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Johannes Weiner 90abee6d78 mm: page_alloc: speed up fallbacks in rmqueue_bulk() 
The test robot identified c2f6ea38fc ("mm: page_alloc: don't steal
single pages from biggest buddy") as the root cause of a 56.4% regression
in vm-scalability::lru-file-mmap-read.

Carlos reports an earlier patch, c0cd6f557b ("mm: page_alloc: fix
freelist movement during block conversion"), as the root cause for a
regression in worst-case zone->lock+irqoff hold times.

Both of these patches modify the page allocator's fallback path to be less
greedy in an effort to stave off fragmentation.  The flip side of this is
that fallbacks are also less productive each time around, which means the
fallback search can run much more frequently.

Carlos' traces point to rmqueue_bulk() specifically, which tries to refill
the percpu cache by allocating a large batch of pages in a loop.  It
highlights how once the native freelists are exhausted, the fallback code
first scans orders top-down for whole blocks to claim, then falls back to
a bottom-up search for the smallest buddy to steal.  For the next batch
page, it goes through the same thing again.

This can be made more efficient.  Since rmqueue_bulk() holds the
zone->lock over the entire batch, the freelists are not subject to outside
changes; when the search for a block to claim has already failed, there is
no point in trying again for the next page.

Modify __rmqueue() to remember the last successful fallback mode, and
restart directly from there on the next rmqueue_bulk() iteration.

Oliver confirms that this improves beyond the regression that the test
robot reported against c2f6ea38fc1b:

commit:
  f3b92176f4 ("tools/selftests: add guard region test for /proc/$pid/pagemap")
  c2f6ea38fc ("mm: page_alloc: don't steal single pages from biggest buddy")
  acc4d5ff0b ("Merge tag 'net-6.15-rc0' of git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")
  2c847f27c3 ("mm: page_alloc: speed up fallbacks in rmqueue_bulk()")   <--- your patch

f3b92176f4 c2f6ea38fc acc4d5ff0b 2c847f27c37da65a93d23c237c5
---------------- --------------------------- --------------------------- ---------------------------
         %stddev     %change         %stddev     %change         %stddev     %change         %stddev
             \          |                \          |                \          |                \
  25525364 ±  3%     -56.4%   11135467           -57.8%   10779336           +31.6%   33581409        vm-scalability.throughput

Carlos confirms that worst-case times are almost fully recovered
compared to before the earlier culprit patch:

  2dd482ba62 (before freelist hygiene):    1ms
  c0cd6f557b  (after freelist hygiene):   90ms
 next-20250319    (steal smallest buddy):  280ms
    this patch                          :    8ms

[jackmanb@google.com: comment updates]
  Link: https://lkml.kernel.org/r/D92AC0P9594X.3BML64MUKTF8Z@google.com
[hannes@cmpxchg.org: reset rmqueue_mode in rmqueue_buddy() error loop, per Yunsheng Lin]
  Link: https://lkml.kernel.org/r/20250409140023.GA2313@cmpxchg.org
Link: https://lkml.kernel.org/r/20250407180154.63348-1-hannes@cmpxchg.org
Fixes: c0cd6f557b ("mm: page_alloc: fix freelist movement during block conversion")
Fixes: c2f6ea38fc ("mm: page_alloc: don't steal single pages from biggest buddy")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reported-by: kernel test robot <oliver.sang@intel.com>
Reported-by: Carlos Song <carlos.song@nxp.com>
Tested-by: Carlos Song <carlos.song@nxp.com>
Tested-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202503271547.fc08b188-lkp@intel.com
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Tested-by: Shivank Garg <shivankg@amd.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>	[6.10+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2025-04-11 17:32:39 -07:00
arch A set of final cleanups for the timer subsystem: 2025-04-06 08:35:37 -07:00
block treewide: Switch/rename to timer_delete[_sync]() 2025-04-05 10:30:12 +02:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto This push fixes reverts the multibuffer hash testing as it is buggy. 2025-04-02 09:14:59 -07:00
Documentation mseal: fix typo and style in documentation 2025-04-11 17:32:35 -07:00
drivers fpga: tests: add module descriptions 2025-04-11 17:32:38 -07:00
fs Miscellaneous scheduler fixes/updates: 2025-04-06 10:44:58 -07:00
include locking/local_lock, mm: replace localtry_ helpers with local_trylock_t type 2025-04-11 17:32:35 -07:00
init Miscellaneous scheduler fixes/updates: 2025-04-06 10:44:58 -07:00
io_uring io_uring: always do atomic put from iowq 2025-04-03 08:31:57 -06:00
ipc treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
kernel Fix a perf events time accounting bug. 2025-04-06 10:48:12 -07:00
lib kunit: slub: add module description 2025-04-11 17:32:39 -07:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm mm: page_alloc: speed up fallbacks in rmqueue_bulk() 2025-04-11 17:32:39 -07:00
net treewide: Switch/rename to timer_delete[_sync]() 2025-04-05 10:30:12 +02:00
rust Kbuild updates for v6.15 2025-04-05 15:46:50 -07:00
samples samples/livepatch: add module descriptions 2025-04-11 17:32:38 -07:00
scripts Disable SLUB_TINY for build testing 2025-04-06 10:00:04 -07:00
security mseal sysmap: kernel config and header change 2025-04-01 15:17:14 -07:00
sound soundwire fix for 6.15-rc1 2025-04-06 12:04:53 -07:00
tools selftests/mm: generate a temporary mountpoint for cgroup filesystem 2025-04-11 17:32:37 -07:00
usr kbuild: hdrcheck: fix cross build with clang 2025-03-05 04:06:45 +09:00
virt ARM: 2025-03-25 14:22:07 -07:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.clippy.toml rust: give Clippy the minimum supported Rust version 2025-01-10 00:17:25 +01:00
.cocciconfig
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: Retire Ralf Baechle 2024-11-12 15:48:59 +01:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore kbuild: Create intermediate vmlinux build with relocations preserved 2025-03-17 00:29:50 +09:00
.mailmap mailmap: map Loic Poulain's old email addresses 2025-04-11 17:32:34 -07:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING
CREDITS Including fixes from netfilter. 2025-04-04 09:15:35 -07:00
Kbuild drm: ensure drm headers are self-contained and pass kernel-doc 2025-02-12 10:44:43 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS MAINTAINERS: add Andrew and Baoquan as kexec maintainers 2025-04-11 17:32:37 -07:00
Makefile Linux 6.15-rc1 2025-04-06 13:11:33 -07:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.