Re: [PATCH 1/5] locking/rtmutex: Stop spinning on the owner when the waiter is woken

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Nov 07, 2026 at 12:29:27AM -0500, Steven Rostedt wrote:
> On Fri,  6 Nov 2026 18:59:27 -0500
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
> > The top waiter of an rt_mutex spins while the owner runs,
> > rtmutex_spin_on_owner(), and does not stop when it is woken: its timeout
> > and a signal find it running, set it TASK_RUNNING, and the spin goes on
> > until the owner leaves its CPU or the waiter's own CPU has to
> > reschedule. A PI futex owner is a user task that may run as long as it
> > likes. On v7.4-rc1, in 8-CPU VMs, behind an owner spinning in userspace,
> > a 1ms pthread_mutex_timedlock() returns ETIMEDOUT 126ms to 631ms late on
> > average, 1.4s at most, a SIGUSR1 is handled up to 1.9s late, and a
> > waiter killed with SIGKILL is reaped 0.5s late on average. With
> > PREEMPT_RT the wound of a ww_mutex is missed the same way: two contexts
> > in an ABBA on two CPUs at SCHED_FIFO spin on each other, and the older
> > one gets its second lock after 96ms on average, 0.95s at most.
> > 
> 
> The above is very verbose. Are you starting to use AI? ;-)

He definitely is. I can't read the above, its word soup. Part of using
AI is making sure the Changelog is actually readable. This is not.