[RFC PATCH v2 0/3] disable optimistic spinning for ftrace_lock

[Yafang Shao]

Recently, we resolved a latency spike issue caused by concurrently running
bpftrace processes. The root cause was high contention on the ftrace_lock
due to optimistic spinning. We can optimize this by disabling optimistic
spinning for ftrace_lock.

While semaphores may present similar challenges, I'm not currently aware of
specific instances that exhibit this exact issue. Should we encounter
problematic semaphores in production workloads, we can address them at that
time.

PATCH #1: introduce slow_mutex_[un]lock to disable optimistic spinning
PATCH #2: add variant for rtmutex
PATCH #3: disable optimistic spinning for ftrace_lock

v1->v2:
- add slow_mutex_[un]lock (Steven)
- add variant for rtmutex (Waiman)
- revise commit log for clarity and accuracy (Waiman, Peter)
- note that semaphores may present similar challenges (David)

RFC v1: https://lore.kernel.org/bpf/20260304074650.58165-1-laoar.shao@xxxxxxxxx/

Yafang Shao (3):
  locking/mutex: Add slow path variants for lock/unlock
  locking/rtmutex: Add slow path variants for lock/unlock
  ftrace: Disable optimistic spinning for ftrace_lock

 include/linux/mutex.h        |   4 ++
 include/linux/rtmutex.h      |   3 +
 kernel/locking/mutex.c       |  41 +++++++++++---
 kernel/locking/rtmutex.c     |  37 +++++++-----
 kernel/locking/rtmutex_api.c |  47 +++++++++++++---
 kernel/trace/ftrace.c        | 106 +++++++++++++++++------------------
 6 files changed, 157 insertions(+), 81 deletions(-)

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2.47.3