This is a note to let you know that I've just added the patch titled
sched/numa: Fix mm numa_scan_seq based unconditional scan
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
sched-numa-fix-mm-numa_scan_seq-based-unconditional-.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 364e7e94105819ed30f3ba32ed806e0d06715107
Author: Raghavendra K T <raghavendra.kt@xxxxxxx>
Date: Fri Oct 20 21:27:46 2023 +0530
sched/numa: Fix mm numa_scan_seq based unconditional scan
[ Upstream commit 84db47ca7146d7bd00eb5cf2b93989a971c84650 ]
Since commit fc137c0ddab2 ("sched/numa: enhance vma scanning logic")
NUMA Balancing allows updating PTEs to trap NUMA hinting faults if the
task had previously accessed VMA. However unconditional scan of VMAs are
allowed during initial phase of VMA creation until process's
mm numa_scan_seq reaches 2 even though current task had not accessed VMA.
Rationale:
- Without initial scan subsequent PTE update may never happen.
- Give fair opportunity to all the VMAs to be scanned and subsequently
understand the access pattern of all the VMAs.
But it has a corner case where, if a VMA is created after some time,
process's mm numa_scan_seq could be already greater than 2.
For e.g., values of mm numa_scan_seq when VMAs are created by running
mmtest autonuma benchmark briefly looks like:
start_seq=0 : 459
start_seq=2 : 138
start_seq=3 : 144
start_seq=4 : 8
start_seq=8 : 1
start_seq=9 : 1
This results in no unconditional PTE updates for those VMAs created after
some time.
Fix:
- Note down the initial value of mm numa_scan_seq in per VMA start_seq.
- Allow unconditional scan till start_seq + 2.
Result:
SUT: AMD EPYC Milan with 2 NUMA nodes 256 cpus.
base kernel: upstream 6.6-rc6 with Mels patches [1] applied.
kernbench
========== base patched %gain
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 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
Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Link: https://lore.kernel.org/r/2ea7cbce80ac7c62e90cbfb9653a7972f902439f.1697816692.git.raghavendra.kt@xxxxxxx
Stable-dep-of: 5f1b64e9a9b7 ("sched/numa: fix memory leak due to the overwritten vma->numab_state")
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 43c19d85dfe7f..20c96ce98751a 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -576,6 +576,9 @@ struct vma_numab_state {
*/
unsigned long pids_active[2];
+ /* MM scan sequence ID when scan first started after VMA creation */
+ int start_scan_seq;
+
/*
* MM scan sequence ID when the VMA was last completely scanned.
* A VMA is not eligible for scanning if prev_scan_seq == numa_scan_seq
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index db59bf549c644..934d6f198b073 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3197,7 +3197,7 @@ static bool vma_is_accessed(struct mm_struct *mm, struct vm_area_struct *vma)
* This is also done to avoid any side effect of task scanning
* amplifying the unfairness of disjoint set of VMAs' access.
*/
- if (READ_ONCE(current->mm->numa_scan_seq) < 2)
+ if ((READ_ONCE(current->mm->numa_scan_seq) - vma->numab_state->start_scan_seq) < 2)
return true;
pids = vma->numab_state->pids_active[0] | vma->numab_state->pids_active[1];
@@ -3349,6 +3349,8 @@ static void task_numa_work(struct callback_head *work)
if (!vma->numab_state)
continue;
+ vma->numab_state->start_scan_seq = mm->numa_scan_seq;
+
vma->numab_state->next_scan = now +
msecs_to_jiffies(sysctl_numa_balancing_scan_delay);
