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Nysal Jan K.A. 7ab02bd36e sched/membarrier: Fix redundant load of membarrier_state 
On architectures where ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
is not selected, sync_core_before_usermode() is a no-op.
In membarrier_mm_sync_core_before_usermode() the compiler does not
eliminate redundant branches and load of mm->membarrier_state
for this case as the atomic_read() cannot be optimized away.

Here's a snippet of the code generated for finish_task_switch() on powerpc
prior to this change:

  1b786c:   ld      r26,2624(r30)   # mm = rq->prev_mm;
  .......
  1b78c8:   cmpdi   cr7,r26,0
  1b78cc:   beq     cr7,1b78e4 <finish_task_switch+0xd0>
  1b78d0:   ld      r9,2312(r13)    # current
  1b78d4:   ld      r9,1888(r9)     # current->mm
  1b78d8:   cmpd    cr7,r26,r9
  1b78dc:   beq     cr7,1b7a70 <finish_task_switch+0x25c>
  1b78e0:   hwsync
  1b78e4:   cmplwi  cr7,r27,128
  .......
  1b7a70:   lwz     r9,176(r26)     # atomic_read(&mm->membarrier_state)
  1b7a74:   b       1b78e0 <finish_task_switch+0xcc>

This was found while analyzing "perf c2c" reports on kernels prior
to commit c1753fd02a ("mm: move mm_count into its own cache line")
where mm_count was false sharing with membarrier_state.

There is a minor improvement in the size of finish_task_switch().
The following are results from bloat-o-meter for ppc64le:

  GCC 7.5.0
  ---------
  add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
  Function                                     old     new   delta
  finish_task_switch                           884     852     -32

  GCC 12.2.1
  ----------
  add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
  Function                                     old     new   delta
  finish_task_switch.isra                      852     820     -32

  LLVM 17.0.6
  -----------
  add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-36 (-36)
  Function                                     old     new   delta
  rt_mutex_schedule                            120     104     -16
  finish_task_switch                           792     772     -20

Results on aarch64:

  GCC 14.1.1
  ----------
  add/remove: 0/2 grow/shrink: 1/1 up/down: 4/-60 (-56)
  Function                                     old     new   delta
  get_nohz_timer_target                        352     356      +4
  e843419@0b02_0000d7e7_408                      8       -      -8
  e843419@01bb_000021d2_868                      8       -      -8
  finish_task_switch.isra                      592     548     -44

Signed-off-by: Nysal Jan K.A. <nysal@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Link: https://lore.kernel.org/r/20250303060457.531293-1-nysal@linux.ibm.com
History 2025-03-03 11:25:40 +01:00
arch
block block-6.14-20250131 2025-01-31 11:49:30 -08:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
Documentation
drivers Additional power management fix for 6.14-rc2 2025-02-09 09:47:06 -08:00
fs
include
init
io_uring
ipc treewide: const qualify ctl_tables where applicable 2025-01-28 13:48:37 +01:00
kernel
lib
LICENSES
mm
net vfs-6.14-rc2.fixes 2025-02-07 09:22:31 -08:00
rust rust: kbuild: do not export generated KASAN ODR symbols 2025-02-06 19:47:04 +01:00
samples
scripts
security
sound sound fixes for 6.14-rc1 2025-01-31 09:17:02 -08:00
tools
usr
virt KVM: remove kvm_arch_post_init_vm 2025-02-04 11:27:45 -05:00
.clang-format
.clippy.toml
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig
.get_maintainer.ignore MAINTAINERS: Retire Ralf Baechle 2024-11-12 15:48:59 +01:00
.gitattributes
.gitignore rust: use host dylib naming convention to support macOS 2025-01-10 01:01:24 +01:00
.mailmap 21 hotfixes. 8 are cc:stable and the remainder address post-6.13 issues. 2025-02-01 09:49:20 -08:00
.rustfmt.toml
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS
Kbuild
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS
Makefile
README

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.