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Finn Thain bc13958889 sched/core: Optimize in_task() and in_interrupt() a bit 
[ Upstream commit 87c3a5893e ]

Except on x86, preempt_count is always accessed with READ_ONCE().
Repeated invocations in macros like irq_count() produce repeated loads.
These redundant instructions appear in various fast paths. In the one
shown below, for example, irq_count() is evaluated during kernel entry
if !tick_nohz_full_cpu(smp_processor_id()).

0001ed0a <irq_enter_rcu>:
   1ed0a:       4e56 0000       linkw %fp,#0
   1ed0e:       200f            movel %sp,%d0
   1ed10:       0280 ffff e000  andil #-8192,%d0
   1ed16:       2040            moveal %d0,%a0
   1ed18:       2028 0008       movel %a0@(8),%d0
   1ed1c:       0680 0001 0000  addil #65536,%d0
   1ed22:       2140 0008       movel %d0,%a0@(8)
   1ed26:       082a 0001 000f  btst #1,%a2@(15)
   1ed2c:       670c            beqs 1ed3a <irq_enter_rcu+0x30>
   1ed2e:       2028 0008       movel %a0@(8),%d0
   1ed32:       2028 0008       movel %a0@(8),%d0
   1ed36:       2028 0008       movel %a0@(8),%d0
   1ed3a:       4e5e            unlk %fp
   1ed3c:       4e75            rts

This patch doesn't prevent the pointless btst and beqs instructions
above, but it does eliminate 2 of the 3 pointless move instructions
here and elsewhere.

On x86, preempt_count is per-cpu data and the problem does not arise
presumably because the compiler is free to optimize more effectively.

This patch was tested on m68k and x86. I was expecting no changes
to object code for x86 and mostly that's what I saw. However, there
were a few places where code generation was perturbed for some reason.

The performance issue addressed here is minor on uniprocessor m68k. I
got a 0.01% improvement from this patch for a simple "find /sys -false"
benchmark. For architectures and workloads susceptible to cache line bounce
the improvement is expected to be larger. The only SMP architecture I have
is x86, and as x86 unaffected I have not done any further measurements.

Fixes: 15115830c8 ("preempt: Cleanup the macro maze a bit")
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/0a403120a682a525e6db2d81d1a3ffcc137c3742.1694756831.git.fthain@linux-m68k.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2023-11-28 17:19:48 +00:00
arch riscv: VMAP_STACK overflow detection thread-safe 2023-11-28 17:19:47 +00:00
block blk-core: use pr_warn_ratelimited() in bio_check_ro() 2023-11-20 11:59:36 +01:00
certs certs: Reference revocation list for all keyrings 2023-08-17 20:12:41 +00:00
crypto crypto: pcrypt - Fix hungtask for PADATA_RESET 2023-11-28 17:19:42 +00:00
Documentation i2c: i801: Add support for Intel Birch Stream SoC 2023-11-28 17:19:46 +00:00
drivers wifi: iwlwifi: Use FW rate for non-data frames 2023-11-28 17:19:48 +00:00
fs gfs2: Fix slab-use-after-free in gfs2_qd_dealloc 2023-11-28 17:19:47 +00:00
include sched/core: Optimize in_task() and in_interrupt() a bit 2023-11-28 17:19:48 +00:00
init x86/retpoline: Make sure there are no unconverted return thunks due to KCSAN 2023-11-28 17:19:36 +00:00
io_uring io_uring/net: ensure socket is marked connected on connect retry 2023-11-20 11:59:38 +01:00
ipc Add x86 shadow stack support 2023-08-31 12:20:12 -07:00
kernel kgdb: Flush console before entering kgdb on panic 2023-11-28 17:19:47 +00:00
lib lib/generic-radix-tree.c: Don't overflow in peek() 2023-11-28 17:19:35 +00:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm vfs: fix readahead(2) on block devices 2023-11-20 11:58:52 +01:00
net 9p: v9fs_listxattr: fix %s null argument warning 2023-11-28 17:19:46 +00:00
rust rust: docs: fix logo replacement 2023-10-19 16:40:00 +02:00
samples VFIO updates for v6.6-rc1 2023-08-30 20:36:01 -07:00
scripts x86/retpoline: Make sure there are no unconverted return thunks due to KCSAN 2023-11-28 17:19:36 +00:00
security apparmor: fix invalid reference on profile->disconnected 2023-11-20 11:59:24 +01:00
sound ALSA: hda/realtek: Add quirk for ASUS UX7602ZM 2023-11-28 17:19:47 +00:00
tools selftests/efivarfs: create-read: fix a resource leak 2023-11-28 17:19:42 +00:00
usr initramfs: Encode dependency on KBUILD_BUILD_TIMESTAMP 2023-06-06 17:54:49 +09:00
virt ARM: 2023-09-07 13:52:20 -07:00
.clang-format iommu: Add for_each_group_device() 2023-05-23 08:15:51 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore kbuild: rpm-pkg: rename binkernel.spec to kernel.spec 2023-07-25 00:59:33 +09:00
.mailmap 20 hotfixes. 12 are cc:stable and the remainder address post-6.5 issues 2023-10-24 09:52:16 -10:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS USB: Remove Wireless USB and UWB documentation 2023-08-09 14:17:32 +02:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Char/Misc driver fixes for 6.6-final 2023-10-28 07:51:27 -10:00
Makefile Linux 6.6.2 2023-11-20 11:59:39 +01:00
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.