linux-kernel - Re: [PATCH v2 4/5] sched: Add task enqueue/dequeue trace points

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <20250819082443.GF3245006@noisy.programming.kicks-ass.net>
Date: Tue, 19 Aug 2025 10:24:43 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Nam Cao <namcao@...utronix.de>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Gabriele Monaco <gmonaco@...hat.com>,
	linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH v2 4/5] sched: Add task enqueue/dequeue trace points

On Tue, Aug 19, 2025 at 09:49:20AM +0200, Nam Cao wrote:
> On Fri, Aug 15, 2025 at 03:40:16PM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 06, 2025 at 10:01:20AM +0200, Nam Cao wrote:
> > 
> > > +/*
> > > + * The two trace points below may not work as expected for fair tasks due
> > > + * to delayed dequeue. See:
> > > + * https://lore.kernel.org/lkml/179674c6-f82a-4718-ace2-67b5e672fdee@amd.com/
> > > + */
> > 
> > > +DECLARE_TRACE(dequeue_task,
> > > +	TP_PROTO(int cpu, struct task_struct *task),
> > > +	TP_ARGS(cpu, task));
> > > +
> > 
> > > @@ -2119,7 +2121,11 @@ inline bool dequeue_task(struct rq *rq, struct task_struct *p, int flags)
> > >  	 * and mark the task ->sched_delayed.
> > >  	 */
> > >  	uclamp_rq_dec(rq, p);
> > > -	return p->sched_class->dequeue_task(rq, p, flags);
> > > +	if (p->sched_class->dequeue_task(rq, p, flags)) {
> > > +		trace_dequeue_task_tp(rq->cpu, p);
> > > +		return true;
> > > +	}
> > > +	return false;
> > >  }
> > 
> > Hurmpff.. that's not very nice.
> > 
> > How about something like:
> > 
> > dequeue_task():
> > 	...
> > 	ret = p->sched_class->dequeue_task(rq, p, flags);
> > 	if (trace_dequeue_task_p_enabled() && !(flags & DEQUEUE_SLEEP))
> > 		__trace_dequeue_task_tp(rq->cpu, p);
> > 	return ret;
> > 
> > 
> > __block_task():
> > 	trace_dequeue_task_tp(rq->cpu, p);
> > 	...
> > 
> > 
> > Specifically, only DEQUEUE_SLEEP is allowed to fail, and DEQUEUE_SLEEP
> > will eventually cause __block_task() to be called, either directly, or
> > delayed.
> 
> Thanks for the suggestion, this makes sense.
> 
> From my understanding, it makes the tracepoints work correctly for fair
> tasks too, so I will get rid of the comment.

Just so indeed :-)