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

[Andrea Righi <arighi@nvidia.com>]

Hi Gabriele,

On Sat, Sep 12, 2026 at 07:13:48AM +0000, Gabriele Monaco wrote:
> Il 11 settembre 2026 21:33:00 UTC, Andrea Righi <arighi@nvidia.com> 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@nvidia.com>
> >---
> 
> 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.
> 

It is **exactly** the same fix! I hit the same issue and missed Sechang's
series. There's also a v3:
https://lore.kernel.org/all/20260630084750.2792851-1-rhkrqnwk98@gmail.com/

We can ignore this one and go with Sechang's. Sorry for the noise.

Thanks,
-Andrea

> Thanks,
> Gabriele
> 
> [1] - https://lore.kernel.org/lkml/20260627081657.499781-1-rhkrqnwk98@gmail.com
> 
> > 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 {
>