Re: [PATCH 1/3] rv/rtapp/sleep: Make the error more informative for user

[Gabriele Monaco <gmonaco@redhat.com>]

On Tue, 2026-05-19 at 09:49 +0200, Nam Cao wrote:
> The rtapp/sleep monitor detects real-time tasks which go to sleep in an
> real-time-unsafe manner. If this happen, the monitor triggers a trace event
> in the sched_wakeup tracepoint's handler.
> 

Ok so here WAKE is no longer tied to the wakeup event but to the end of the task
switch.

So what happens if a task was not sleeping but just got preempted? Wouldn't that
trigger WAKE (though that isn't a real wakeup) without RT_FRIENDLY_WAKE ?

Thanks,
Gabriele

> However, the invoking context of that trace event is not the most
> informative, because of the stack trace of that event is the wakeup's code
> path which is not very helpful:
> 
> 74.669317: rv:error_sleep: condvar[254]: violation detected
>     ltl_validate+0x345 ([kernel.kallsyms])
>     handle_sched_wakeup+0x34 ([kernel.kallsyms])
>     ttwu_do_activate+0xff ([kernel.kallsyms])
>     sched_ttwu_pending+0x104 ([kernel.kallsyms])
>     __flush_smp_call_function_queue+0x15b ([kernel.kallsyms])
>     __sysvec_call_function_single+0x18 ([kernel.kallsyms])
>     sysvec_call_function_single+0x66 ([kernel.kallsyms])
>     asm_sysvec_call_function_single+0x1a ([kernel.kallsyms])
>     pv_native_safe_halt+0xf ([kernel.kallsyms])
>     default_idle+0x9 ([kernel.kallsyms])
>     default_idle_call+0x33 ([kernel.kallsyms])
>     do_idle+0x234 ([kernel.kallsyms])
>     cpu_startup_entry+0x24 ([kernel.kallsyms])
>     start_secondary+0xf8 ([kernel.kallsyms])
>     common_startup_64+0x13e ([kernel.kallsyms])
> 
> What would be much more valuable is the stack trace of the task itself.
> 
> Change the update of WAKEUP from being in sched_wakeup trace point's
> handler to sched_exit trace point's handler. This makes the event happen in
> the task's context, making the stack trace far more informative for user:
> 
> rv:error_sleep: condvar[254]: violation detected
>     ltl_validate+0x345 ([kernel.kallsyms])
>     handle_sched_exit+0x39 ([kernel.kallsyms])
>     __schedule+0x80f ([kernel.kallsyms])
>     schedule+0x22 ([kernel.kallsyms])
>     futex_do_wait+0x33 ([kernel.kallsyms])
>     __futex_wait+0x8c ([kernel.kallsyms])
>     futex_wait+0x73 ([kernel.kallsyms])
>     do_futex+0xc6 ([kernel.kallsyms])
>     __x64_sys_futex+0x121 ([kernel.kallsyms])
>     do_syscall_64+0xf3 ([kernel.kallsyms])
>     entry_SYSCALL_64_after_hwframe+0x77 ([kernel.kallsyms])
>     __futex_abstimed_wait_common64+0xc6 (inlined)
>     __futex_abstimed_wait_common+0xc6 (/usr/lib/x86_64-linux-gnu/libc.so.6)
> 
> Signed-off-by: Nam Cao <namcao@linutronix.de>
> ---
>  kernel/trace/rv/monitors/sleep/sleep.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/rv/monitors/sleep/sleep.c
> b/kernel/trace/rv/monitors/sleep/sleep.c
> index 8dfe5ec13e19..0a36f5519e6b 100644
> --- a/kernel/trace/rv/monitors/sleep/sleep.c
> +++ b/kernel/trace/rv/monitors/sleep/sleep.c
> @@ -92,9 +92,9 @@ static void handle_sched_set_state(void *data, struct
> task_struct *task, int sta
>  		ltl_atom_pulse(task, LTL_ABORT_SLEEP, true);
>  }
>  
> -static void handle_sched_wakeup(void *data, struct task_struct *task)
> +static void handle_sched_exit(void *data, bool is_switch)
>  {
> -	ltl_atom_pulse(task, LTL_WAKE, true);
> +	ltl_atom_pulse(current, LTL_WAKE, true);
>  }
>  
>  static void handle_sched_waking(void *data, struct task_struct *task)
> @@ -200,7 +200,7 @@ static int enable_sleep(void)
>  		return retval;
>  
>  	rv_attach_trace_probe("rtapp_sleep", sched_waking,
> handle_sched_waking);
> -	rv_attach_trace_probe("rtapp_sleep", sched_wakeup,
> handle_sched_wakeup);
> +	rv_attach_trace_probe("rtapp_sleep", sched_exit_tp,
> handle_sched_exit);
>  	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);
> @@ -213,7 +213,7 @@ static int enable_sleep(void)
>  static void disable_sleep(void)
>  {
>  	rv_detach_trace_probe("rtapp_sleep", sched_waking,
> handle_sched_waking);
> -	rv_detach_trace_probe("rtapp_sleep", sched_wakeup,
> handle_sched_wakeup);
> +	rv_detach_trace_probe("rtapp_sleep", sched_exit_tp,
> handle_sched_exit);
>  	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);