG_MIGRATION
 	"pgmigrate_success",