Re: [PATCH] sched: Set need-resched flags before tracing

[Gabriele Monaco]

Il 11 settembre 2026 21:33:00 UTC, Andrea Righi <arighi@xxxxxxxxxx> ha scritto:
>sched_set_need_resched_tp is emitted before the corresponding thread
>flag is set. This allows a BPF tracepoint program to recursively invoke
>the same tracepoint while leaving its RCU read-side critical section.
>
>If rcu_read_unlock_special() must defer a quiescent state while
>preemption or interrupts are disabled, it calls
>set_need_resched_current(). Since TIF_NEED_RESCHED is still clear, this
>emits the tracepoint again and repeats until the kernel stack overflows:
>
>  __trace_set_need_resched()
>    bpf_trace_run3()
>      rcu_read_unlock_migrate()
>        rcu_read_unlock_special()
>          set_need_resched_current()
>            set_tsk_need_resched()
>              __trace_set_need_resched()
>
>Set the thread flag before emitting the tracepoint in both
>set_tsk_need_resched() and __resched_curr(). For a remote reschedule,
>retain the result of set_nr_and_not_polling() so that tracing remains
>ahead of IPI delivery while still observing the updated flag.
>
>Fixes: adcc3bfa8806 ("sched: Adapt sched tracepoints for RV task model")
>Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
>---

Hi Andrea,

Isn't this mostly what was done in [1]? I wonder if that patch was just forgotten.

There was a discussion that apparently didn't go anywhere, but it doesn't look like a blocker for the change to me.

Thanks,
Gabriele

[1] - https://lore.kernel.org/lkml/20260627081657.499781-1-rhkrqnwk98@xxxxxxxxx

> include/linux/sched.h | 5 ++++-
> kernel/sched/core.c   | 7 +++++--
> 2 files changed, 9 insertions(+), 3 deletions(-)
>
>diff --git a/include/linux/sched.h b/include/linux/sched.h
>index 59f6366fdf501..6003dcd080e6c 100644
>--- a/include/linux/sched.h
>+++ b/include/linux/sched.h
>@@ -2105,8 +2105,11 @@ static inline int test_tsk_thread_flag(struct task_struct *tsk, int flag)
> static inline void set_tsk_need_resched(struct task_struct *tsk)
> {
> 	if (tracepoint_enabled(sched_set_need_resched_tp) &&
>-	    !test_tsk_thread_flag(tsk, TIF_NEED_RESCHED))
>+	    !test_tsk_thread_flag(tsk, TIF_NEED_RESCHED)) {
>+		set_tsk_thread_flag(tsk, TIF_NEED_RESCHED);
> 		__trace_set_need_resched(tsk, TIF_NEED_RESCHED);
>+		return;
>+	}
> 	set_tsk_thread_flag(tsk,TIF_NEED_RESCHED);
> }
> 
>diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>index 91f059a556950..aed403fd2f82e 100644
>--- a/kernel/sched/core.c
>+++ b/kernel/sched/core.c
>@@ -1196,6 +1196,7 @@ static void __resched_curr(struct rq *rq, int tif)
> {
> 	struct task_struct *curr = rq->curr;
> 	struct thread_info *cti = task_thread_info(curr);
>+	bool send_ipi;
> 	int cpu;
> 
> 	lockdep_assert_rq_held(rq);
>@@ -1212,15 +1213,17 @@ static void __resched_curr(struct rq *rq, int tif)
> 
> 	cpu = cpu_of(rq);
> 
>-	trace_sched_set_need_resched_tp(curr, cpu, tif);
> 	if (cpu == smp_processor_id()) {
> 		set_ti_thread_flag(cti, tif);
> 		if (tif == TIF_NEED_RESCHED)
> 			set_preempt_need_resched();
>+		trace_sched_set_need_resched_tp(curr, cpu, tif);
> 		return;
> 	}
> 
>-	if (set_nr_and_not_polling(cti, tif)) {
>+	send_ipi = set_nr_and_not_polling(cti, tif);
>+	trace_sched_set_need_resched_tp(curr, cpu, tif);
>+	if (send_ipi) {
> 		if (tif == TIF_NEED_RESCHED)
> 			smp_send_reschedule(cpu);
> 	} else {