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Message-ID: <a249f3ea-7663-d906-b69f-2ea0e3a99a55@amd.com>
Date: Wed, 13 Sep 2023 11:54:21 +0530
From: Raghavendra K T <raghavendra.kt@....com>
To: Swapnil Sapkal <Swapnil.Sapkal@....com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
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>,
kernel test robot <oliver.sang@...el.com>,
K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [RFC PATCH V1 0/6] sched/numa: Enhance disjoint VMA scanning
On 9/13/2023 10:58 AM, Swapnil Sapkal wrote:
> Hello Raghu,
>
> On 8/29/2023 11:36 AM, Raghavendra K T wrote:
>> Since commit fc137c0ddab2 ("sched/numa: enhance vma scanning logic") [1]
>> VMA scanning is allowed if:
>> 1) The task had accessed the VMA.
>> Rationale: Reduce overhead for the tasks that had not
>> touched VMA. Also filter out unnecessary scanning.
>>
>> 2) Early phase of the VMA scan where mm->numa_scan_seq is less than 2.
>> Rationale: Understanding initial characteristics of VMAs and also
>> prevent VMA scanning unfairness.
>>
>> While that works for most of the times to reduce scanning overhead,
>> there are some corner cases associated with it.
>>
>> This was found in an internal LKP run and also reported by [2]. There was
>> an attempt to fix.
>>
>> Link:
>> https://lore.kernel.org/linux-mm/cover.1685506205.git.raghavendra.kt@amd.com/T/
>>
>>
>> This is a fully different series after Mel's feedback to address the
>> issue
>> and also a continuation of enhancing VMA scanning for NUMA balancing.
>>
>> Problem statement (Disjoint VMA set):
>> ======================================
>> Let's look at some of the corner cases with a below example of tasks
>> and their
>> access pattern.
>>
>> Consider N tasks (threads) of a process.
>> Set1 tasks accessing vma_x (group of VMAs)
>> Set2 tasks accessing vma_y (group of VMAs)
>>
>> Set1 Set2
>> ------------------- --------------------
>> | task_1..task_n/2 | | task_n/2+1..task_n |
>> ------------------- --------------------
>> | |
>> V V
>> ------------------- --------------------
>> | vma_x | | vma_y |
>> ------------------- --------------------
>>
>> Corner cases:
>> (a) Out of N tasks, not all of them gets fair opportunity to scan.
>> (PeterZ).
>> suppose Set1 tasks gets more opportunity to scan (May be because of the
>> activity pattern of tasks or other reasons in current design) in the
>> above
>> example, then vma_x gets scanned more number of times than vma_y.
>>
>> some experiment is also done here which illustrates this unfairness:
>> Link:
>> https://lore.kernel.org/lkml/c730dee0-a711-8a8e-3eb1-1bfdd21e6add@amd.com/
>>
>>
>> (b) Sizes of vmas can differ.
>> Suppose size of vma_y is far greater than the size of vma_x, then a
>> bigger
>> portion of vma_y can potentially be left unscanned since scanning is
>> bounded
>> by scan_size of 256MB (default) for each iteration.
>>
>> (c) Highly active threads trap a few VMAs frequently, and some of the
>> VMAs not
>> accessed for long time can potentially get starved of scanning
>> indefinitely
>> (Mel). There is a possibility of lack of enough hints/details about
>> VMAs if it
>> is needed later for migration.
>>
>> (d) Allocation of memory in some specific manner (Mel).
>> One example could be, Suppose a main thread allocates memory and it is
>> not
>> active. When other threads tries to act upon it, they may not have much
>> hints about it, if the corresponding VMA was not scanned.
>>
>> (e) VMAs that are created after two full scans of mm
>> (mm->numa_scan_seq > 2)
>> will never get scanned. (Observed rarely but very much possible
>> depending on
>> workload behaviour).
>>
>> Above this, a combination of some of the above (e.g., (a) and (b)) can
>> potentially amplifyi/worsen the side effect.
>>
>> This patchset, tries to address the above issues by enhancing
>> unconditional
>> VMA scanning logic.
>>
>> High level ideas:
>> =================
>> Idea-1) Depending on vma_size, populate a per vma_scan_select value,
>> decrement it
>> and when it hits zero do force scan (Mel).
>> vma_scan_select value is again repopulated when it hits zero.
>>
>> This is how VMA scanning phases looks like after implementation:
>>
>> |<---p1--->|<-----p2----->|<-----p2----->|...
>>
>> Algorithm:
>> p1: New VMA, initial phase do not scan till scan_delay.
>>
>> p2: Allow scanning if the task has accessed VMA or vma_scan_select hit
>> zero.
>>
>> Reinitialize vma_scan_select and repeat p2.
>>
>> pros/cons:
>> + : Ratelimiting is inbuilt to the approach
>> + : vma_size is taken into account for scanning
>> +/-: Scanning continues forever
>> - : Changes in vma size is taken care after force scan. i.e.,
>> vma_scan_select is repopulated only after vma_scan_select hits zero.
>>
>> Idea-1 can potentially cover all the issues mentioned above.
>>
>> Idea-2) Take bitmask_weight of latest access_pids value (suggested by
>> Bharata).
>> If number of tasks accessing vma is >= 1, unconditionally allow scanning.
>>
>> Idea-3 ) Take bitmask_weight of access_pid history of VMA. If number
>> of tasks
>> accessing VMA is > THRESHOLD (=3), unconditionally allow scanning.
>>
>> Rationale (Idea-2,3): Do not miss out scanning of critical VMAs.
>>
>> Idea-4) Have a per vma_scan_seq. allow the unconditional scan till
>> vma_scan_seq
>> reaches a value proportional (or equal) to vma_size/scan_size.
>> This a complimentary to Idea-1.
>>
>> this is how VMA scanning phases looks like after implementation:
>>
>> |<--p1--->|<-----p2----->|<-----p3----->|<-----p4----->...||<-----p2----->|<-----p3----->|<-----p4----->
>> ...||
>>
>> RESET RESET
>> Algorithm:
>> p1: New VMA, initial phase do not scan till scan_delay.
>>
>> p2: Allow scanning if task has accessed VMA or vma_scan_seq has
>> reached till
>> f(vma_size)/scan_size) for e.g., f = 1/2 * vma_size/scan_size.
>>
>> p3: Allow scanning if task has accessed VMA or vma_scan_seq has
>> reached till
>> f(vma_size)/scan_size in a rate limited manner. This is an optional
>> phase.
>>
>> p4: Allow scanning iff task has accessed VMA.
>>
>> Reset after p4 (optional).
>>
>> Repeat p2, p3 p4
>>
>> Motivation: Allow agressive scanning in the beginning followed by a rate
>> limited scanning. And then completely disallow scanning to avoid
>> unnecessary
>> scanning. Reset time could be a function of scan_delay and chosen long
>> enough
>> to aid long running task to forget history and start afresh.
>>
>> + : Ratelimiting need to be taken care separately if needed.
>> +/-: Scanning continues only if RESET of vma_scan_seq is implemented.
>> + : changes in vma size is taken care in every scan.
>>
>> Current patch series implements Ideas 1, 2, 3 + extension of access
>> PID history
>> idea from PeterZ.
>>
>> Results:
>> ======
>> Base: 6.5.0-rc6+ (4853c74bd7ab)
>> SUT: Milan w/ 2 numa nodes 256 cpus
>>
>> mmtest numa01_THREAD_ALLOC manual run:
>>
>> base patched
>> real 1m22.758s 1m9.200s
>> user 249m49.540s 229m30.039s
>> sys 0m25.040s 3m10.451s
>>
>> numa_pte_updates 6985 1573363
>> numa_hint_faults 2705 1022623
>> numa_hint_faults_local 2279 389633
>> numa_pages_migrated 426 632990
>>
>> kernbench
>> base patched
>> Amean user-256 21989.09 ( 0.00%) 21677.36 * 1.42%*
>> Amean syst-256 10171.34 ( 0.00%) 10818.28 * -6.36%*
>> Amean elsp-256 166.81 ( 0.00%) 168.40 * -0.95%*
>>
>> Duration User 65973.18 65038.00
>> Duration System 30538.92 32478.59
>> Duration Elapsed 529.52 533.09
>>
>> Ops NUMA PTE updates 976844.00 962680.00
>> Ops NUMA hint faults 226763.00 245620.00
>> Ops NUMA pages migrated 220146.00 207025.00
>> Ops AutoNUMA cost 1144.84 1238.77
>>
>> Improvements in other benchmarks I have tested.
>> Time based:
>> Hashjoin 4.21%
>> Btree 2.04%
>> XSbench 0.36%
>>
>> Throughput based:
>> Graph500 -3.62%
>> Nas.bt 3.69%
>> Nas.ft 21.91%
>>
>> Note: VMA scanning improvements [1] has refined scanning so much that
>> system overhead we re-introduce with additional scan look glaringly
>> high. But If we consider the difference between before [1] and current
>> series, overall scanning overhead is considerably reduced.
>>
>> 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/
>>
>> Note: Patch description is again repeated in some patches to avoid any
>> need to copy from cover letter again.
>>
>> Peter Zijlstra (1):
>> sched/numa: Increase tasks' access history
>>
>> Raghavendra K T (5):
>> sched/numa: Move up the access pid reset logic
>> sched/numa: Add disjoint vma unconditional scan logic
>> sched/numa: Remove unconditional scan logic using mm numa_scan_seq
>> sched/numa: Allow recently accessed VMAs to be scanned
>> sched/numa: Allow scanning of shared VMAs
>>
>> include/linux/mm.h | 12 +++--
>> include/linux/mm_types.h | 5 +-
>> kernel/sched/fair.c | 109 ++++++++++++++++++++++++++++++++-------
>> 3 files changed, 102 insertions(+), 24 deletions(-)
>>
>
> I have tested this series on 4th generation EPYC processor. I am seeing
> improvement in autonuma-benchmark with the series.
>
> o System Details
>
> - 4th Generation EPYC System
> - 2 x 128C/256T
> - NPS1 mode
>
> o Kernels
>
> base: 4853c74bd7ab Merge tag 'parisc-for-6.5-rc7' of
> git://git.kernel.org/pub/scm/linux/kernel/git/deller/parisc-linux
>
>
> ==================================================================
> Test : autonuma-benchmark
> Units : Time in seconds
> Interpretation: Lower is better
> Statistic : AMean
> ==================================================================
> commit:
> base (4853c74bd7ab)
> base + this_series
>
> base (4853c74bd7ab) base + this_series
> ---------------- ---------------------------
> %stddev %change %stddev
> \ | \
> 522.58 -11.2% 464.23
> autonuma-benchmark.numa01.seconds
> 273.93 -1.2% 270.75
> autonuma-benchmark.numa01_THREAD_ALLOC.seconds
> 0.60 +0.0% 0.60
> autonuma-benchmark.numa02.seconds
> 102.68 +3.7% 106.50
> autonuma-benchmark.numa02_SMT.seconds
>
> Tested-by: Swapnil Sapkal <Swapnil.Sapkal@....com>
>
Thank you Swapnil.
Regards
- Raghu
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