This is a note to let you know that I've just added the patch titled
sched/numa: Complete scanning of inactive VMAs when there is no alternative
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-complete-scanning-of-inactive-vmas-when-t.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 a5145866409098262ff88a4fba9602b1c2435ad9
Author: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Date: Tue Oct 10 09:31:43 2023 +0100
sched/numa: Complete scanning of inactive VMAs when there is no alternative
[ Upstream commit f169c62ff7cd1acf8bac8ae17bfeafa307d9e6fa ]
VMAs are skipped if there is no recent fault activity but this represents
a chicken-and-egg problem as there may be no fault activity if the PTEs
are never updated to trap NUMA hints. There is an indirect reliance on
scanning to be forced early in the lifetime of a task but this may fail
to detect changes in phase behaviour. Force inactive VMAs to be scanned
when all other eligible VMAs have been updated within the same scan
sequence.
Test results in general look good with some changes in performance, both
negative and positive, depending on whether the additional scanning and
faulting was beneficial or not to the workload. The autonuma benchmark
workload NUMA01_THREADLOCAL was picked for closer examination. The workload
creates two processes with numerous threads and thread-local storage that
is zero-filled in a loop. It exercises the corner case where unrelated
threads may skip VMAs that are thread-local to another thread and still
has some VMAs that inactive while the workload executes.
The VMA skipping activity frequency with and without the patch:
6.6.0-rc2-sched-numabtrace-v1
=============================
649 reason=scan_delay
9,094 reason=unsuitable
48,915 reason=shared_ro
143,919 reason=inaccessible
193,050 reason=pid_inactive
6.6.0-rc2-sched-numabselective-v1
=============================
146 reason=seq_completed
622 reason=ignore_pid_inactive
624 reason=scan_delay
6,570 reason=unsuitable
16,101 reason=shared_ro
27,608 reason=inaccessible
41,939 reason=pid_inactive
Note that with the patch applied, the PID activity is ignored
(ignore_pid_inactive) to ensure a VMA with some activity is completely
scanned. In addition, a small number of VMAs are scanned when no other
eligible VMA is available during a single scan window (seq_completed).
The number of times a VMA is skipped due to no PID activity from the
scanning task (pid_inactive) drops dramatically. It is expected that
this will increase the number of PTEs updated for NUMA hinting faults
as well as hinting faults but these represent PTEs that would otherwise
have been missed. The tradeoff is scan+fault overhead versus improving
locality due to migration.
On a 2-socket Cascade Lake test machine, the time to complete the
workload is as follows;
6.6.0-rc2 6.6.0-rc2
sched-numabtrace-v1 sched-numabselective-v1
Min elsp-NUMA01_THREADLOCAL 174.22 ( 0.00%) 117.64 ( 32.48%)
Amean elsp-NUMA01_THREADLOCAL 175.68 ( 0.00%) 123.34 * 29.79%*
Stddev elsp-NUMA01_THREADLOCAL 1.20 ( 0.00%) 4.06 (-238.20%)
CoeffVar elsp-NUMA01_THREADLOCAL 0.68 ( 0.00%) 3.29 (-381.70%)
Max elsp-NUMA01_THREADLOCAL 177.18 ( 0.00%) 128.03 ( 27.74%)
The time to complete the workload is reduced by almost 30%:
6.6.0-rc2 6.6.0-rc2
sched-numabtrace-v1 sched-numabselective-v1 /
Duration User 91201.80 63506.64
Duration System 2015.53 1819.78
Duration Elapsed 1234.77 868.37
In this specific case, system CPU time was not increased but it's not
universally true.
From vmstat, the NUMA scanning and fault activity is as follows;
6.6.0-rc2 6.6.0-rc2
sched-numabtrace-v1 sched-numabselective-v1
Ops NUMA base-page range updates 64272.00 26374386.00
Ops NUMA PTE updates 36624.00 55538.00
Ops NUMA PMD updates 54.00 51404.00
Ops NUMA hint faults 15504.00 75786.00
Ops NUMA hint local faults % 14860.00 56763.00
Ops NUMA hint local percent 95.85 74.90
Ops NUMA pages migrated 1629.00 6469222.00
Both the number of PTE updates and hint faults is dramatically
increased. While this is superficially unfortunate, it represents
ranges that were simply skipped without the patch. As a result
of the scanning and hinting faults, many more pages were also
migrated but as the time to completion is reduced, the overhead
is offset by the gain.
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Tested-by: Raghavendra K T <raghavendra.kt@xxxxxxx>
Link: https://lore.kernel.org/r/20231010083143.19593-7-mgorman@xxxxxxxxxxxxxxxxxxx
Stable-dep-of: f22cde4371f3 ("sched/numa: Fix the vma scan starving issue")
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 80d9d1b7685c6..43c19d85dfe7f 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -575,6 +575,12 @@ struct vma_numab_state {
* every VMA_PID_RESET_PERIOD jiffies:
*/
unsigned long pids_active[2];
+
+ /*
+ * 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
+ */
+ int prev_scan_seq;
};
/*
diff --git a/include/linux/sched/numa_balancing.h b/include/linux/sched/numa_balancing.h
index 7dcc0bdfddbbf..b69afb8630db4 100644
--- a/include/linux/sched/numa_balancing.h
+++ b/include/linux/sched/numa_balancing.h
@@ -22,6 +22,7 @@ enum numa_vmaskip_reason {
NUMAB_SKIP_SCAN_DELAY,
NUMAB_SKIP_PID_INACTIVE,
NUMAB_SKIP_IGNORE_PID,
+ NUMAB_SKIP_SEQ_COMPLETED,
};
#ifdef CONFIG_NUMA_BALANCING
diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 27b51c81b1067..010ba1b7cb0ea 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -671,7 +671,8 @@ DEFINE_EVENT(sched_numa_pair_template, sched_swap_numa,
EM( NUMAB_SKIP_INACCESSIBLE, "inaccessible" ) \
EM( NUMAB_SKIP_SCAN_DELAY, "scan_delay" ) \
EM( NUMAB_SKIP_PID_INACTIVE, "pid_inactive" ) \
- EMe(NUMAB_SKIP_IGNORE_PID, "ignore_pid_inactive" )
+ EM( NUMAB_SKIP_IGNORE_PID, "ignore_pid_inactive" ) \
+ EMe(NUMAB_SKIP_SEQ_COMPLETED, "seq_completed" )
/* Redefine for export. */
#undef EM
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 03eb1cab320d8..0af2be3ee849e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3233,6 +3233,8 @@ static void task_numa_work(struct callback_head *work)
unsigned long nr_pte_updates = 0;
long pages, virtpages;
struct vma_iterator vmi;
+ bool vma_pids_skipped;
+ bool vma_pids_forced = false;
SCHED_WARN_ON(p != container_of(work, struct task_struct, numa_work));
@@ -3275,7 +3277,6 @@ static void task_numa_work(struct callback_head *work)
*/
p->node_stamp += 2 * TICK_NSEC;
- start = mm->numa_scan_offset;
pages = sysctl_numa_balancing_scan_size;
pages <<= 20 - PAGE_SHIFT; /* MB in pages */
virtpages = pages * 8; /* Scan up to this much virtual space */
@@ -3285,6 +3286,16 @@ static void task_numa_work(struct callback_head *work)
if (!mmap_read_trylock(mm))
return;
+
+ /*
+ * VMAs are skipped if the current PID has not trapped a fault within
+ * the VMA recently. Allow scanning to be forced if there is no
+ * suitable VMA remaining.
+ */
+ vma_pids_skipped = false;
+
+retry_pids:
+ start = mm->numa_scan_offset;
vma_iter_init(&vmi, mm, start);
vma = vma_next(&vmi);
if (!vma) {
@@ -3335,6 +3346,13 @@ static void task_numa_work(struct callback_head *work)
/* Reset happens after 4 times scan delay of scan start */
vma->numab_state->pids_active_reset = vma->numab_state->next_scan +
msecs_to_jiffies(VMA_PID_RESET_PERIOD);
+
+ /*
+ * Ensure prev_scan_seq does not match numa_scan_seq,
+ * to prevent VMAs being skipped prematurely on the
+ * first scan:
+ */
+ vma->numab_state->prev_scan_seq = mm->numa_scan_seq - 1;
}
/*
@@ -3356,8 +3374,19 @@ static void task_numa_work(struct callback_head *work)
vma->numab_state->pids_active[1] = 0;
}
- /* Do not scan the VMA if task has not accessed */
- if (!vma_is_accessed(mm, vma)) {
+ /* Do not rescan VMAs twice within the same sequence. */
+ if (vma->numab_state->prev_scan_seq == mm->numa_scan_seq) {
+ mm->numa_scan_offset = vma->vm_end;
+ trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_SEQ_COMPLETED);
+ continue;
+ }
+
+ /*
+ * Do not scan the VMA if task has not accessed it, unless no other
+ * VMA candidate exists.
+ */
+ if (!vma_pids_forced && !vma_is_accessed(mm, vma)) {
+ vma_pids_skipped = true;
trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_PID_INACTIVE);
continue;
}
@@ -3386,8 +3415,28 @@ static void task_numa_work(struct callback_head *work)
cond_resched();
} while (end != vma->vm_end);
+
+ /* VMA scan is complete, do not scan until next sequence. */
+ vma->numab_state->prev_scan_seq = mm->numa_scan_seq;
+
+ /*
+ * Only force scan within one VMA at a time, to limit the
+ * cost of scanning a potentially uninteresting VMA.
+ */
+ if (vma_pids_forced)
+ break;
} for_each_vma(vmi, vma);
+ /*
+ * If no VMAs are remaining and VMAs were skipped due to the PID
+ * not accessing the VMA previously, then force a scan to ensure
+ * forward progress:
+ */
+ if (!vma && !vma_pids_forced && vma_pids_skipped) {
+ vma_pids_forced = true;
+ goto retry_pids;
+ }
+
out:
/*
* It is possible to reach the end of the VMA list but the last few
