Re: [RFC PATCH V3 0/1] sched/numa: Fix disjoint set vma scan regression
From: Raghavendra K T
Date: Thu Jun 08 2023 - 00:01:28 EST
On 6/7/2023 5:10 PM, Sapkal Swapnil wrote:
Hello Raghavendra,
On 5/31/2023 9:55 AM, Raghavendra K T wrote:
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, where 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 (for 128 threads on my machine).
This was found in internal LKP run and also reported by [4].
While RFC V1 [2] tried to address this issue, the logic had more
heuristics.
RFC V2 [3] was rewritten based on vma_size.
Current implementation drops some of additional logic for long running
task
and relooked some of the usage of READ_ONCE/WRITE_ONCE().
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.
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.
Your comments/Ideas are welcome.
Changes since:
RFC V2:
1) Drop reset of scan counter that tried to take care of long running
workloads
2) Correct usage of READ_ONCE/WRITE_ONCE (Bharata)
3) Base is 6.4.0-rc2
RFC 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 on 6.4.0-rc2 (that has w/ numascan
enhancement)
base-numascan base base+fix
real 1m1.507s 1m23.259s 1m2.632s
user 213m51.336s 251m46.363s 220m35.528s
sys 3m3.397s 0m12.492s 2m41.393s
numa_hit 5615517 4560123 4963875
numa_local 5615505 4560024 4963700
numa_other 12 99 175
numa_pte_updates 1822797 493 1559111
numa_hint_faults 1307113 523 1469031
numa_hint_faults_local 612617 488 884829
numa_pages_migrated 694370 35 584202
We can see regression in base real time recovered, but with some
additional
system time overhead.
Below is the mmtest autonuma performance
autonumabench
===========
(base 6.4.0-rc2 that has numascan enhancement)
base-numascan base base+fix
Amean syst-NUMA01 300.46 ( 0.00%) 23.97
* 92.02%* 67.18 * 77.64%*
Amean syst-NUMA01_THREADLOCAL 0.20 ( 0.00%) 0.22
* -9.15%* 0.22 * -9.15%*
Amean syst-NUMA02 0.70 ( 0.00%) 0.71
* -0.61%* 0.70 * 0.41%*
Amean syst-NUMA02_SMT 0.58 ( 0.00%) 0.62
* -5.38%* 0.61 * -3.67%*
Amean elsp-NUMA01 320.92 ( 0.00%) 276.13
* 13.96%* 324.11 * -0.99%*
Amean elsp-NUMA01_THREADLOCAL 1.02 ( 0.00%) 1.03
* -1.83%* 1.03 * -1.83%*
Amean elsp-NUMA02 3.16 ( 0.00%) 3.93 *
-24.20%* 3.14 * 0.81%*
Amean elsp-NUMA02_SMT 3.82 ( 0.00%) 3.87
* -1.27%* 3.44 * 9.90%*
Duration User 403532.43 279173.53 359098.23
Duration System 2114.31 179.20 481.54
Duration Elapsed 2312.20 2004.48 2335.84
Ops NUMA alloc hit 55795455.00 45452739.00
45500387.00
Ops NUMA alloc local 55794177.00 45435858.00
45500070.00
Ops NUMA base-page range updates 147858285.00 18601.00
42043107.00
Ops NUMA PTE updates 147858285.00 18601.00
42043107.00
Ops NUMA hint faults 150531983.00 18254.00
42450080.00
Ops NUMA hint local faults % 125691825.00 11964.00
32993313.00
Ops NUMA hint local percent 83.50
65.54 77.72
Ops NUMA pages migrated 13535786.00 2207.00
4654628.00
Ops AutoNUMA cost 753952.10 91.44
212633.14
Please note there is a system time overhead added for numa01 but we
still have very
good improvement w.r.t base without numascan.
I tested the patch with lkp autonuma benchmark on a dual socket 4th
Generation EPYC server (2 X 96C/192T) running in NPS1 mode. Below are
the results:
commit:
6.4.0-rc2
6.4.0-rc2+patch
6.4.0-rc2 6.4.0-rc2+patch
---------------- ---------------------------
%stddev %change %stddev
\ | \
501.84 -12.5% 439.14 numa01.seconds
228.66 -1.8% 224.44 numa01_THREAD_ALLOC.seconds
0.51 +21.6% 0.62 numa02.seconds
107.17 +0.0% 107.17 numa02_SMT.seconds
2936 -9.1% 2669 elapsed_time
794910 +3.7% 824178 system_time
474520 -17.5% 391331 user_time
Tested-by: Swapnil Sapkal <swapnil.sapkal@xxxxxxx>
[1] Link:
https://lore.kernel.org/lkml/cover.1677672277.git.raghavendra.kt@xxxxxxx/T/#t
[2] Link:
https://lore.kernel.org/lkml/cover.1683033105.git.raghavendra.kt@xxxxxxx/
[3] Link:
https://lore.kernel.org/lkml/cover.1684228065.git.raghavendra.kt@xxxxxxx/T/
[4] Link:
https://lore.kernel.org/lkml/db995c11-08ba-9abf-812f-01407f70a5d4@xxxxxxx/T/
Raghavendra K T (1):
sched/numa: Fix disjoint set vma scan regression
include/linux/mm_types.h | 1 +
kernel/sched/fair.c | 31 ++++++++++++++++++++++++-------
2 files changed, 25 insertions(+), 7 deletions(-)
--
Thanks and regards,
Swapnil
Thank you Swapnil.
It reminds again that LKP's numa01 = numa01_THREAD_ALLOC which has
regained numbers.
I will also wait if kernel-test-robot also sees issue fixed. and also
if Mel/Peter have any objections/comment on the direction.
Regards