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Message-ID: <301ffc14-84be-461a-91d8-ff5a97cef981@amd.com>
Date: Thu, 8 Jun 2023 09:31:01 +0530
From: Raghavendra K T <raghavendra.kt@....com>
To: Sapkal Swapnil <Swapnil.Sapkal@....com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
kernel test robot <oliver.sang@...el.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Mel Gorman <mgorman@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>, rppt@...nel.org,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Bharata B Rao <bharata@....com>,
Aithal Srikanth <sraithal@....com>
Subject: Re: [RFC PATCH V3 0/1] sched/numa: Fix disjoint set vma scan
regression
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@....com>
>
>> [1] Link:
>> https://lore.kernel.org/lkml/cover.1677672277.git.raghavendra.kt@amd.com/T/#t
>>
>> [2] Link:
>> https://lore.kernel.org/lkml/cover.1683033105.git.raghavendra.kt@amd.com/
>> [3] Link:
>> https://lore.kernel.org/lkml/cover.1684228065.git.raghavendra.kt@amd.com/T/
>>
>> [4] Link:
>> https://lore.kernel.org/lkml/db995c11-08ba-9abf-812f-01407f70a5d4@amd.com/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
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