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Subject[tip:sched/core] sched/numa: Stagger NUMA balancing scan periods for new threads
Commit-ID:  1378447598432513d94ce2c607c412dc4f260f31
Gitweb: https://git.kernel.org/tip/1378447598432513d94ce2c607c412dc4f260f31
Author: Mel Gorman <mgorman@techsingularity.net>
AuthorDate: Fri, 4 May 2018 16:41:09 +0100
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 14 May 2018 09:12:24 +0200

sched/numa: Stagger NUMA balancing scan periods for new threads

Threads share an address space and each can change the protections of the
same address space to trap NUMA faults. This is redundant and potentially
counter-productive as any thread doing the update will suffice. Potentially
only one thread is required but that thread may be idle or it may not have
any locality concerns and pick an unsuitable scan rate.

This patch uses independent scan period but they are staggered based on
the number of address space users when the thread is created. The intent
is that threads will avoid scanning at the same time and have a chance
to adapt their scan rate later if necessary. This reduces the total scan
activity early in the lifetime of the threads.

The different in headline performance across a range of machines and
workloads is marginal but the system CPU usage is reduced as well as overall
scan activity. The following is the time reported by NAS Parallel Benchmark
using unbound openmp threads and a D size class:

4.17.0-rc1 4.17.0-rc1
vanilla stagger-v1r1
Time bt.D 442.77 ( 0.00%) 419.70 ( 5.21%)
Time cg.D 171.90 ( 0.00%) 180.85 ( -5.21%)
Time ep.D 33.10 ( 0.00%) 32.90 ( 0.60%)
Time is.D 9.59 ( 0.00%) 9.42 ( 1.77%)
Time lu.D 306.75 ( 0.00%) 304.65 ( 0.68%)
Time mg.D 54.56 ( 0.00%) 52.38 ( 4.00%)
Time sp.D 1020.03 ( 0.00%) 903.77 ( 11.40%)
Time ua.D 400.58 ( 0.00%) 386.49 ( 3.52%)

Note it's not a universal win but we have no prior knowledge of which
thread matters but the number of threads created often exceeds the size
of the node when the threads are not bound. However, there is a reducation
of overall system CPU usage:

4.17.0-rc1 4.17.0-rc1
vanilla stagger-v1r1
sys-time-bt.D 48.78 ( 0.00%) 48.22 ( 1.15%)
sys-time-cg.D 25.31 ( 0.00%) 26.63 ( -5.22%)
sys-time-ep.D 1.65 ( 0.00%) 0.62 ( 62.42%)
sys-time-is.D 40.05 ( 0.00%) 24.45 ( 38.95%)
sys-time-lu.D 37.55 ( 0.00%) 29.02 ( 22.72%)
sys-time-mg.D 47.52 ( 0.00%) 34.92 ( 26.52%)
sys-time-sp.D 119.01 ( 0.00%) 109.05 ( 8.37%)
sys-time-ua.D 51.52 ( 0.00%) 45.13 ( 12.40%)

NUMA scan activity is also reduced:

NUMA alloc local 1042828 1342670
NUMA base PTE updates 140481138 93577468
NUMA huge PMD updates 272171 180766
NUMA page range updates 279832690 186129660
NUMA hint faults 1395972 1193897
NUMA hint local faults 877925 855053
NUMA hint local percent 62 71
NUMA pages migrated 12057909 9158023

Similar observations are made for other thread-intensive workloads. System
CPU usage is lower even though the headline gains in performance tend to be
small. For example, specjbb 2005 shows almost no difference in performance
but scan activity is reduced by a third on a 4-socket box. I didn't find
a workload (thread intensive or otherwise) that suffered badly.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/20180504154109.mvrha2qo5wdl65vr@techsingularity.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/sched/core.c | 22 +---------------------
kernel/sched/fair.c | 41 +++++++++++++++++++++++++++++++++++++++++
kernel/sched/sched.h | 6 ++++++
3 files changed, 48 insertions(+), 21 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4e0ebae045dc..102c36c317dc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2177,27 +2177,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
INIT_HLIST_HEAD(&p->preempt_notifiers);
#endif

-#ifdef CONFIG_NUMA_BALANCING
- if (p->mm && atomic_read(&p->mm->mm_users) == 1) {
- p->mm->numa_next_scan = jiffies + msecs_to_jiffies(sysctl_numa_balancing_scan_delay);
- p->mm->numa_scan_seq = 0;
- }
-
- if (clone_flags & CLONE_VM)
- p->numa_preferred_nid = current->numa_preferred_nid;
- else
- p->numa_preferred_nid = -1;
-
- p->node_stamp = 0ULL;
- p->numa_scan_seq = p->mm ? p->mm->numa_scan_seq : 0;
- p->numa_scan_period = sysctl_numa_balancing_scan_delay;
- p->numa_work.next = &p->numa_work;
- p->numa_faults = NULL;
- p->last_task_numa_placement = 0;
- p->last_sum_exec_runtime = 0;
-
- p->numa_group = NULL;
-#endif /* CONFIG_NUMA_BALANCING */
+ init_numa_balancing(clone_flags, p);
}

DEFINE_STATIC_KEY_FALSE(sched_numa_balancing);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 43c7b45f20be..f32b97d4c63b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1139,6 +1139,47 @@ static unsigned int task_scan_max(struct task_struct *p)
return max(smin, smax);
}

+void init_numa_balancing(unsigned long clone_flags, struct task_struct *p)
+{
+ int mm_users = 0;
+ struct mm_struct *mm = p->mm;
+
+ if (mm) {
+ mm_users = atomic_read(&mm->mm_users);
+ if (mm_users == 1) {
+ mm->numa_next_scan = jiffies + msecs_to_jiffies(sysctl_numa_balancing_scan_delay);
+ mm->numa_scan_seq = 0;
+ }
+ }
+ p->node_stamp = 0;
+ p->numa_scan_seq = mm ? mm->numa_scan_seq : 0;
+ p->numa_scan_period = sysctl_numa_balancing_scan_delay;
+ p->numa_work.next = &p->numa_work;
+ p->numa_faults = NULL;
+ p->numa_group = NULL;
+ p->last_task_numa_placement = 0;
+ p->last_sum_exec_runtime = 0;
+
+ /* New address space, reset the preferred nid */
+ if (!(clone_flags & CLONE_VM)) {
+ p->numa_preferred_nid = -1;
+ return;
+ }
+
+ /*
+ * New thread, keep existing numa_preferred_nid which should be copied
+ * already by arch_dup_task_struct but stagger when scans start.
+ */
+ if (mm) {
+ unsigned int delay;
+
+ delay = min_t(unsigned int, task_scan_max(current),
+ current->numa_scan_period * mm_users * NSEC_PER_MSEC);
+ delay += 2 * TICK_NSEC;
+ p->node_stamp = delay;
+ }
+}
+
static void account_numa_enqueue(struct rq *rq, struct task_struct *p)
{
rq->nr_numa_running += (p->numa_preferred_nid != -1);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 15750c222ca2..c9895d35c5f7 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1069,6 +1069,12 @@ enum numa_faults_stats {
extern void sched_setnuma(struct task_struct *p, int node);
extern int migrate_task_to(struct task_struct *p, int cpu);
extern int migrate_swap(struct task_struct *, struct task_struct *);
+extern void init_numa_balancing(unsigned long clone_flags, struct task_struct *p);
+#else
+static inline void
+init_numa_balancing(unsigned long clone_flags, struct task_struct *p)
+{
+}
#endif /* CONFIG_NUMA_BALANCING */

#ifdef CONFIG_SMP
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