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K Prateek Nayak 3dd65ffa2d softirq: Allow raising SCHED_SOFTIRQ from SMP-call-function on RT kernel 
[ Upstream commit 6675ce2004 ]

do_softirq_post_smp_call_flush() on PREEMPT_RT kernels carries a
WARN_ON_ONCE() for any SOFTIRQ being raised from an SMP-call-function.
Since do_softirq_post_smp_call_flush() is called with preempt disabled,
raising a SOFTIRQ during flush_smp_call_function_queue() can lead to
longer preempt disabled sections.

Since commit b2a02fc43a ("smp: Optimize
send_call_function_single_ipi()") IPIs to an idle CPU in
TIF_POLLING_NRFLAG mode can be optimized out by instead setting
TIF_NEED_RESCHED bit in idle task's thread_info and relying on the
flush_smp_call_function_queue() in the idle-exit path to run the
SMP-call-function.

To trigger an idle load balancing, the scheduler queues
nohz_csd_function() responsible for triggering an idle load balancing on
a target nohz idle CPU and sends an IPI. Only now, this IPI is optimized
out and the SMP-call-function is executed from
flush_smp_call_function_queue() in do_idle() which can raise a
SCHED_SOFTIRQ to trigger the balancing.

So far, this went undetected since, the need_resched() check in
nohz_csd_function() would make it bail out of idle load balancing early
as the idle thread does not clear TIF_POLLING_NRFLAG before calling
flush_smp_call_function_queue(). The need_resched() check was added with
the intent to catch a new task wakeup, however, it has recently
discovered to be unnecessary and will be removed in the subsequent
commit after which nohz_csd_function() can raise a SCHED_SOFTIRQ from
flush_smp_call_function_queue() to trigger an idle load balance on an
idle target in TIF_POLLING_NRFLAG mode.

nohz_csd_function() bails out early if "idle_cpu()" check for the
target CPU, and does not lock the target CPU's rq until the very end,
once it has found tasks to run on the CPU and will not inhibit the
wakeup of, or running of a newly woken up higher priority task. Account
for this and prevent a WARN_ON_ONCE() when SCHED_SOFTIRQ is raised from
flush_smp_call_function_queue().

Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241119054432.6405-2-kprateek.nayak@amd.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-01-09 13:31:48 +01:00
arch NUMA: optimize detection of memory with no node id assigned by firmware 2025-01-09 13:31:40 +01:00
block block: avoid to reuse hctx not removed from cpuhp callback list 2025-01-02 10:32:11 +01:00
certs certs: Reference revocation list for all keyrings 2023-08-17 20:12:41 +00:00
crypto crypto: ecdsa - Avoid signed integer overflow on signature decoding 2025-01-09 13:31:41 +01:00
Documentation i2c: i801: Add support for Intel Panther Lake 2025-01-09 13:31:43 +01:00
drivers net/mlx5: unique names for per device caches 2025-01-09 13:31:48 +01:00
fs fs/ntfs3: Fix warning in ni_fiemap 2025-01-09 13:31:46 +01:00
include usb: chipidea: add CI_HDRC_HAS_SHORT_PKT_LIMIT flag 2025-01-09 13:31:46 +01:00
init Compiler Attributes: disable __counted_by for clang < 19.1.3 2024-12-09 10:32:46 +01:00
io_uring io_uring/sqpoll: fix sqpoll error handling races 2025-01-02 10:32:10 +01:00
ipc ipc: fix memleak if msg_init_ns failed in create_ipc_ns 2024-12-09 10:32:54 +01:00
kernel softirq: Allow raising SCHED_SOFTIRQ from SMP-call-function on RT kernel 2025-01-09 13:31:48 +01:00
lib lib: stackinit: hide never-taken branch from compiler 2024-12-14 19:59:57 +01:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm memblock: allow zero threshold in validate_numa_converage() 2025-01-09 13:31:40 +01:00
net Bluetooth: hci_conn: Reduce hci_conn_drop() calls in two functions 2025-01-09 13:31:43 +01:00
rust rust: macros: fix documentation of the paste! macro 2024-12-09 10:32:25 +01:00
samples samples/bpf: Fix a resource leak 2024-12-14 20:00:03 +01:00
scripts setlocalversion: work around "git describe" performance 2024-12-14 20:00:17 +01:00
security apparmor: test: Fix memory leak for aa_unpack_strdup() 2024-12-09 10:32:45 +01:00
sound ALSA: hda/realtek: Fix spelling mistake "Firelfy" -> "Firefly" 2025-01-02 10:32:11 +01:00
tools stddef: make __struct_group() UAPI C++-friendly 2025-01-02 10:32:03 +01:00
usr initramfs: Encode dependency on KBUILD_BUILD_TIMESTAMP 2023-06-06 17:54:49 +09:00
virt KVM: Use dedicated mutex to protect kvm_usage_count to avoid deadlock 2024-10-04 16:29:47 +02:00
.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 Remove *.orig pattern from .gitignore 2024-10-04 16:29:44 +02:00
.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 membarrier: riscv: Add full memory barrier in switch_mm() 2024-09-12 11:11:45 +02:00
Makefile Linux 6.6.69 2025-01-02 10:32:11 +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.