Re: [PATCH v3 3/3] softirq: Use a dedicated thread for timer wakeups on PREEMPT_RT.

["Luis Claudio R. Goncalves" <lgoncalv@redhat.com>]

On Mon, Dec 01, 2025 at 10:51:50PM +0100, Jan Kiszka wrote:
> On 06.11.24 15:51, Sebastian Andrzej Siewior wrote:
> > A timer/ hrtimer softirq is raised in-IRQ context. With threaded
> > interrupts enabled or on PREEMPT_RT this leads to waking the ksoftirqd
> > for the processing of the softirq. ksoftirqd runs as SCHED_OTHER which
> > means it will compete with other tasks for CPU ressources.
> > This can introduce long delays for timer processing on heavy loaded
> > systems and is not desired.
> > 
> > Split the TIMER_SOFTIRQ and HRTIMER_SOFTIRQ processing into a dedicated
> > timers thread and let it run at the lowest SCHED_FIFO priority.
> > Wake-ups for RT tasks happen from hardirq context so only timer_list timers
> > and hrtimers for "regular" tasks are processed here. The higher priority
> > ensures that wakeups are performed before scheduling SCHED_OTHER tasks.
> > 
> > Using a dedicated variable to store the pending softirq bits values
> > ensure that the timer are not accidentally picked up by ksoftirqd and
> > other threaded interrupts.
> > It shouldn't be picked up by ksoftirqd since it runs at lower priority.
> > However if ksoftirqd is already running while a timer fires, then
> > ksoftird will be PI-boosted due to the BH-lock to ktimer's priority.
> > Ideally we try to avoid having ksoftirqd running.
> > 
> > The timer thread can pick up pending softirqs from ksoftirqd but only
> > if the softirq load is high. It is not be desired that the picked up
> > softirqs are processed at SCHED_FIFO priority under high softirq load
> > but this can already happen by a PI-boost by a force-threaded interrupt.
> > 
> > [ frederic@kernel.org: rcutorture.c fixes, storm fix by introduction of
> >   local_timers_pending() for tick_nohz_next_event() ]
> > 
> > [ junxiao.chang@intel.com: Ensure ktimersd gets woken up even if a
> >   softirq is currently served. ]
> > 
> > Reviewed-by: Paul E. McKenney <paulmck@kernel.org> [rcutorture]
> > Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> 
> This went into 6.13 and was never backported to 6.12-lts. And that is
> why you can easily stall the latter with a workload like this and
> CONFIG_PREEMPT_RT enabled:
> 
> echo "+cpu" >> /sys/fs/cgroup/cgroup.subtree_control
> echo "+cpuset" >> /sys/fs/cgroup/cgroup.subtree_control
> 
> mkdir /sys/fs/cgroup/stalltest.sub1
> mkdir /sys/fs/cgroup/stalltest.sub2
> sleep 10000000 &
> pid=$!
> 
> systemd-run --slice "stalltest.slice" taskset -c 0 sh -c " \
>     while true; do
>         echo $pid > /sys/fs/cgroup/stalltest.sub1/cgroup.procs;
>         echo $pid > /sys/fs/cgroup/stalltest.sub2/cgroup.procs;
>     done"
> 
> echo "1000 20000" > /sys/fs/cgroup/stalltest.slice/cpu.max
> 
> This triggers a lock-up if a holder of cgroup_file_kn_lock with
> SCHED_OTHER is scheduled out after using up its timeslice and then
> cgroup_file_notify_timer fires over a SCHED_OTHER context as well,
> trying to get this lock, failing and then never being able to reactivate
> the lock holder again as well.
> 
> I've nicely reproduced this with upstream 6.12.58 while Debian's lastest
> 6.12-rt does not trigger because it additionally has the downstream -rt
> patches on board.
> 
> How should we handle this? Consider 6.12 mainline with -rt and cgroups
> as potentially broken, asking people to user 6.12-rt? Or port this back?
> 
> BTW, the original report of this issue came from an older
> 5.10.194-cip39-rt16 kernel (based on rt94 for 5.10). When was this
> feature introduced to the -rt patches? Was it ever backported to 5.10-rt
> or other -rt versions?

Hi Jan!

I failed to locate the original discussion (from v5.10-rt) as the V1 of this
patchset is a new thread. Anyway, you are correct, the commit below (and the
other two changes from the series) are not present in v5.10-rt.

AFAICT commit 49a17639508c ("softirq: Use a dedicated thread for timer wakeups
on PREEMPT_RT.") was merged initially to v6.13-rc1, it was never exclusive
to the RT tree.

Luis

> Jan
> 
> -- 
> Siemens AG, Foundational Technologies
> Linux Expert Center
> 
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