[PATCH 2/2] sched,numa: cap pte scanning overhead to 3% of run time

[riel]

From: Rik van Riel <riel@xxxxxxxxxx>

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.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
Reported-and-tested-by: Jan Stancek <jstancek@xxxxxxxxxx>
---
 kernel/sched/fair.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f04fda8f669c..b0924377ab0d 100644
--- 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 @@ void task_numa_work(struct callback_head *work)
 	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;
+	}
 }
 
 /*
-- 
2.1.0

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