This is a note to let you know that I've just added the patch titled
sched/numa: Fix the vma scan starving issue
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-the-vma-scan-starving-issue.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 d086055a82fc3d3e926bd973e024e26792e39e59
Author: Yujie Liu <yujie.liu@xxxxxxxxx>
Date: Tue Aug 27 19:29:58 2024 +0800
sched/numa: Fix the vma scan starving issue
[ Upstream commit f22cde4371f3c624e947a35b075c06c771442a43 ]
Problem statement:
Since commit fc137c0ddab2 ("sched/numa: enhance vma scanning logic"), the
Numa vma scan overhead has been reduced a lot. Meanwhile, the reducing of
the vma scan might create less Numa page fault information. The
insufficient information makes it harder for the Numa balancer to make
decision. Later, commit b7a5b537c55c08 ("sched/numa: Complete scanning of
partial VMAs regardless of PID activity") and commit 84db47ca7146d7
("sched/numa: Fix mm numa_scan_seq based unconditional scan") are found to
bring back part of the performance.
Recently when running SPECcpu omnetpp_r on a 320 CPUs/2 Sockets system, a
long duration of remote Numa node read was observed by PMU events: A few
cores having ~500MB/s remote memory access for ~20 seconds. It causes
high core-to-core variance and performance penalty. After the
investigation, it is found that many vmas are skipped due to the active
PID check. According to the trace events, in most cases,
vma_is_accessed() returns false because the history access info stored in
pids_active array has been cleared.
Proposal:
The main idea is to adjust vma_is_accessed() to let it return true easier.
Thus compare the diff between mm->numa_scan_seq and
vma->numab_state->prev_scan_seq. If the diff has exceeded the threshold,
scan the vma.
This patch especially helps the cases where there are small number of
threads, like the process-based SPECcpu. Without this patch, if the
SPECcpu process access the vma at the beginning, then sleeps for a long
time, the pid_active array will be cleared. A a result, if this process
is woken up again, it never has a chance to set prot_none anymore.
Because only the first 2 times of access is granted for vma scan:
(current->mm->numa_scan_seq) - vma->numab_state->start_scan_seq) < 2 to be
worse, no other threads within the task can help set the prot_none. This
causes information lost.
Raghavendra helped test current patch and got the positive result
on the AMD platform:
autonumabench NUMA01
base patched
Amean syst-NUMA01 194.05 ( 0.00%) 165.11 * 14.92%*
Amean elsp-NUMA01 324.86 ( 0.00%) 315.58 * 2.86%*
Duration User 380345.36 368252.04
Duration System 1358.89 1156.23
Duration Elapsed 2277.45 2213.25
autonumabench NUMA02
Amean syst-NUMA02 1.12 ( 0.00%) 1.09 * 2.93%*
Amean elsp-NUMA02 3.50 ( 0.00%) 3.56 * -1.84%*
Duration User 1513.23 1575.48
Duration System 8.33 8.13
Duration Elapsed 28.59 29.71
kernbench
Amean user-256 22935.42 ( 0.00%) 22535.19 * 1.75%*
Amean syst-256 7284.16 ( 0.00%) 7608.72 * -4.46%*
Amean elsp-256 159.01 ( 0.00%) 158.17 * 0.53%*
Duration User 68816.41 67615.74
Duration System 21873.94 22848.08
Duration Elapsed 506.66 504.55
Intel 256 CPUs/2 Sockets:
autonuma benchmark also shows improvements:
v6.10-rc5 v6.10-rc5
+patch
Amean syst-NUMA01 245.85 ( 0.00%) 230.84 * 6.11%*
Amean syst-NUMA01_THREADLOCAL 205.27 ( 0.00%) 191.86 * 6.53%*
Amean syst-NUMA02 18.57 ( 0.00%) 18.09 * 2.58%*
Amean syst-NUMA02_SMT 2.63 ( 0.00%) 2.54 * 3.47%*
Amean elsp-NUMA01 517.17 ( 0.00%) 526.34 * -1.77%*
Amean elsp-NUMA01_THREADLOCAL 99.92 ( 0.00%) 100.59 * -0.67%*
Amean elsp-NUMA02 15.81 ( 0.00%) 15.72 * 0.59%*
Amean elsp-NUMA02_SMT 13.23 ( 0.00%) 12.89 * 2.53%*
v6.10-rc5 v6.10-rc5
+patch
Duration User 1064010.16 1075416.23
Duration System 3307.64 3104.66
Duration Elapsed 4537.54 4604.73
The SPECcpu remote node access issue disappears with the patch applied.
Link: https://lkml.kernel.org/r/20240827112958.181388-1-yu.c.chen@xxxxxxxxx
Fixes: fc137c0ddab2 ("sched/numa: enhance vma scanning logic")
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Co-developed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Signed-off-by: Yujie Liu <yujie.liu@xxxxxxxxx>
Reported-by: Xiaoping Zhou <xiaoping.zhou@xxxxxxxxx>
Reviewed-and-tested-by: Raghavendra K T <raghavendra.kt@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: "Chen, Tim C" <tim.c.chen@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Raghavendra K T <raghavendra.kt@xxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0af2be3ee849e..5eb4807bad209 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3213,6 +3213,15 @@ static bool vma_is_accessed(struct mm_struct *mm, struct vm_area_struct *vma)
return true;
}
+ /*
+ * This vma has not been accessed for a while, and if the number
+ * the threads in the same process is low, which means no other
+ * threads can help scan this vma, force a vma scan.
+ */
+ if (READ_ONCE(mm->numa_scan_seq) >
+ (vma->numab_state->prev_scan_seq + get_nr_threads(current)))
+ return true;
+
return false;
}
