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Message-Id: <1446756983-28173-3-git-send-email-riel@redhat.com>
Date: Thu, 5 Nov 2015 15:56:23 -0500
From: riel@...hat.com
To: linux-kernel@...r.kernel.org
Cc: peterz@...radead.org, mingo@...nel.org, jstancek@...hat.com,
mgorman@...e.de
Subject: [PATCH 2/2] sched,numa: cap pte scanning overhead to 3% of run time
From: Rik van Riel <riel@...hat.com>
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@...hat.com>
Reported-and-tested-by: Jan Stancek <jstancek@...hat.com>
---
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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