Re: [RFC PATCH V1 0/6] sched/numa: Enhance disjoint VMA scanning

[Raghavendra K T]

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@xxxxxxx/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@xxxxxxx/
>
> (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@xxxxxxx/T/#t
> 2. Link: https://lore.kernel.org/lkml/cover.1683033105.git.raghavendra.kt@xxxxxxx/
>
> 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