Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Ingo writes:
  "scheduler fixes:

   These fixes address a rather involved performance regression between
   v4.17->v4.19 in the sched/numa auto-balancing code. Since distros
   really need this fix we accelerated it to sched/urgent for a faster
   upstream merge.

   NUMA scheduling and balancing performance is now largely back to
   v4.17 levels, without reintroducing the NUMA placement bugs that
   v4.18 and v4.19 fixed.

   Many thanks to Srikar Dronamraju, Mel Gorman and Jirka Hladky, for
   reporting, testing, re-testing and solving this rather complex set of
   bugs."

* 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  sched/numa: Migrate pages to local nodes quicker early in the lifetime of a task
  mm, sched/numa: Remove rate-limiting of automatic NUMA balancing migration
  sched/numa: Avoid task migration for small NUMA improvement
  mm/migrate: Use spin_trylock() while resetting rate limit
  sched/numa: Limit the conditions where scan period is reset
  sched/numa: Reset scan rate whenever task moves across nodes
  sched/numa: Pass destination CPU as a parameter to migrate_task_rq
  sched/numa: Stop multiple tasks from moving to the CPU at the same time

4 files changed, 95 insertions, 16 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 625bc9897f62..ad97f3ba5ec5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1167,7 +1167,7 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 
         if (task_cpu(p) != new_cpu) {
                 if (p->sched_class->migrate_task_rq)
-                        p->sched_class->migrate_task_rq(p);
+                        p->sched_class->migrate_task_rq(p, new_cpu);
                 p->se.nr_migrations++;
                 rseq_migrate(p);
                 perf_event_task_migrate(p);
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 997ea7b839fa..91e4202b0634 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1607,7 +1607,7 @@ out:
         return cpu;
 }
 
-static void migrate_task_rq_dl(struct task_struct *p)
+static void migrate_task_rq_dl(struct task_struct *p, int new_cpu __maybe_unused)
 {
         struct rq *rq;
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f808ddf2a868..7fc4a371bdd2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1392,6 +1392,17 @@ bool should_numa_migrate_memory(struct task_struct *p, struct page * page,
         int last_cpupid, this_cpupid;
 
         this_cpupid = cpu_pid_to_cpupid(dst_cpu, current->pid);
+        last_cpupid = page_cpupid_xchg_last(page, this_cpupid);
+
+        /*
+         * Allow first faults or private faults to migrate immediately early in
+         * the lifetime of a task. The magic number 4 is based on waiting for
+         * two full passes of the "multi-stage node selection" test that is
+         * executed below.
+         */
+        if ((p->numa_preferred_nid == -1 || p->numa_scan_seq <= 4) &&
+            (cpupid_pid_unset(last_cpupid) || cpupid_match_pid(p, last_cpupid)))
+                return true;
 
         /*
          * Multi-stage node selection is used in conjunction with a periodic
@@ -1410,7 +1421,6 @@ bool should_numa_migrate_memory(struct task_struct *p, struct page * page,
          * This quadric squishes small probabilities, making it less likely we
          * act on an unlikely task<->page relation.
          */
-        last_cpupid = page_cpupid_xchg_last(page, this_cpupid);
         if (!cpupid_pid_unset(last_cpupid) &&
                                 cpupid_to_nid(last_cpupid) != dst_nid)
                 return false;
@@ -1514,6 +1524,21 @@ struct task_numa_env {
 static void task_numa_assign(struct task_numa_env *env,
                              struct task_struct *p, long imp)
 {
+        struct rq *rq = cpu_rq(env->dst_cpu);
+
+        /* Bail out if run-queue part of active NUMA balance. */
+        if (xchg(&rq->numa_migrate_on, 1))
+                return;
+
+        /*
+         * Clear previous best_cpu/rq numa-migrate flag, since task now
+         * found a better CPU to move/swap.
+         */
+        if (env->best_cpu != -1) {
+                rq = cpu_rq(env->best_cpu);
+                WRITE_ONCE(rq->numa_migrate_on, 0);
+        }
+
         if (env->best_task)
                 put_task_struct(env->best_task);
         if (p)
@@ -1553,6 +1578,13 @@ static bool load_too_imbalanced(long src_load, long dst_load,
 }
 
 /*
+ * Maximum NUMA importance can be 1998 (2*999);
+ * SMALLIMP @ 30 would be close to 1998/64.
+ * Used to deter task migration.
+ */
+#define SMALLIMP        30
+
+/*
  * This checks if the overall compute and NUMA accesses of the system would
  * be improved if the source tasks was migrated to the target dst_cpu taking
  * into account that it might be best if task running on the dst_cpu should
@@ -1569,6 +1601,9 @@ static void task_numa_compare(struct task_numa_env *env,
         long moveimp = imp;
         int dist = env->dist;
 
+        if (READ_ONCE(dst_rq->numa_migrate_on))
+                return;
+
         rcu_read_lock();
         cur = task_rcu_dereference(&dst_rq->curr);
         if (cur && ((cur->flags & PF_EXITING) || is_idle_task(cur)))
@@ -1582,7 +1617,7 @@ static void task_numa_compare(struct task_numa_env *env,
                 goto unlock;
 
         if (!cur) {
-                if (maymove || imp > env->best_imp)
+                if (maymove && moveimp >= env->best_imp)
                         goto assign;
                 else
                         goto unlock;
@@ -1625,16 +1660,22 @@ static void task_numa_compare(struct task_numa_env *env,
                                task_weight(cur, env->dst_nid, dist);
         }
 
-        if (imp <= env->best_imp)
-                goto unlock;
-
         if (maymove && moveimp > imp && moveimp > env->best_imp) {
-                imp = moveimp - 1;
+                imp = moveimp;
                 cur = NULL;
                 goto assign;
         }
 
         /*
+         * If the NUMA importance is less than SMALLIMP,
+         * task migration might only result in ping pong
+         * of tasks and also hurt performance due to cache
+         * misses.
+         */
+        if (imp < SMALLIMP || imp <= env->best_imp + SMALLIMP / 2)
+                goto unlock;
+
+        /*
          * In the overloaded case, try and keep the load balanced.
          */
         load = task_h_load(env->p) - task_h_load(cur);
@@ -1710,6 +1751,7 @@ static int task_numa_migrate(struct task_struct *p)
                 .best_cpu = -1,
         };
         struct sched_domain *sd;
+        struct rq *best_rq;
         unsigned long taskweight, groupweight;
         int nid, ret, dist;
         long taskimp, groupimp;
@@ -1805,20 +1847,17 @@ static int task_numa_migrate(struct task_struct *p)
         if (env.best_cpu == -1)
                 return -EAGAIN;
 
-        /*
-         * Reset the scan period if the task is being rescheduled on an
-         * alternative node to recheck if the tasks is now properly placed.
-         */
-        p->numa_scan_period = task_scan_start(p);
-
+        best_rq = cpu_rq(env.best_cpu);
         if (env.best_task == NULL) {
                 ret = migrate_task_to(p, env.best_cpu);
+                WRITE_ONCE(best_rq->numa_migrate_on, 0);
                 if (ret != 0)
                         trace_sched_stick_numa(p, env.src_cpu, env.best_cpu);
                 return ret;
         }
 
         ret = migrate_swap(p, env.best_task, env.best_cpu, env.src_cpu);
+        WRITE_ONCE(best_rq->numa_migrate_on, 0);
 
         if (ret != 0)
                 trace_sched_stick_numa(p, env.src_cpu, task_cpu(env.best_task));
@@ -2596,6 +2635,39 @@ void task_tick_numa(struct rq *rq, struct task_struct *curr)
         }
 }
 
+static void update_scan_period(struct task_struct *p, int new_cpu)
+{
+        int src_nid = cpu_to_node(task_cpu(p));
+        int dst_nid = cpu_to_node(new_cpu);
+
+        if (!static_branch_likely(&sched_numa_balancing))
+                return;
+
+        if (!p->mm || !p->numa_faults || (p->flags & PF_EXITING))
+                return;
+
+        if (src_nid == dst_nid)
+                return;
+
+        /*
+         * Allow resets if faults have been trapped before one scan
+         * has completed. This is most likely due to a new task that
+         * is pulled cross-node due to wakeups or load balancing.
+         */
+        if (p->numa_scan_seq) {
+                /*
+                 * Avoid scan adjustments if moving to the preferred
+                 * node or if the task was not previously running on
+                 * the preferred node.
+                 */
+                if (dst_nid == p->numa_preferred_nid ||
+                    (p->numa_preferred_nid != -1 && src_nid != p->numa_preferred_nid))
+                        return;
+        }
+
+        p->numa_scan_period = task_scan_start(p);
+}
+
 #else
 static void task_tick_numa(struct rq *rq, struct task_struct *curr)
 {
@@ -2609,6 +2681,10 @@ static inline void account_numa_dequeue(struct rq *rq, struct task_struct *p)
 {
 }
 
+static inline void update_scan_period(struct task_struct *p, int new_cpu)
+{
+}
+
 #endif /* CONFIG_NUMA_BALANCING */
 
 static void
@@ -6275,7 +6351,7 @@ static void detach_entity_cfs_rq(struct sched_entity *se);
  * cfs_rq_of(p) references at time of call are still valid and identify the
  * previous CPU. The caller guarantees p->pi_lock or task_rq(p)->lock is held.
  */
-static void migrate_task_rq_fair(struct task_struct *p)
+static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
 {
         /*
          * As blocked tasks retain absolute vruntime the migration needs to
@@ -6328,6 +6404,8 @@ static void migrate_task_rq_fair(struct task_struct *p)
 
         /* We have migrated, no longer consider this task hot */
         p->se.exec_start = 0;
+
+        update_scan_period(p, new_cpu);
 }
 
 static void task_dead_fair(struct task_struct *p)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4a2e8cae63c4..455fa330de04 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -783,6 +783,7 @@ struct rq {
 #ifdef CONFIG_NUMA_BALANCING
         unsigned int            nr_numa_running;
         unsigned int            nr_preferred_running;
+        unsigned int            numa_migrate_on;
 #endif
         #define CPU_LOAD_IDX_MAX 5
         unsigned long           cpu_load[CPU_LOAD_IDX_MAX];
@@ -1523,7 +1524,7 @@ struct sched_class {
 
 #ifdef CONFIG_SMP
         int  (*select_task_rq)(struct task_struct *p, int task_cpu, int sd_flag, int flags);
-        void (*migrate_task_rq)(struct task_struct *p);
+        void (*migrate_task_rq)(struct task_struct *p, int new_cpu);
 
         void (*task_woken)(struct rq *this_rq, struct task_struct *task);