linux-kernel - Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic

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Date:   Thu, 4 May 2023 11:55:15 -0300
From:   Wander Lairson Costa <wander@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        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>,
        Brian Cain <bcain@...cinc.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christian Brauner <brauner@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-perf-users@...r.kernel.org>, Hu Chunyu <chuhu@...hat.com>,
        Paul McKenney <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe() function

On Thu, May 4, 2023 at 11:34 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, May 04, 2023 at 02:29:45PM +0200, Oleg Nesterov wrote:
> > On 05/04, Peter Zijlstra wrote:
> > >
> > > Urgh.. that's plenty horrible. And I'm sure everybody plus kitchen sink
> > > has already asked why can't we just rcu free the thing unconditionally.
> > >
> > > Google only found me an earlier version of this same patch set, but I'm
> > > sure we've had that discussion many times over the past several years.
> >
> > Yes... see for example
> >
> > https://lore.kernel.org/lkml/CAHk-=whtj+aSYftniMRG4xvFE8dmmYyrqcJyPmzStsfj5w9r=w@mail.gmail.com/
> >
> > We already have an rcu pass before put_task_struct(zombie), see
> > put_task_struct_rcu_user(), another one look unfortunate.
>
> Ah indeed, it got mentioned there as well. And Linus seems to be arguing
> against doing an rcu free there. So humm..
>
> Then I'm thinking something trivial like so:
>
> static inline void put_task_struct(struct task_struct *t)
> {
>         if (!refcount_dec_and_test(&t->usage))
>                 return;
>
>         if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
>                 call_rcu(&t->rcu, __put_task_struct_rcu);
>
>         __put_task_struct(t);
> }
>

That's what v5 [1] does. What would be the path in this case? Should I
resend it as v8?

> should do, or alternatively use irq_work, which has a much lower
> latency, but meh..
>

Initially, I did that. I switched to call_rcu because the code got much simpler.

[1] https://lore.kernel.org/all/20230210161323.37400-1-wander@redhat.com/