Commit 51170840 authored Nov 05, 2015 by Rik van Riel Committed by Ingo Molnar Nov 23, 2015

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>

parent 525705d1

kernel/sched/fair.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -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;
		}
		}

		/*

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