Re: [PATCH v2 3/4] rv/rtapp/sleep: Stop monitoring kernel threads
From: Gabriele Monaco
Date: Wed Jul 01 2026 - 09:33:52 EST
On Fri, 2026-06-19 at 09:21 +0200, Nam Cao wrote:
> The rtapp/sleep monitor's primary purpose is detecting common mistakes
> with user-space real-time design. Monitoring real-time issues with
> kernel threads is a bonus.
>
> However, accomodating kernel threads complicates the monitor due to
> the edge cases which is seen by the monitor as lower-priority task
> waking higher-priority task:
>
> - kthread_stop() wakes up the task in order to stop it.
>
> - The rcu thread and migration thread can be woken by any task.
>
> - The ktimerd thread is woken near the end of irq_exit_rcu(), where
> the preempt counter is "broken" and falsely says this is task
> context. This requires the monitor to use the hardirq_context flag
> instead of the preempt counter.
>
> Beside complicating the monitor, the final case also requires enabling
> CONFIG_TRACE_IRQFLAGS (so that "hardirq_context" can be used). This
> adds overhead to the kernel even when the monitor is not active. This
> may be an obstacle to enabling this monitor in distros' kernels.
Very good especially for this!
> Furthermore, kernel threads usually are started before the monitor is
> enabled. Consequently, the threads' states (i.o.w. the monitor's
> atomic propositions for the threads) are not fully known to the
> monitor. As a result, the kernel threads mostly cannot be monitored.
>
> Overall, the downsides of accomodating kernel threads outweights the
> benefits. Thus, exclude kernel threads to simplify the monitor.
>
> Signed-off-by: Nam Cao <namcao@xxxxxxxxxxxxx>
Reviewed-by: Gabriele Monaco <gmonaco@xxxxxxxxxx>
Thanks,
Gabriele
> ---
> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
> Documentation/trace/rv/monitor_rtapp.rst | 22 ++---
> kernel/trace/rv/monitors/sleep/Kconfig | 1 -
> kernel/trace/rv/monitors/sleep/sleep.c | 39 +-------
> kernel/trace/rv/monitors/sleep/sleep.h | 104 +++++++++-------------
> tools/verification/models/rtapp/sleep.ltl | 7 +-
> 5 files changed, 54 insertions(+), 119 deletions(-)
>
> diff --git a/Documentation/trace/rv/monitor_rtapp.rst
> b/Documentation/trace/rv/monitor_rtapp.rst
> index 570be67a8f3b..502d3ea412eb 100644
> --- a/Documentation/trace/rv/monitor_rtapp.rst
> +++ b/Documentation/trace/rv/monitor_rtapp.rst
> @@ -93,9 +93,9 @@ assessment.
>
> The monitor's specification is::
>
> - RULE = always ((RT and SLEEP) imply (RT_FRIENDLY_SLEEP or ALLOWLIST))
> + RULE = always ((RT and SLEEP and USER_THREAD) imply (RT_FRIENDLY_SLEEP or
> ALLOWLIST))
>
> - RT_FRIENDLY_SLEEP = (RT_VALID_SLEEP_REASON or KERNEL_THREAD)
> + RT_FRIENDLY_SLEEP = RT_VALID_SLEEP_REASON
> and ((not SCHEDULE_IN) until RT_FRIENDLY_WAKE)
>
> RT_VALID_SLEEP_REASON = FUTEX_WAIT
> @@ -110,23 +110,13 @@ The monitor's specification is::
> or WOKEN_BY_HARDIRQ
> or WOKEN_BY_NMI
> or ABORT_SLEEP
> - or KTHREAD_SHOULD_STOP
>
> ALLOWLIST = BLOCK_ON_RT_MUTEX
> or FUTEX_LOCK_PI
> - or TASK_IS_RCU
> - or TASK_IS_MIGRATION
> -
> -Beside the scenarios described above, this specification also handle some
> -special cases:
> -
> - - `KERNEL_THREAD`: kernel tasks do not have any pattern that can be
> recognized
> - as valid real-time sleeping reasons. Therefore sleeping reason is not
> - checked for kernel tasks.
> - - `KTHREAD_SHOULD_STOP`: a non-real-time thread may stop a real-time kernel
> - thread by waking it and waiting for it to exit (`kthread_stop()`). This
> - wakeup is safe for real-time.
> - - `ALLOWLIST`: to handle known false positives with the kernel.
> +
> +Beside the scenarios described above, this specification also defines an
> allow list
> +to handle some special cases:
> +
> - `BLOCK_ON_RT_MUTEX` is included in the allowlist due to its
> implementation.
> In the release path of rt_mutex, a boosted task is de-boosted before
> waking
> the rt_mutex's waiter. Consequently, the monitor may see a real-time-
> unsafe
> diff --git a/kernel/trace/rv/monitors/sleep/Kconfig
> b/kernel/trace/rv/monitors/sleep/Kconfig
> index 6b7a122e7b47..d6ec3e9a91b6 100644
> --- a/kernel/trace/rv/monitors/sleep/Kconfig
> +++ b/kernel/trace/rv/monitors/sleep/Kconfig
> @@ -5,7 +5,6 @@ config RV_MON_SLEEP
> select RV_LTL_MONITOR
> depends on HAVE_SYSCALL_TRACEPOINTS
> depends on RV_MON_RTAPP
> - select TRACE_IRQFLAGS
> default y
> select LTL_MON_EVENTS_ID
> bool "sleep monitor"
> diff --git a/kernel/trace/rv/monitors/sleep/sleep.c
> b/kernel/trace/rv/monitors/sleep/sleep.c
> index 638be7d8747f..aa5a984853b5 100644
> --- a/kernel/trace/rv/monitors/sleep/sleep.c
> +++ b/kernel/trace/rv/monitors/sleep/sleep.c
> @@ -43,7 +43,6 @@ static void ltl_atoms_init(struct task_struct *task, struct
> ltl_monitor *mon, bo
> ltl_atom_set(mon, LTL_WOKEN_BY_EQUAL_OR_HIGHER_PRIO, false);
>
> if (task_creation) {
> - ltl_atom_set(mon, LTL_KTHREAD_SHOULD_STOP, false);
> ltl_atom_set(mon, LTL_NANOSLEEP_CLOCK_REALTIME, false);
> ltl_atom_set(mon, LTL_NANOSLEEP_TIMER_ABSTIME, false);
> ltl_atom_set(mon, LTL_CLOCK_NANOSLEEP, false);
> @@ -53,33 +52,7 @@ static void ltl_atoms_init(struct task_struct *task, struct
> ltl_monitor *mon, bo
> ltl_atom_set(mon, LTL_BLOCK_ON_RT_MUTEX, false);
> }
>
> - if (task->flags & PF_KTHREAD) {
> - ltl_atom_set(mon, LTL_KERNEL_THREAD, true);
> -
> - /* kernel tasks do not do syscall */
> - ltl_atom_set(mon, LTL_FUTEX_WAIT, false);
> - ltl_atom_set(mon, LTL_FUTEX_LOCK_PI, false);
> - ltl_atom_set(mon, LTL_NANOSLEEP_CLOCK_REALTIME, false);
> - ltl_atom_set(mon, LTL_NANOSLEEP_TIMER_ABSTIME, false);
> - ltl_atom_set(mon, LTL_CLOCK_NANOSLEEP, false);
> - ltl_atom_set(mon, LTL_EPOLL_WAIT, false);
> -
> - if (strstarts(task->comm, "migration/"))
> - ltl_atom_set(mon, LTL_TASK_IS_MIGRATION, true);
> - else
> - ltl_atom_set(mon, LTL_TASK_IS_MIGRATION, false);
> -
> - if (strstarts(task->comm, "rcu"))
> - ltl_atom_set(mon, LTL_TASK_IS_RCU, true);
> - else
> - ltl_atom_set(mon, LTL_TASK_IS_RCU, false);
> - } else {
> - ltl_atom_set(mon, LTL_KTHREAD_SHOULD_STOP, false);
> - ltl_atom_set(mon, LTL_KERNEL_THREAD, false);
> - ltl_atom_set(mon, LTL_TASK_IS_RCU, false);
> - ltl_atom_set(mon, LTL_TASK_IS_MIGRATION, false);
> - }
> -
> + ltl_atom_set(mon, LTL_USER_THREAD, !(task->flags & PF_KTHREAD));
> }
>
> static void handle_sched_set_state(void *data, struct task_struct *task, int
> state)
> @@ -97,7 +70,7 @@ static void handle_sched_exit(void *data, bool is_switch)
>
> static void handle_sched_waking(void *data, struct task_struct *task)
> {
> - if (this_cpu_read(hardirq_context)) {
> + if (in_hardirq()) {
> ltl_atom_pulse(task, LTL_WOKEN_BY_HARDIRQ, true);
> } else if (in_task()) {
> if (current->prio <= task->prio)
> @@ -181,12 +154,6 @@ static void handle_sys_exit(void *data, struct pt_regs
> *regs, long ret)
> ltl_atom_update(current, LTL_CLOCK_NANOSLEEP, false);
> }
>
> -static void handle_kthread_stop(void *data, struct task_struct *task)
> -{
> - /* FIXME: this could race with other tracepoint handlers */
> - ltl_atom_update(task, LTL_KTHREAD_SHOULD_STOP, true);
> -}
> -
> static int enable_sleep(void)
> {
> int retval;
> @@ -200,7 +167,6 @@ static int enable_sleep(void)
> rv_attach_trace_probe("rtapp_sleep", sched_set_state_tp,
> handle_sched_set_state);
> rv_attach_trace_probe("rtapp_sleep", contention_begin,
> handle_contention_begin);
> rv_attach_trace_probe("rtapp_sleep", contention_end,
> handle_contention_end);
> - rv_attach_trace_probe("rtapp_sleep", sched_kthread_stop,
> handle_kthread_stop);
> rv_attach_trace_probe("rtapp_sleep", sys_enter, handle_sys_enter);
> rv_attach_trace_probe("rtapp_sleep", sys_exit, handle_sys_exit);
> return 0;
> @@ -213,7 +179,6 @@ static void disable_sleep(void)
> rv_detach_trace_probe("rtapp_sleep", sched_set_state_tp,
> handle_sched_set_state);
> rv_detach_trace_probe("rtapp_sleep", contention_begin,
> handle_contention_begin);
> rv_detach_trace_probe("rtapp_sleep", contention_end,
> handle_contention_end);
> - rv_detach_trace_probe("rtapp_sleep", sched_kthread_stop,
> handle_kthread_stop);
> rv_detach_trace_probe("rtapp_sleep", sys_enter, handle_sys_enter);
> rv_detach_trace_probe("rtapp_sleep", sys_exit, handle_sys_exit);
>
> diff --git a/kernel/trace/rv/monitors/sleep/sleep.h
> b/kernel/trace/rv/monitors/sleep/sleep.h
> index 2fe2ec7edae8..44e593f41e6a 100644
> --- a/kernel/trace/rv/monitors/sleep/sleep.h
> +++ b/kernel/trace/rv/monitors/sleep/sleep.h
> @@ -18,15 +18,12 @@ enum ltl_atom {
> LTL_EPOLL_WAIT,
> LTL_FUTEX_LOCK_PI,
> LTL_FUTEX_WAIT,
> - LTL_KERNEL_THREAD,
> - LTL_KTHREAD_SHOULD_STOP,
> LTL_NANOSLEEP_CLOCK_REALTIME,
> LTL_NANOSLEEP_TIMER_ABSTIME,
> LTL_RT,
> LTL_SCHEDULE_IN,
> LTL_SLEEP,
> - LTL_TASK_IS_MIGRATION,
> - LTL_TASK_IS_RCU,
> + LTL_USER_THREAD,
> LTL_WOKEN_BY_EQUAL_OR_HIGHER_PRIO,
> LTL_WOKEN_BY_HARDIRQ,
> LTL_WOKEN_BY_NMI,
> @@ -43,15 +40,12 @@ static const char *ltl_atom_str(enum ltl_atom atom)
> "ep_wa",
> "fu_lo_pi",
> "fu_wa",
> - "ker_th",
> - "kth_sh_st",
> "na_cl_re",
> "na_ti_ab",
> "rt",
> "sch_in",
> "sle",
> - "ta_mi",
> - "ta_rc",
> + "us_th",
> "wo_eq_hi_pr",
> "wo_ha",
> "wo_nm",
> @@ -79,46 +73,41 @@ static void ltl_start(struct task_struct *task, struct
> ltl_monitor *mon)
> bool woken_by_hardirq = test_bit(LTL_WOKEN_BY_HARDIRQ, mon->atoms);
> bool woken_by_equal_or_higher_prio =
> test_bit(LTL_WOKEN_BY_EQUAL_OR_HIGHER_PRIO,
> mon->atoms);
> - bool task_is_rcu = test_bit(LTL_TASK_IS_RCU, mon->atoms);
> - bool task_is_migration = test_bit(LTL_TASK_IS_MIGRATION, mon->atoms);
> + bool user_thread = test_bit(LTL_USER_THREAD, mon->atoms);
> bool sleep = test_bit(LTL_SLEEP, mon->atoms);
> bool schedule_in = test_bit(LTL_SCHEDULE_IN, mon->atoms);
> bool rt = test_bit(LTL_RT, mon->atoms);
> bool nanosleep_timer_abstime = test_bit(LTL_NANOSLEEP_TIMER_ABSTIME,
> mon->atoms);
> bool nanosleep_clock_realtime =
> test_bit(LTL_NANOSLEEP_CLOCK_REALTIME, mon->atoms);
> - bool kthread_should_stop = test_bit(LTL_KTHREAD_SHOULD_STOP, mon-
> >atoms);
> - bool kernel_thread = test_bit(LTL_KERNEL_THREAD, mon->atoms);
> bool futex_wait = test_bit(LTL_FUTEX_WAIT, mon->atoms);
> bool futex_lock_pi = test_bit(LTL_FUTEX_LOCK_PI, mon->atoms);
> bool epoll_wait = test_bit(LTL_EPOLL_WAIT, mon->atoms);
> bool clock_nanosleep = test_bit(LTL_CLOCK_NANOSLEEP, mon->atoms);
> bool block_on_rt_mutex = test_bit(LTL_BLOCK_ON_RT_MUTEX, mon->atoms);
> bool abort_sleep = test_bit(LTL_ABORT_SLEEP, mon->atoms);
> - bool val41 = task_is_rcu || task_is_migration;
> - bool val42 = futex_lock_pi || val41;
> - bool val5 = block_on_rt_mutex || val42;
> - bool val33 = abort_sleep || kthread_should_stop;
> - bool val34 = woken_by_nmi || val33;
> - bool val35 = woken_by_hardirq || val34;
> - bool val14 = woken_by_equal_or_higher_prio || val35;
> + bool val7 = block_on_rt_mutex || futex_lock_pi;
> + bool val32 = woken_by_nmi || abort_sleep;
> + bool val33 = woken_by_hardirq || val32;
> + bool val14 = woken_by_equal_or_higher_prio || val33;
> bool val13 = !schedule_in;
> bool val25 = !nanosleep_clock_realtime;
> bool val26 = nanosleep_timer_abstime && val25;
> bool val18 = clock_nanosleep && val26;
> bool val20 = val18 || epoll_wait;
> - bool val9 = futex_wait || val20;
> - bool val11 = val9 || kernel_thread;
> + bool val11 = futex_wait || val20;
> + bool val3 = !user_thread;
> bool val2 = !sleep;
> + bool val4 = val2 || val3;
> bool val1 = !rt;
> - bool val3 = val1 || val2;
> + bool val5 = val1 || val4;
>
> - if (val3)
> + if (val5)
> __set_bit(S0, mon->states);
> if (val11 && val13)
> __set_bit(S1, mon->states);
> if (val11 && val14)
> __set_bit(S4, mon->states);
> - if (val5)
> + if (val7)
> __set_bit(S5, mon->states);
> }
>
> @@ -129,130 +118,125 @@ ltl_possible_next_states(struct ltl_monitor *mon,
> unsigned int state, unsigned l
> bool woken_by_hardirq = test_bit(LTL_WOKEN_BY_HARDIRQ, mon->atoms);
> bool woken_by_equal_or_higher_prio =
> test_bit(LTL_WOKEN_BY_EQUAL_OR_HIGHER_PRIO,
> mon->atoms);
> - bool task_is_rcu = test_bit(LTL_TASK_IS_RCU, mon->atoms);
> - bool task_is_migration = test_bit(LTL_TASK_IS_MIGRATION, mon->atoms);
> + bool user_thread = test_bit(LTL_USER_THREAD, mon->atoms);
> bool sleep = test_bit(LTL_SLEEP, mon->atoms);
> bool schedule_in = test_bit(LTL_SCHEDULE_IN, mon->atoms);
> bool rt = test_bit(LTL_RT, mon->atoms);
> bool nanosleep_timer_abstime = test_bit(LTL_NANOSLEEP_TIMER_ABSTIME,
> mon->atoms);
> bool nanosleep_clock_realtime =
> test_bit(LTL_NANOSLEEP_CLOCK_REALTIME, mon->atoms);
> - bool kthread_should_stop = test_bit(LTL_KTHREAD_SHOULD_STOP, mon-
> >atoms);
> - bool kernel_thread = test_bit(LTL_KERNEL_THREAD, mon->atoms);
> bool futex_wait = test_bit(LTL_FUTEX_WAIT, mon->atoms);
> bool futex_lock_pi = test_bit(LTL_FUTEX_LOCK_PI, mon->atoms);
> bool epoll_wait = test_bit(LTL_EPOLL_WAIT, mon->atoms);
> bool clock_nanosleep = test_bit(LTL_CLOCK_NANOSLEEP, mon->atoms);
> bool block_on_rt_mutex = test_bit(LTL_BLOCK_ON_RT_MUTEX, mon->atoms);
> bool abort_sleep = test_bit(LTL_ABORT_SLEEP, mon->atoms);
> - bool val41 = task_is_rcu || task_is_migration;
> - bool val42 = futex_lock_pi || val41;
> - bool val5 = block_on_rt_mutex || val42;
> - bool val33 = abort_sleep || kthread_should_stop;
> - bool val34 = woken_by_nmi || val33;
> - bool val35 = woken_by_hardirq || val34;
> - bool val14 = woken_by_equal_or_higher_prio || val35;
> + bool val7 = block_on_rt_mutex || futex_lock_pi;
> + bool val32 = woken_by_nmi || abort_sleep;
> + bool val33 = woken_by_hardirq || val32;
> + bool val14 = woken_by_equal_or_higher_prio || val33;
> bool val13 = !schedule_in;
> bool val25 = !nanosleep_clock_realtime;
> bool val26 = nanosleep_timer_abstime && val25;
> bool val18 = clock_nanosleep && val26;
> bool val20 = val18 || epoll_wait;
> - bool val9 = futex_wait || val20;
> - bool val11 = val9 || kernel_thread;
> + bool val11 = futex_wait || val20;
> + bool val3 = !user_thread;
> bool val2 = !sleep;
> + bool val4 = val2 || val3;
> bool val1 = !rt;
> - bool val3 = val1 || val2;
> + bool val5 = val1 || val4;
>
> switch (state) {
> case S0:
> - if (val3)
> + if (val5)
> __set_bit(S0, next);
> if (val11 && val13)
> __set_bit(S1, next);
> if (val11 && val14)
> __set_bit(S4, next);
> - if (val5)
> + if (val7)
> __set_bit(S5, next);
> break;
> case S1:
> if (val11 && val13)
> __set_bit(S1, next);
> - if (val13 && val3)
> + if (val13 && val5)
> __set_bit(S2, next);
> - if (val14 && val3)
> + if (val14 && val5)
> __set_bit(S3, next);
> if (val11 && val14)
> __set_bit(S4, next);
> - if (val13 && val5)
> + if (val13 && val7)
> __set_bit(S6, next);
> - if (val14 && val5)
> + if (val14 && val7)
> __set_bit(S7, next);
> break;
> case S2:
> if (val11 && val13)
> __set_bit(S1, next);
> - if (val13 && val3)
> + if (val13 && val5)
> __set_bit(S2, next);
> - if (val14 && val3)
> + if (val14 && val5)
> __set_bit(S3, next);
> if (val11 && val14)
> __set_bit(S4, next);
> - if (val13 && val5)
> + if (val13 && val7)
> __set_bit(S6, next);
> - if (val14 && val5)
> + if (val14 && val7)
> __set_bit(S7, next);
> break;
> case S3:
> - if (val3)
> + if (val5)
> __set_bit(S0, next);
> if (val11 && val13)
> __set_bit(S1, next);
> if (val11 && val14)
> __set_bit(S4, next);
> - if (val5)
> + if (val7)
> __set_bit(S5, next);
> break;
> case S4:
> - if (val3)
> + if (val5)
> __set_bit(S0, next);
> if (val11 && val13)
> __set_bit(S1, next);
> if (val11 && val14)
> __set_bit(S4, next);
> - if (val5)
> + if (val7)
> __set_bit(S5, next);
> break;
> case S5:
> - if (val3)
> + if (val5)
> __set_bit(S0, next);
> if (val11 && val13)
> __set_bit(S1, next);
> if (val11 && val14)
> __set_bit(S4, next);
> - if (val5)
> + if (val7)
> __set_bit(S5, next);
> break;
> case S6:
> if (val11 && val13)
> __set_bit(S1, next);
> - if (val13 && val3)
> + if (val13 && val5)
> __set_bit(S2, next);
> - if (val14 && val3)
> + if (val14 && val5)
> __set_bit(S3, next);
> if (val11 && val14)
> __set_bit(S4, next);
> - if (val13 && val5)
> + if (val13 && val7)
> __set_bit(S6, next);
> - if (val14 && val5)
> + if (val14 && val7)
> __set_bit(S7, next);
> break;
> case S7:
> - if (val3)
> + if (val5)
> __set_bit(S0, next);
> if (val11 && val13)
> __set_bit(S1, next);
> if (val11 && val14)
> __set_bit(S4, next);
> - if (val5)
> + if (val7)
> __set_bit(S5, next);
> break;
> }
> diff --git a/tools/verification/models/rtapp/sleep.ltl
> b/tools/verification/models/rtapp/sleep.ltl
> index 5923e58d7810..4d78fdd204c0 100644
> --- a/tools/verification/models/rtapp/sleep.ltl
> +++ b/tools/verification/models/rtapp/sleep.ltl
> @@ -1,6 +1,6 @@
> -RULE = always ((RT and SLEEP) imply (RT_FRIENDLY_SLEEP or ALLOWLIST))
> +RULE = always ((RT and SLEEP and USER_THREAD) imply (RT_FRIENDLY_SLEEP or
> ALLOWLIST))
>
> -RT_FRIENDLY_SLEEP = (RT_VALID_SLEEP_REASON or KERNEL_THREAD)
> +RT_FRIENDLY_SLEEP = RT_VALID_SLEEP_REASON
> and ((not SCHEDULE_IN) until RT_FRIENDLY_WAKE)
>
> RT_VALID_SLEEP_REASON = FUTEX_WAIT
> @@ -15,9 +15,6 @@ RT_FRIENDLY_WAKE = WOKEN_BY_EQUAL_OR_HIGHER_PRIO
> or WOKEN_BY_HARDIRQ
> or WOKEN_BY_NMI
> or ABORT_SLEEP
> - or KTHREAD_SHOULD_STOP
>
> ALLOWLIST = BLOCK_ON_RT_MUTEX
> or FUTEX_LOCK_PI
> - or TASK_IS_RCU
> - or TASK_IS_MIGRATION