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Date: Tue, 19 Sep 2023 09:15:10 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Raghavendra K T <raghavendra.kt@....com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
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
* Raghavendra K T <raghavendra.kt@....com> wrote:
> 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.
Just for the record, a final version of this series should be submitted via
the scheduler tree, not -mm.
Thanks,
Ingo
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