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Uday Shankar b349a3d1b1 io-wq: backoff when retrying worker creation 
[ Upstream commit 13918315c5 ]

When io_uring submission goes async for the first time on a given task,
we'll try to create a worker thread to handle the submission. Creating
this worker thread can fail due to various transient conditions, such as
an outstanding signal in the forking thread, so we have retry logic with
a limit of 3 retries. However, this retry logic appears to be too
aggressive/fast - we've observed a thread blowing through the retry
limit while having the same outstanding signal the whole time. Here's an
excerpt of some tracing that demonstrates the issue:

First, signal 26 is generated for the process. It ends up getting routed
to thread 92942.

 0)   cbd-92284    /* signal_generate: sig=26 errno=0 code=-2 comm=psblkdASD pid=92934 grp=1 res=0 */

This causes create_io_thread in the signalled thread to fail with
ERESTARTNOINTR, and thus a retry is queued.

13) task_th-92942  /* io_uring_queue_async_work: ring 000000007325c9ae, request 0000000080c96d8e, user_data 0x0, opcode URING_CMD, flags 0x8240001, normal queue, work 000000006e96dd3f */
13) task_th-92942  io_wq_enqueue() {
13) task_th-92942    _raw_spin_lock();
13) task_th-92942    io_wq_activate_free_worker();
13) task_th-92942    _raw_spin_lock();
13) task_th-92942    create_io_worker() {
13) task_th-92942      __kmalloc_cache_noprof();
13) task_th-92942      __init_swait_queue_head();
13) task_th-92942      kprobe_ftrace_handler() {
13) task_th-92942        get_kprobe();
13) task_th-92942        aggr_pre_handler() {
13) task_th-92942          pre_handler_kretprobe();
13) task_th-92942          /* create_enter: (create_io_thread+0x0/0x50) fn=0xffffffff8172c0e0 arg=0xffff888996bb69c0 node=-1 */
13) task_th-92942        } /* aggr_pre_handler */
...
13) task_th-92942        } /* copy_process */
13) task_th-92942      } /* create_io_thread */
13) task_th-92942      kretprobe_rethook_handler() {
13) task_th-92942        /* create_exit: (create_io_worker+0x8a/0x1a0 <- create_io_thread) arg1=0xfffffffffffffdff */
13) task_th-92942      } /* kretprobe_rethook_handler */
13) task_th-92942    queue_work_on() {
...

The CPU is then handed to a kworker to process the queued retry:

------------------------------------------
 13) task_th-92942  => kworker-54154
------------------------------------------
13) kworker-54154  io_workqueue_create() {
13) kworker-54154    io_queue_worker_create() {
13) kworker-54154      task_work_add() {
13) kworker-54154        wake_up_state() {
13) kworker-54154          try_to_wake_up() {
13) kworker-54154            _raw_spin_lock_irqsave();
13) kworker-54154            _raw_spin_unlock_irqrestore();
13) kworker-54154          } /* try_to_wake_up */
13) kworker-54154        } /* wake_up_state */
13) kworker-54154        kick_process();
13) kworker-54154      } /* task_work_add */
13) kworker-54154    } /* io_queue_worker_create */
13) kworker-54154  } /* io_workqueue_create */

And then we immediately switch back to the original task to try creating
a worker again. This fails, because the original task still hasn't
handled its signal.

-----------------------------------------
 13) kworker-54154  => task_th-92942
------------------------------------------
13) task_th-92942  create_worker_cont() {
13) task_th-92942    kprobe_ftrace_handler() {
13) task_th-92942      get_kprobe();
13) task_th-92942      aggr_pre_handler() {
13) task_th-92942        pre_handler_kretprobe();
13) task_th-92942        /* create_enter: (create_io_thread+0x0/0x50) fn=0xffffffff8172c0e0 arg=0xffff888996bb69c0 node=-1 */
13) task_th-92942      } /* aggr_pre_handler */
13) task_th-92942    } /* kprobe_ftrace_handler */
13) task_th-92942    create_io_thread() {
13) task_th-92942      copy_process() {
13) task_th-92942        task_active_pid_ns();
13) task_th-92942        _raw_spin_lock_irq();
13) task_th-92942        recalc_sigpending();
13) task_th-92942        _raw_spin_lock_irq();
13) task_th-92942      } /* copy_process */
13) task_th-92942    } /* create_io_thread */
13) task_th-92942    kretprobe_rethook_handler() {
13) task_th-92942      /* create_exit: (create_worker_cont+0x35/0x1b0 <- create_io_thread) arg1=0xfffffffffffffdff */
13) task_th-92942    } /* kretprobe_rethook_handler */
13) task_th-92942    io_worker_release();
13) task_th-92942    queue_work_on() {
13) task_th-92942      clear_pending_if_disabled();
13) task_th-92942      __queue_work() {
13) task_th-92942      } /* __queue_work */
13) task_th-92942    } /* queue_work_on */
13) task_th-92942  } /* create_worker_cont */

The pattern repeats another couple times until we blow through the retry
counter, at which point we give up. All outstanding work is canceled,
and the io_uring command which triggered all this is failed with
ECANCELED:

13) task_th-92942  io_acct_cancel_pending_work() {
...
13) task_th-92942  /* io_uring_complete: ring 000000007325c9ae, req 0000000080c96d8e, user_data 0x0, result -125, cflags 0x0 extra1 0 extra2 0  */

Finally, the task gets around to processing its outstanding signal 26,
but it's too late.

13) task_th-92942  /* signal_deliver: sig=26 errno=0 code=-2 sa_handler=59566a0 sa_flags=14000000 */

Try to address this issue by adding a small scaling delay when retrying
worker creation. This should give the forking thread time to handle its
signal in the above case. This isn't a particularly satisfying solution,
as sufficiently paradoxical scheduling would still have us hitting the
same issue, and I'm open to suggestions for something better. But this
is likely to prevent this (already rare) issue from hitting in practice.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Link: https://lore.kernel.org/r/20250208-wq_retry-v2-1-4f6f5041d303@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-03-22 12:54:19 -07:00
arch arm64: amu: Delay allocating cpumask for AMU FIE support 2025-03-22 12:54:17 -07:00
block block: fix conversion of GPT partition name to 7-bit 2025-03-13 13:01:58 +01:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: api - Fix boot-up self-test race 2025-02-08 09:57:28 +01:00
Documentation docs: rust: remove spurious item in expect list 2025-03-13 13:02:18 +01:00
drivers net: wwan: mhi_wwan_mbim: Silence sequence number glitch errors 2025-03-22 12:54:19 -07:00
fs btrfs: fix two misuses of folio_shift() 2025-03-22 12:54:16 -07:00
include PCI: pci_ids: add INTEL_HDA_PTL_H 2025-03-22 12:54:19 -07:00
init Compiler Attributes: disable __counted_by for clang < 19.1.3 2024-12-05 14:02:45 +01:00
io_uring io-wq: backoff when retrying worker creation 2025-03-22 12:54:19 -07:00
ipc ipc: fix memleak if msg_init_ns failed in create_ipc_ns 2024-12-05 14:03:02 +01:00
kernel sched: Clarify wake_up_q()'s write to task->wake_q.next 2025-03-22 12:54:16 -07:00
lib rcuref: Plug slowpath race in rcuref_put() 2025-03-07 18:25:43 +01:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm userfaultfd: fix PTE unmapping stack-allocated PTE copies 2025-03-22 12:54:09 -07:00
net Bluetooth: L2CAP: Fix slab-use-after-free Read in l2cap_send_cmd 2025-03-22 12:54:17 -07:00
rust rust: map long to isize and char to u8 2025-03-13 13:02:13 +01:00
samples rust: treewide: switch to the kernel Vec type 2025-03-13 13:01:46 +01:00
scripts kbuild: rust: remove the alloc crate and GlobalAlloc 2025-03-13 13:01:47 +01:00
security ima: Reset IMA_NONACTION_RULE_FLAGS after post_setattr 2025-03-07 18:25:44 +01:00
sound ASoC: SOF: amd: Handle IPC replies before FW_BOOT_COMPLETE 2025-03-22 12:54:19 -07:00
tools selftests/bpf: Fix invalid flag of recv() 2025-03-22 12:54:18 -07:00
usr kbuild: hdrcheck: fix cross build with clang 2025-03-13 13:02:18 +01:00
virt KVM: x86: switch hugepage recovery thread to vhost_task 2024-12-05 14:02:43 +01:00
.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: enable Clippy's check-private-items 2025-03-13 13:01:42 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore Add Jeff Kirsher to .get_maintainer.ignore 2024-03-08 11:36:54 +00:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore rust: introduce .clippy.toml 2025-03-13 13:01:42 +01:00
.mailmap mailmap: add entry for Thorsten Blum 2024-11-07 14:14:59 -08:00
.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 MAINTAINERS: Remove self from DSA entry 2024-11-03 12:52:38 -08:00
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 MAINTAINERS: add entry for the Rust alloc module 2025-03-13 13:01:47 +01:00
Makefile Linux 6.12.19 2025-03-13 13:02:20 +01:00
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.