sched/numa: Cap PTE scanning overhead to 3% of run time

There is a fundamental mismatch between the runtime based NUMA scanning
at the task level, and the wall clock time NUMA scanning at the mm level.
On a severely overloaded system, with very large processes, this mismatch
can cause the system to spend all of its time in change_prot_numa().

This can happen if the task spends at least two ticks in change_prot_numa(),
and only gets two ticks of CPU time in the real time between two scan
intervals of the mm.

This patch ensures that a task never spends more than 3% of run
time scanning PTEs. It does that by ensuring that in-between
task_numa_work() runs, the task spends at least 32x as much time on
other things than it did on task_numa_work().

This is done stochastically: if a timer tick happens, or the task
gets rescheduled during task_numa_work(), we delay a future run of
task_numa_work() until the task has spent at least 32x the amount of
CPU time doing something else, as it spent inside task_numa_work().
The longer task_numa_work() takes, the more likely it is this happens.

If task_numa_work() takes very little time, chances are low that that
code will do anything, but we will not care.

Reported-and-tested-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: mgorman@suse.de
Link: http://lkml.kernel.org/r/1446756983-28173-3-git-send-email-riel@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 309b1d551f25a0ea568ce38b589b9632e6648ec2..95b944ecf7e4c90cb8d4721a234e6f8566bb6b8e 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2155,6 +2155,7 @@ void task_numa_work(struct callback_head *work)
        unsigned long migrate, next_scan, now = jiffies;
        struct task_struct *p = current;
        struct mm_struct *mm = p->mm;
+       u64 runtime = p->se.sum_exec_runtime;
        struct vm_area_struct *vma;
        unsigned long start, end;
        unsigned long nr_pte_updates = 0;
@@ -2277,6 +2278,17 @@ out:
        else
                reset_ptenuma_scan(p);
        up_read(&mm->mmap_sem);
+
+       /*
+        * Make sure tasks use at least 32x as much time to run other code
+        * than they used here, to limit NUMA PTE scanning overhead to 3% max.
+        * Usually update_task_scan_period slows down scanning enough; on an
+        * overloaded system we need to limit overhead on a per task basis.
+        */
+       if (unlikely(p->se.sum_exec_runtime != runtime)) {
+               u64 diff = p->se.sum_exec_runtime - runtime;
+               p->node_stamp += 32 * diff;
+       }
 }
 
 /*