[PATCH V1 0/1] sched/numa: Fix mm numa_scan_seq based unconditional scan

From: Raghavendra K T
Date: Fri Oct 20 2023 - 11:57:37 EST

NUMA balancing code that updates PTEs by allowing unconditional scan
based on the value of processes' mm numa_scan_seq is not perfect.

More description is in patch1.

Have used the below patch to identify the corner case.

Detailed Result: (Only part of the result is updated
in patch1 to save space in commit log)

Detailed Result:

SUT: AMD EPYC Milan with 2 NUMA nodes 256 cpus.

Base kernel: upstream 6.6-rc6 (dd72f9c7e512) with Mels patch-series
from tip/sched/core [1] applied.

Summary: Some benchmarks imrove. There is increase in system
time due to additional scanning. But elapsed time shows gain.

However there is also some overhead seen for benchmarks like NUMA01.

kernbench
========== base patched
Amean user-128 13799.58 ( 0.00%) 13789.86 * 0.07%*
Amean syst-128 3280.80 ( 0.00%) 3249.67 * 0.95%*
Amean elsp-128 165.09 ( 0.00%) 164.78 * 0.19%*

Duration User 41404.28 41375.08
Duration System 9862.22 9768.48
Duration Elapsed 519.87 518.72

Ops NUMA PTE updates 1041416.00 831536.00
Ops NUMA hint faults 263296.00 220966.00
Ops NUMA pages migrated 258021.00 212769.00
Ops AutoNUMA cost 1328.67 1114.69

autonumabench

NUMA01_THREADLOCAL
==================
Amean syst-NUMA01_THREADLOCAL 10.65 ( 0.00%) 26.47 *-148.59%*
Amean elsp-NUMA01_THREADLOCAL 81.79 ( 0.00%) 67.74 * 17.18%*

Duration User 54832.73 47379.67
Duration System 75.00 185.75
Duration Elapsed 576.72 476.09

Ops NUMA PTE updates 394429.00 11121044.00
Ops NUMA hint faults 1001.00 8906404.00
Ops NUMA pages migrated 288.00 2998694.00
Ops AutoNUMA cost 7.77 44666.84

NUMA01
=====
Amean syst-NUMA01 31.97 ( 0.00%) 52.95 * -65.62%*
Amean elsp-NUMA01 143.16 ( 0.00%) 150.81 * -5.34%*

Duration User 84839.49 91342.19
Duration System 224.26 371.12
Duration Elapsed 1005.64 1059.01

Ops NUMA PTE updates 33929508.00 50116313.00
Ops NUMA hint faults 34993820.00 52895783.00
Ops NUMA pages migrated 5456115.00 7441228.00
Ops AutoNUMA cost 175310.27 264971.11

NUMA02
=========
Amean syst-NUMA02 0.86 ( 0.00%) 0.86 * -0.50%*
Amean elsp-NUMA02 3.99 ( 0.00%) 3.82 * 4.40%*

Duration User 1186.06 1092.07
Duration System 6.44 6.47
Duration Elapsed 31.28 30.30

Ops NUMA PTE updates 776.00 731.00
Ops NUMA hint faults 527.00 490.00
Ops NUMA pages migrated 183.00 153.00
Ops AutoNUMA cost 2.64 2.46

Link: https://lore.kernel.org/linux-mm/ZSXF3AFZgIld1meX@xxxxxxxxx/T/

Raghavendra K T (1):
sched/numa: Fix mm numa_scan_seq based unconditional scan

include/linux/mm_types.h | 3 +++
kernel/sched/fair.c | 4 +++-
2 files changed, 6 insertions(+), 1 deletion(-)

---8<---
diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 010ba1b7cb0e..a4870b01c8a1 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -10,6 +10,30 @@
#include <linux/tracepoint.h>
#include <linux/binfmts.h>

+TRACE_EVENT(sched_vma_start_seq,
+
+ TP_PROTO(struct task_struct *t, struct vm_area_struct *vma, int start_seq),
+
+ TP_ARGS(t, vma, start_seq),
+
+ TP_STRUCT__entry(
+ __array( char, comm, TASK_COMM_LEN )
+ __field( pid_t, pid )
+ __field( void *, vma )
+ __field( int, start_seq )
+ ),
+
+ TP_fast_assign(
+ memcpy(__entry->comm, t->comm, TASK_COMM_LEN);
+ __entry->pid = t->pid;
+ __entry->vma = vma;
+ __entry->start_seq = start_seq;
+ ),
+
+ TP_printk("comm=%s pid=%d vma = %px start_seq=%d", __entry->comm, __entry->pid, __entry->vma,
+ __entry->start_seq)
+);
+
/*
* Tracepoint for calling kthread_stop, performed to end a kthread:
*/
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c8af3a7ccba7..e0c16ea8470b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3335,6 +3335,7 @@ static void task_numa_work(struct callback_head *work)
continue;

vma->numab_state->start_scan_seq = mm->numa_scan_seq;
+ trace_sched_vma_start_seq(p, vma, mm->numa_scan_seq);

vma->numab_state->next_scan = now +
msecs_to_jiffies(sysctl_numa_balancing_scan_delay);


--
2.34.1