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

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Message-ID: <122cfd4ba6b0805e91ff09526d5d159ff3871964.camel@redhat.com>
Date: Wed, 16 Jul 2025 15:40:13 +0200
From: Gabriele Monaco <gmonaco@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>, Steven
 Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, 
	linux-trace-kernel@...r.kernel.org, Nam Cao <namcao@...utronix.de>, Tomas
 Glozar	 <tglozar@...hat.com>, Juri Lelli <jlelli@...hat.com>, Clark
 Williams	 <williams@...hat.com>, John Kacur <jkacur@...hat.com>
Subject: 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:
> > 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
> 
> > @@ -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() ?
> 
> > @@ -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