sched/numa: Be more careful about joining numa groups

Due to the way the pid is truncated, and tasks are moved between
CPUs by the scheduler, it is possible for the current task_numa_fault
to group together tasks that do not actually share memory together.

This patch adds a few easy sanity checks to task_numa_fault, joining
tasks together if they share the same tsk->mm, or if the fault was on
a page with an elevated mapcount, in a shared VMA.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1381141781-10992-57-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

3 files changed, 19 insertions, 5 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1127a46ac3d2..59f953b2e413 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1454,6 +1454,7 @@ struct task_struct {
 
 #define TNF_MIGRATED    0x01
 #define TNF_NO_GROUP    0x02
+#define TNF_SHARED      0x04
 
 #ifdef CONFIG_NUMA_BALANCING
 extern void task_numa_fault(int last_node, int node, int pages, int flags);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5166b9b1af70..222c2d0b6ae2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1381,7 +1381,7 @@ static void double_lock(spinlock_t *l1, spinlock_t *l2)
         spin_lock_nested(l2, SINGLE_DEPTH_NESTING);
 }
 
-static void task_numa_group(struct task_struct *p, int cpupid)
+static void task_numa_group(struct task_struct *p, int cpupid, int flags)
 {
         struct numa_group *grp, *my_grp;
         struct task_struct *tsk;
@@ -1439,10 +1439,16 @@ static void task_numa_group(struct task_struct *p, int cpupid)
         if (my_grp->nr_tasks == grp->nr_tasks && my_grp > grp)
                 goto unlock;
 
-        if (!get_numa_group(grp))
-                goto unlock;
+        /* Always join threads in the same process. */
+        if (tsk->mm == current->mm)
+                join = true;
+
+        /* Simple filter to avoid false positives due to PID collisions */
+        if (flags & TNF_SHARED)
+                join = true;
 
-        join = true;
+        if (join && !get_numa_group(grp))
+                join = false;
 
 unlock:
         rcu_read_unlock();
@@ -1539,7 +1545,7 @@ void task_numa_fault(int last_cpupid, int node, int pages, int flags)
         } else {
                 priv = cpupid_match_pid(p, last_cpupid);
                 if (!priv && !(flags & TNF_NO_GROUP))
-                        task_numa_group(p, last_cpupid);
+                        task_numa_group(p, last_cpupid, flags);
         }
 
         /*
diff --git a/mm/memory.c b/mm/memory.c
index 9898eeb9a21c..823720c43ea9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3584,6 +3584,13 @@ int do_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
         if (!pte_write(pte))
                 flags |= TNF_NO_GROUP;
 
+        /*
+         * Flag if the page is shared between multiple address spaces. This
+         * is later used when determining whether to group tasks together
+         */
+        if (page_mapcount(page) > 1 && (vma->vm_flags & VM_SHARED))
+                flags |= TNF_SHARED;
+
         last_cpupid = page_cpupid_last(page);
         page_nid = page_to_nid(page);
         target_nid = numa_migrate_prep(page, vma, addr, page_nid);