linux-kernel - [tip:sched/core] sched/numa: Mitigate chance that same task always updates PTEs

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Date:	Wed, 9 Oct 2013 10:26:02 -0700
From:	tip-bot for Peter Zijlstra <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	peterz@...radead.org, hannes@...xchg.org, riel@...hat.com,
	srikar@...ux.vnet.ibm.com, aarcange@...hat.com, mgorman@...e.de,
	tglx@...utronix.de
Subject: [tip:sched/core] sched/numa:
  Mitigate chance that same task always updates PTEs

Commit-ID:  19a78d110d7a8045aeb90d38ee8fe9743ce88c2d
Gitweb:     http://git.kernel.org/tip/19a78d110d7a8045aeb90d38ee8fe9743ce88c2d
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Mon, 7 Oct 2013 11:28:51 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 9 Oct 2013 12:39:56 +0200

sched/numa: Mitigate chance that same task always updates PTEs

With a trace_printk("working\n"); right after the cmpxchg in
task_numa_work() we can see that of a 4 thread process, its always the
same task winning the race and doing the protection change.

This is a problem since the task doing the protection change has a
penalty for taking faults -- it is busy when marking the PTEs. If its
always the same task the ->numa_faults[] get severely skewed.

Avoid this by delaying the task doing the protection change such that
it is unlikely to win the privilege again.

Before:

root@...erlagos:~# grep "thread 0/.*working" /debug/tracing/trace | tail -15
      thread 0/0-3232  [022] ....   212.787402: task_numa_work: working
      thread 0/0-3232  [022] ....   212.888473: task_numa_work: working
      thread 0/0-3232  [022] ....   212.989538: task_numa_work: working
      thread 0/0-3232  [022] ....   213.090602: task_numa_work: working
      thread 0/0-3232  [022] ....   213.191667: task_numa_work: working
      thread 0/0-3232  [022] ....   213.292734: task_numa_work: working
      thread 0/0-3232  [022] ....   213.393804: task_numa_work: working
      thread 0/0-3232  [022] ....   213.494869: task_numa_work: working
      thread 0/0-3232  [022] ....   213.596937: task_numa_work: working
      thread 0/0-3232  [022] ....   213.699000: task_numa_work: working
      thread 0/0-3232  [022] ....   213.801067: task_numa_work: working
      thread 0/0-3232  [022] ....   213.903155: task_numa_work: working
      thread 0/0-3232  [022] ....   214.005201: task_numa_work: working
      thread 0/0-3232  [022] ....   214.107266: task_numa_work: working
      thread 0/0-3232  [022] ....   214.209342: task_numa_work: working

After:

root@...erlagos:~# grep "thread 0/.*working" /debug/tracing/trace | tail -15
      thread 0/0-3253  [005] ....   136.865051: task_numa_work: working
      thread 0/2-3255  [026] ....   136.965134: task_numa_work: working
      thread 0/3-3256  [024] ....   137.065217: task_numa_work: working
      thread 0/3-3256  [024] ....   137.165302: task_numa_work: working
      thread 0/3-3256  [024] ....   137.265382: task_numa_work: working
      thread 0/0-3253  [004] ....   137.366465: task_numa_work: working
      thread 0/2-3255  [026] ....   137.466549: task_numa_work: working
      thread 0/0-3253  [004] ....   137.566629: task_numa_work: working
      thread 0/0-3253  [004] ....   137.666711: task_numa_work: working
      thread 0/1-3254  [028] ....   137.766799: task_numa_work: working
      thread 0/0-3253  [004] ....   137.866876: task_numa_work: working
      thread 0/2-3255  [026] ....   137.966960: task_numa_work: working
      thread 0/1-3254  [028] ....   138.067041: task_numa_work: working
      thread 0/2-3255  [026] ....   138.167123: task_numa_work: working
      thread 0/3-3256  [024] ....   138.267207: task_numa_work: working

Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Mel Gorman <mgorman@...e.de>
Reviewed-by: Rik van Riel <riel@...hat.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1381141781-10992-14-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 817cd7b..573d815e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -946,6 +946,12 @@ void task_numa_work(struct callback_head *work)
 		return;
 
 	/*
+	 * Delay this task enough that another task of this mm will likely win
+	 * the next time around.
+	 */
+	p->node_stamp += 2 * TICK_NSEC;
+
+	/*
 	 * Do not set pte_numa if the current running node is rate-limited.
 	 * This loses statistics on the fault but if we are unwilling to
 	 * migrate to this node, it is less likely we can do useful work
@@ -1026,7 +1032,7 @@ void task_tick_numa(struct rq *rq, struct task_struct *curr)
 	if (now - curr->node_stamp > period) {
 		if (!curr->node_stamp)
 			curr->numa_scan_period = sysctl_numa_balancing_scan_period_min;
-		curr->node_stamp = now;
+		curr->node_stamp += period;
 
 		if (!time_before(jiffies, curr->mm->numa_next_scan)) {
 			init_task_work(work, task_numa_work); /* TODO: move this into sched_fork() */
--
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