Re: [PATCH v2] sched: set TIF_NEED_RESCHED before calling __trace_set_need_resched()
From: K Prateek Nayak
Date: Wed Jul 01 2026 - 02:54:09 EST
Hello Peter, Gabriele,
On 7/1/2026 2:04 AM, Peter Zijlstra wrote:
>> -static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
>> +static inline bool set_nr_and_not_polling(struct rq *rq, int tif)
>> {
>> + struct task_struct *curr = rq->curr;
>> + struct thread_info *ti = task_thread_info(curr);
>> +
>> set_ti_thread_flag(ti, tif);
>> return true;
>> }
>
> This !POLLING thing also needs tracing, no?
Indeed! I'm stupid.
Here is the full diff with set_nr_if_polling() covered too on top of
tip:sched/core + Sechang's v3 at
https://lore.kernel.org/lkml/20260630084750.2792851-1-rhkrqnwk98@xxxxxxxxx/
for completeness.
Let me know if you prefer folding it in or if this needs to be a
separate patch.
(lightly testet)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7cbd541f656f..bd2f7fb87dc9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1049,9 +1049,16 @@ static inline void hrtick_schedule_exit(struct rq *rq) { }
* this avoids any races wrt polling state changes and thereby avoids
* spurious IPIs.
*/
-static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
+static inline bool set_nr_and_not_polling(struct rq *rq, int tif)
{
- return !(fetch_or(&ti->flags, 1 << tif) & _TIF_POLLING_NRFLAG);
+ struct task_struct *curr = rq->curr;
+ struct thread_info *ti = task_thread_info(curr);
+ unsigned long old_flags = fetch_or(&ti->flags, 1 << tif);
+
+ if (trace_sched_set_need_resched_tp_enabled() && !(old_flags & (1 << tif)))
+ trace_call__sched_set_need_resched_tp(curr, cpu_of(rq), tif);
+
+ return !(old_flags & _TIF_POLLING_NRFLAG);
}
/*
@@ -1072,13 +1079,20 @@ static bool set_nr_if_polling(struct task_struct *p)
return true;
} while (!try_cmpxchg(&ti->flags, &val, val | _TIF_NEED_RESCHED));
+ trace_sched_set_need_resched_tp(p, task_cpu(p), TIF_NEED_RESCHED);
return true;
}
#else
-static inline bool set_nr_and_not_polling(struct thread_info *ti, int tif)
+static inline bool set_nr_and_not_polling(struct rq *rq, int tif)
{
- set_ti_thread_flag(ti, tif);
+ struct task_struct *curr = rq->curr;
+ struct thread_info *ti = task_thread_info(curr);
+ int set = test_and_set_ti_thread_flag(ti, tif);
+
+ if (trace_sched_set_need_resched_tp_enabled() && !set)
+ trace_call__sched_set_need_resched_tp(curr, cpu_of(rq), tif);
+
return true;
}
@@ -1186,7 +1200,6 @@ static void __resched_curr(struct rq *rq, int tif)
{
struct task_struct *curr = rq->curr;
struct thread_info *cti = task_thread_info(curr);
- bool need_ipi;
int cpu;
lockdep_assert_rq_held(rq);
@@ -1204,16 +1217,16 @@ static void __resched_curr(struct rq *rq, int tif)
cpu = cpu_of(rq);
if (cpu == smp_processor_id()) {
- set_ti_thread_flag(cti, tif);
+ int set = test_and_set_ti_thread_flag(cti, tif);
+
+ if (trace_sched_set_need_resched_tp_enabled() && !set)
+ trace_call__sched_set_need_resched_tp(curr, cpu, tif);
if (tif == TIF_NEED_RESCHED)
set_preempt_need_resched();
- trace_sched_set_need_resched_tp(curr, cpu, tif);
return;
}
- need_ipi = set_nr_and_not_polling(cti, tif);
- trace_sched_set_need_resched_tp(curr, cpu, tif);
- if (need_ipi) {
+ if (set_nr_and_not_polling(rq, tif)) {
if (tif == TIF_NEED_RESCHED)
smp_send_reschedule(cpu);
} else {
@@ -1353,7 +1366,7 @@ static void wake_up_idle_cpu(int cpu)
* and testing of the above solutions didn't appear to report
* much benefits.
*/
- if (set_nr_and_not_polling(task_thread_info(rq->idle), TIF_NEED_RESCHED))
+ if (set_nr_and_not_polling(rq, TIF_NEED_RESCHED))
smp_send_reschedule(cpu);
else
trace_sched_wake_idle_without_ipi(cpu);
---
Now, on a separate note to Gabriele, this trips up RV's sched:nrp monitor
when running sched-messaging because of the following race:
CPU0 CPU1
==== ====
<Any thread running>
... /* Wakes up a task on CPU0 */
<IRQ> rq_lock(rq0)
__resched_curr(rq0)
set = test_and_set_ti_thread_flag(curr, 0, TIF_NEED_RESCHED);
<... interrupted by something before tracepoint hits>
!!! rq0->curr has NEED_RESCHED set but trace_sched_set_need_resched_tp() is not called !!!
</IRQ>
raw_irqentry_exit_cond_resched()
if (!preempt_count() /* Masks PREEMPT_NEED_RESCHED */) {
if (need_resched() /* Only check TIF flags */) {
preempt_schedule_irq()
schedule(SM_PREEMPT)
trace_sched_entry_tp(true /* SM_PREEMPT */)
!!! SPLAT: rv: monitor nrp does not allow event schedule_entry_preempt on state any_thread_running !!!
Easy way to solve this is by moving the trace_sched_entry_tp() within
the rq_lock critical section. SM_PREEMPT has to happen on a
!POLLING CPU and __resched_curr() is always under the rq_lock() so
the __schedule() on a remote CPU cannot race with it before
trace_sched_set_need_resched_tp() is executed.
(lightly tested again)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bd2f7fb87dc93..961c325feca84 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7086,9 +7086,6 @@ static void __sched notrace __schedule(int sched_mode)
struct rq *rq;
int cpu;
- /* Trace preemptions consistently with task switches */
- trace_sched_entry_tp(sched_mode == SM_PREEMPT);
-
cpu = smp_processor_id();
rq = cpu_rq(cpu);
prev = rq->curr;
@@ -7121,6 +7118,9 @@ static void __sched notrace __schedule(int sched_mode)
rq_lock(rq, &rf);
smp_mb__after_spinlock();
+ /* Trace preemptions consistently with task switches */
+ trace_sched_entry_tp(sched_mode == SM_PREEMPT);
+
hrtick_schedule_enter(rq);
/* Promote REQ to ACT */
---
I'm not sure if this is a problem with current mainline but I did trip
it in my testing and the above seems to solve it - your mileage may vary
:-)
--
Thanks and Regards,
Prateek