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

From: Raghavendra K T
Date: Tue Aug 29 2023 - 02:07:42 EST

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(-)

--
2.34.1