Re: [PATCH v3 12/17] sched: Adapt sched tracepoints for RV task model

On Wed, 2025-07-16 at 14:38 +0200, Peter Zijlstra wrote:

On Tue, Jul 15, 2025 at 09:14:29AM +0200, Gabriele Monaco wrote:

quoted

Add the following tracepoints:
* sched_set_need_resched(tsk, cpu, tif)
    Called when a task is set the need resched [lazy] flag
* sched_switch_vain(preempt, tsk, tsk_state)
    Called when a task is selected again during __schedule
    i.e. prev == next == tsk : no real context switch

quoted

@@ -6592,6 +6598,7 @@ static bool try_to_block_task(struct rq *rq,

struct task_struct *p,
 	int flags = DEQUEUE_NOCLOCK;
 
 	if (signal_pending_state(task_state, p)) {
+		trace_sched_set_state_tp(p, TASK_RUNNING, true);
 		WRITE_ONCE(p->__state, TASK_RUNNING);
 		*task_state_p = TASK_RUNNING;
 		return false;

I'm confused on the purpose of this. How does this relate to say the
wakeup in signal_wake_up_state() ?

quoted

@@ -6786,6 +6793,7 @@ static void __sched notrace __schedule(int

sched_mode)
 		rq = context_switch(rq, prev, next, &rf);
 	} else {
 		rq_unpin_lock(rq, &rf);
+		trace_sched_switch_vain_tp(preempt, prev,
prev_state);
 		__balance_callbacks(rq);
 		raw_spin_rq_unlock_irq(rq);
 	}

Hurmph... don't you already have this covered by:
trace_sched_exit_tp() ?

Specifically, the only case where is_switch := false, is this case.

Mostly, it may work in some cases, but sched_exit happens with
interrupt enabled while all types of switches (including the vain ones)
must occur with interrupt disabled.

Some assumptions don't stand without this tracepoint, but I guess I
could adapt monitors to live without this if you believe it's not worth
adding a new tracepoint there.

Thanks,
Gabriele