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Date:   Thu, 4 May 2023 16:33:03 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Wander Lairson Costa <wander@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
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        Namhyung Kim <namhyung@kernel.org>,
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        Adrian Hunter <adrian.hunter@intel.com>,
        Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
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        "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>,
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        Shakeel Butt <shakeelb@google.com>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-perf-users@vger.kernel.org>, Hu Chunyu <chuhu@redhat.com>,
        Paul McKenney <paulmck@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe()
 function
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References: <20230425114307.36889-1-wander@redhat.com>
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 <20230504122945.GA28757@redhat.com>
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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);
}

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