[PATCH 43/43] sched: numa: Increase and decrease a tasks scanning period based on task fault statistics

[Mel Gorman <mgorman@suse.de>]

Currently the rate of scanning for an address space is controlled by the
individual tasks. The next scan is determined by p->numa_scan_period
and slowly increases as NUMA faults are handled. This assumes there are
no phase changes.

Now that there is a policy in place that guesses if a task or process
is properly placed, use that information to grow/shrink the scanning
window on a per-task basis.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/sched/fair.c |   22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6d2ccd3..598f657 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1063,18 +1063,25 @@ static void task_numa_placement(struct task_struct *p)
 	}
 
 	/*
+	 * If this NUMA node is the selected on based on task NUMA
+	 * faults then increase the time before it scans again
+	 */
+	if (task_selected_nid == this_nid)
+		p->numa_scan_period = min(sysctl_balance_numa_scan_period_max,
+					  p->numa_scan_period * 2);
+
+	/*
 	 * If this NUMA node is the selected one based on process
 	 * memory and task NUMA faults then set the home node.
 	 * There should be no need to requeue the task.
 	 */
 	if (task_selected_nid == this_nid && mm_selected_nid == this_nid) {
-		p->numa_scan_period = min(sysctl_balance_numa_scan_period_max,
-					  p->numa_scan_period * 2);
 		p->home_node = this_nid;
 		return;
 	}
 
-	p->numa_scan_period = sysctl_balance_numa_scan_period_min;
+	p->numa_scan_period = max(sysctl_balance_numa_scan_period_min,
+				p->numa_scan_period / 2);
 	task_numa_find_placement(p);
 }
 
@@ -1110,15 +1117,6 @@ void task_numa_fault(int node, int pages)
 	p->mm->mm_balancenuma->mm_numa_fault_tot++;
 	p->mm->mm_balancenuma->mm_numa_fault[node]++;
 
-	/*
-	 * Assume that as faults occur that pages are getting properly placed
-	 * and fewer NUMA hints are required. Note that this is a big
-	 * assumption, it assumes processes reach a steady steady with no
-	 * further phase changes.
-	 */
-	p->numa_scan_period = min(sysctl_balance_numa_scan_period_max,
-				p->numa_scan_period + jiffies_to_msecs(2));
-
 	task_numa_placement(p);
 }
 
-- 
1.7.9.2

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