[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAAq0SUk1vfNDuzGbXNftgW4wq4PC_EzMhpq4E=RBQNkOB3f4YQ@mail.gmail.com>
Date: Mon, 23 Jan 2023 14:24:40 -0300
From: Wander Lairson Costa <wander@...hat.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Stafford Horne <shorne@...il.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Andy Lutomirski <luto@...nel.org>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Fenghua Yu <fenghua.yu@...el.com>,
Andrei Vagin <avagin@...il.com>,
open list <linux-kernel@...r.kernel.org>,
Paul McKenney <paulmck@...nel.org>
Subject: Re: [PATCH v2 1/4] sched/task: Add the put_task_struct_atomic_safe function
On Mon, Jan 23, 2023 at 1:30 PM Oleg Nesterov <oleg@...hat.com> wrote:
>
> On 01/20, Wander Lairson Costa wrote:
> >
> > +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> > +{
> > + if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > + /*
> > + * Decrement the refcount explicitly to avoid unnecessarily
> > + * calling call_rcu.
> > + */
> > + if (refcount_dec_and_test(&task->usage))
> > + /*
> > + * under PREEMPT_RT, we can't call put_task_struct
> > + * in atomic context because it will indirectly
> > + * acquire sleeping locks.
> > + */
> > + call_rcu(&task->rcu, __delayed_put_task_struct);
> ^^^^^^^^^
> I am not sure the usage of task->rcu is safe...
>
> Suppose that, before __delayed_put_task_struct() is called by RCU, this task
> does the last schedule and calls put_task_struct_rcu_user().
>
> And, can't we simply turn put_task_struct() into something like
>
> put_task_struct(struct task_struct *t)
> {
> if (refcount_dec_and_test(&t->usage)) {
> if (IS_ENABLED(CONFIG_PREEMPT_RT)
> && (in_atomic() || irqs_disabled()))
> call_rcu(...);
> else
> __put_task_struct(t);
> }
> }
>
> ?
Yeah, that was one approach I thought about. I chose to use an
explicit function because I assumed calling __put_task_struct() from a
non-preemptable context should be the exception, not the rule.
Therefore (if I am correct in my assumption), it would make sense for
only some call sites to pay the overhead price for it. But this is
just a guess, and I have no evidence to support my claim.
Powered by blists - more mailing lists