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>

1 files changed, 11 insertions, 5 deletions

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);
         }
 
         /*