[PATCH] Revert "sched/numa: add statistics of numa balance task"

[Chen Yu <yu.c.chen@intel.com>]

This reverts commit ad6b26b6a0a79166b53209df2ca1cf8636296382.

This commit introduces per-memcg/task NUMA balance statistics,
but unfortunately it introduced a NULL pointer exception due
to the following race condition: After a swap task candidate
was chosen, its mm_struct pointer was set to NULL due to task
exit. Later, when performing the actual task swapping, the
p->mm caused the problem.

CPU0                                   CPU1
:
...
task_numa_migrate
     task_numa_find_cpu
      task_numa_compare
        # a normal task p is chosen
        env->best_task = p

                                          # p exit:
                                          exit_signals(p);
                                             p->flags |= PF_EXITING
                                          exit_mm
                                             p->mm = NULL;

      migrate_swap_stop
        __migrate_swap_task((arg->src_task, arg->dst_cpu)
         count_memcg_event_mm(p->mm, NUMA_TASK_SWAP)# p->mm is NULL

task_lock() should be held and the PF_EXITING flag needs to
be checked to prevent this from happening. After discussion,
the conclusion was that adding a lock is not worthwhile for
some statistics calculations. Revert the change and rely on
the tracepoint for this purpose.

Fixes: ad6b26b6a0a7 ("sched/numa: add statistics of numa balance task")
Reported-by: Jirka Hladky <jhladky@redhat.com>
Closes: https://lore.kernel.org/all/CAE4VaGBLJxpd=NeRJXpSCuw=REhC5LWJpC29kDy-Zh2ZDyzQZA@mail.gmail.com/
Reported-by: Srikanth Aithal <Srikanth.Aithal@amd.com>
Reported-by: Suneeth D <Suneeth.D@amd.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
---
 Documentation/admin-guide/cgroup-v2.rst | 6 ------
 include/linux/sched.h                   | 4 ----
 include/linux/vm_event_item.h           | 2 --
 kernel/sched/core.c                     | 9 ++-------
 kernel/sched/debug.c                    | 4 ----
 mm/memcontrol.c                         | 2 --
 mm/vmstat.c                             | 2 --
 7 files changed, 2 insertions(+), 27 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 0cc35a14afbe..bd98ea3175ec 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1732,12 +1732,6 @@ 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.
 
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4f78a64beb52..aa9c5be7a632 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -548,10 +548,6 @@ 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;
diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 91a3ce9a2687..9e15a088ba38 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -66,8 +66,6 @@ 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,
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8988d38d46a3..ca0be74e865b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3362,10 +3362,6 @@ 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;
@@ -7934,9 +7930,8 @@ int migrate_task_to(struct task_struct *p, int target_cpu)
 	if (!cpumask_test_cpu(target_cpu, p->cpus_ptr))
 		return -EINVAL;
 
-	__schedstat_inc(p->stats.numa_task_migrated);
-	count_vm_numa_event(NUMA_TASK_MIGRATE);
-	count_memcg_event_mm(p->mm, NUMA_TASK_MIGRATE);
+	/* TODO: This is not properly updating schedstats */
+
 	trace_sched_move_numa(p, curr_cpu, target_cpu);
 	return stop_one_cpu(curr_cpu, migration_cpu_stop, &arg);
 }
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 9d71baf08075..557246880a7e 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -1210,10 +1210,6 @@ 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);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 902da8a9c643..70fdeda1120b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -474,8 +474,6 @@ static const unsigned int memcg_vm_event_stat[] = {
 	NUMA_PAGE_MIGRATE,
 	NUMA_PTE_UPDATES,
 	NUMA_HINT_FAULTS,
-	NUMA_TASK_MIGRATE,
-	NUMA_TASK_SWAP,
 #endif
 };
 
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 429ae5339bfe..a78d70ddeacd 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1346,8 +1346,6 @@ 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",
-- 
2.25.1