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Message-ID: <719f0729-d28f-d12f-cff4-ab8115861d30@amd.com>
Date: Tue, 19 Sep 2023 12:00:01 +0530
From: Raghavendra K T <raghavendra.kt@....com>
To: 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>,
Sapkal Swapnil <Swapnil.Sapkal@....com>,
K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [RFC PATCH V1 0/6] sched/numa: Enhance disjoint VMA scanning
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(-)
>
Hello Andrew,
I am Resending patch rebasing to mm-unstable, adding results from Oliver
and Swapnil.
(so that it is ready to merge once we get go ahead/ no objection from
Mel, Peter ... Okay to work any further suggestions if any).
Hope that is Okay.
Thanks and Regards
- Raghu
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