[RFC PATCH V2 0/1] sched/numa: Fix disjoint set vma scan regression
From: Raghavendra K T
Date: Tue May 16 2023 - 05:20:50 EST
With the numa scan enhancements [1], only the threads which had previously
accessed vma are allowed to scan.
While this had improved significant system time overhead, there were corner
cases, which genuinely need some relaxation for e.g., concern raised by
PeterZ where unfairness amongst the thread belonging to disjoint set of vmas,
that can potentially amplify the side effects of vma regions belonging to some
of the tasks being left unscanned.
[1] had handled that issue by allowing first two scans at mm level
(mm->numa_scan_seq) unconditionally. But that was not enough.
One of the test that exercise similar side effect is numa01_THREAD_ALLOC where
allocation happen by main thread and it is divided into memory chunks of 24MB
to be continuously bzeroed.
(this is run default by LKP tests while numa01 is run default in mmtests which
operate on full 3GB region by each thread)
While RFC [2] tried to address this issue, the logic had more heuristics.
After posting [2], [3] also confirmed same regression.
The current patch addresses the same issue in a more accurate way as
follows:
(1) Any disjoint vma which is not associated with a task, that tries to
scan is now allowed to induce prot_none faults. Total number of such
unconditional scans allowed per vma is derived based on the exact vma size
as follows:
total scans allowed = 1/2 * vma_size / scan_size.
(2) Total scans already done is maintained using a per vma scan counter.
(3) For a very long running task, this scan counter is reset after 16
times whole mm of task scanning took place (using mm->numa_scan_seq).
With above patch, numa01_THREAD_ALLOC regression reported is resolved,
but please note that with [1] there was a drastic decrease in system time
for mmtest numa01, this patch adds back some of the system time.
Summary: numa scan enhancement patch [1] togethor with the current patchset
improves overall system time by filtering unnecessary numa scan
while still retaining necessary scanning in some corner cases which
involves disjoint set vmas.
(Mel, PeterZ this patch looks more precise handling of the issue)
Your comments/Ideas are welcome.
Changes since V1:
1) Rewrite entire logic based on actual vma size than heuristics
2) Added Reported-by kernel test robot and internal LKP test
3) Rebased to 6.4.-rc1 (ba0ad6ed89)
Result:
SUT: Milan w/ 2 numa nodes 256 cpus
Run of numa01_THREAD__ALLOC 6.4.0-rc1 (that has w/ numascan enhancement)
base-numascan base base+fix
real 1m3.025s 1m24.163s 1m3.551s
user 213m44.232s 251m3.638s 219m55.662s
sys 6m26.598s 0m13.056s 2m35.767s
numa_hit 5478165 4395752 4907431
numa_local 5478103 4395366 4907044
numa_other 62 386 387
numa_pte_updates 1989274 11606 1265014
numa_hint_faults 1756059 515 1135804
numa_hint_faults_local 971500 486 558076
numa_pages_migrated 784211 29 577728
Below is the mmtest autonuma performance
autonuma
===========
base: 6.4.0-rc1+
base w/o numascan base(=w/ numascan) base + fix
Amean syst-NUMA01 247.46 ( 0.00%) 18.52 * 92.51%* 148.18 * 40.12%*
Amean syst-NUMA01_THREADLOCAL 0.23 ( 0.00%) 0.21 * 5.06%* 0.22 * 1.90%*
Amean syst-NUMA02 0.70 ( 0.00%) 0.70 * 1.02%* 0.73 * -3.46%*
Amean syst-NUMA02_SMT 0.59 ( 0.00%) 0.59 * 0.00%* 0.58 * 2.42%*
Amean elsp-NUMA01 309.54 ( 0.00%) 284.57 * 8.07%* 306.84 * 0.87%*
Amean elsp-NUMA01_THREADLOCAL 1.02 ( 0.00%) 1.02 * 0.42%* 1.04 * -1.53%*
Amean elsp-NUMA02 3.22 ( 0.00%) 3.55 * -10.21%* 3.32 * -3.15%*
Amean elsp-NUMA02_SMT 3.71 ( 0.00%) 3.86 * -4.08%* 3.74 * -0.69%*
Duration User 383183.43 294971.18 357446.52
Duration System 1743.53 140.85 1048.57
Duration Elapsed 2232.09 2062.33 2214.44
Ops NUMA alloc hit 57057379.00 43378289.00 51885613.00
Ops NUMA alloc local 57055256.00 43377265.00 51884407.00
Ops NUMA base-page range updates 137882746.00 25895.00 83600214.00
Ops NUMA PTE updates 137882746.00 25895.00 83600214.00
Ops NUMA hint faults 139609832.00 22651.00 84634363.00
Ops NUMA hint local faults % 113091055.00 18200.00 65809169.00
Ops NUMA hint local percent 81.01 80.35 77.76
Ops NUMA pages migrated 13415929.00 1798.00 9638327.00
Ops AutoNUMA cost 699269.24 113.47 423940.14
links:
[1] https://lore.kernel.org/lkml/cover.1677672277.git.raghavendra.kt@xxxxxxx/T/#t
[2] https://lore.kernel.org/lkml/cover.1683033105.git.raghavendra.kt@xxxxxxx/
[3] https://lore.kernel.org/lkml/db995c11-08ba-9abf-812f-01407f70a5d4@xxxxxxx/T/
Note: I have updated patch-1 with appropriate log required for the commit, so some of
above result/info is duplicated.
Raghavendra K T (1):
sched/numa: Fix disjoint set vma scan regression
include/linux/mm_types.h | 1 +
kernel/sched/fair.c | 41 ++++++++++++++++++++++++++++++++--------
2 files changed, 34 insertions(+), 8 deletions(-)
--
2.34.1