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Rafael Aquini 0365c9029a ipc: replace costly bailout check in sysvipc_find_ipc() 
commit 20401d1058f3f841f35a594ac2fc1293710e55b9 upstream.

sysvipc_find_ipc() was left with a costly way to check if the offset
position fed to it is bigger than the total number of IPC IDs in use.  So
much so that the time it takes to iterate over /proc/sysvipc/* files grows
exponentially for a custom benchmark that creates "N" SYSV shm segments
and then times the read of /proc/sysvipc/shm (milliseconds):

    12 msecs to read   1024 segs from /proc/sysvipc/shm
    18 msecs to read   2048 segs from /proc/sysvipc/shm
    65 msecs to read   4096 segs from /proc/sysvipc/shm
   325 msecs to read   8192 segs from /proc/sysvipc/shm
  1303 msecs to read  16384 segs from /proc/sysvipc/shm
  5182 msecs to read  32768 segs from /proc/sysvipc/shm

The root problem lies with the loop that computes the total amount of ids
in use to check if the "pos" feeded to sysvipc_find_ipc() grew bigger than
"ids->in_use".  That is a quite inneficient way to get to the maximum
index in the id lookup table, specially when that value is already
provided by struct ipc_ids.max_idx.

This patch follows up on the optimization introduced via commit
15df03c879 ("sysvipc: make get_maxid O(1) again") and gets rid of the
aforementioned costly loop replacing it by a simpler checkpoint based on
ipc_get_maxidx() returned value, which allows for a smooth linear increase
in time complexity for the same custom benchmark:

     2 msecs to read   1024 segs from /proc/sysvipc/shm
     2 msecs to read   2048 segs from /proc/sysvipc/shm
     4 msecs to read   4096 segs from /proc/sysvipc/shm
     9 msecs to read   8192 segs from /proc/sysvipc/shm
    19 msecs to read  16384 segs from /proc/sysvipc/shm
    39 msecs to read  32768 segs from /proc/sysvipc/shm

Link: https://lkml.kernel.org/r/20210809203554.1562989-1-aquini@redhat.com
Signed-off-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Waiman Long <llong@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Hugo SIMELIERE <hsimeliere.opensource@witekio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2024-09-04 13:17:44 +02:00
arch KVM: arm64: Don't use cbz/adr with external symbols 2024-09-04 13:17:44 +02:00
block block/ioctl: prefer different overflow check 2024-07-05 09:12:33 +02:00
certs certs/blacklist_hashes.c: fix const confusion in certs blacklist 2022-06-22 14:13:17 +02:00
crypto crypto: aead,cipher - zeroize key buffer after use 2024-07-18 13:05:38 +02:00
Documentation arm64: cpufeature: Fix the visibility of compat hwcaps 2024-08-19 05:41:23 +02:00
drivers wifi: mwifiex: duplicate static structs used in driver instances 2024-09-04 13:17:44 +02:00
fs binfmt_misc: pass binfmt_misc flags to the interpreter 2024-09-04 13:17:42 +02:00
include binfmt_misc: pass binfmt_misc flags to the interpreter 2024-09-04 13:17:42 +02:00
init init/main.c: Fix potential static_command_line memory overflow 2024-05-02 16:23:39 +02:00
io_uring io_uring/io-wq: limit retrying worker initialisation 2024-08-19 05:41:03 +02:00
ipc ipc: replace costly bailout check in sysvipc_find_ipc() 2024-09-04 13:17:44 +02:00
kernel hrtimer: Prevent queuing of hrtimer without a function callback 2024-09-04 13:17:39 +02:00
lib bitmap: introduce generic optimized bitmap_size() 2024-09-04 13:17:30 +02:00
LICENSES LICENSES/deprecated: add Zlib license text 2020-09-16 14:33:49 +02:00
mm memcg_write_event_control(): fix a user-triggerable oops 2024-09-04 13:17:30 +02:00
net mptcp: sched: check both backup in retrans 2024-09-04 13:17:44 +02:00
samples Add gitignore file for samples/fanotify/ subdirectory 2024-08-19 05:41:21 +02:00
scripts kbuild: Fix '-S -c' in x86 stack protector scripts 2024-08-19 05:40:59 +02:00
security selinux: fix potential counting error in avc_add_xperms_decision() 2024-09-04 13:17:30 +02:00
sound ALSA: timer: Relax start tick time check for slave timer elements 2024-09-04 13:17:43 +02:00
tools tools: move alignment-related macros to new <linux/align.h> 2024-09-04 13:17:44 +02:00
usr usr/include/Makefile: add linux/nfc.h to the compile-test coverage 2022-02-01 17:25:48 +01:00
virt KVM: Always flush async #PF workqueue when vCPU is being destroyed 2024-04-13 12:58:04 +02:00
.clang-format RDMA 5.10 pull request 2020-10-17 11:18:18 -07:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore kbuild: generate Module.symvers only when vmlinux exists 2021-05-19 10:12:59 +02:00
.mailmap mailmap: add two more addresses of Uwe Kleine-König 2020-12-06 10:19:07 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Jason Cooper to CREDITS 2020-11-30 10:20:34 +01:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Remove DECnet support from kernel 2023-06-21 15:45:38 +02:00
Makefile Linux 5.10.224 2024-08-19 05:41:24 +02:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -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 Restructured Text 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.