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From:   Wander Lairson Costa <wander@redhat.com>
Date:   Mon, 30 Jan 2023 11:58:52 -0300
Message-ID: <CAAq0SUkATfafSdoKK+u4MYwtJpYjp7jY_f5t+fvp69x-iP=CXg@mail.gmail.com>
Subject: Re: [PATCH v2 1/4] sched/task: Add the put_task_struct_atomic_safe function
To:     Valentin Schneider <vschneid@redhat.com>
Cc:     Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Stafford Horne <shorne@gmail.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Andrei Vagin <avagin@gmail.com>,
        open list <linux-kernel@vger.kernel.org>,
        Paul McKenney <paulmck@kernel.org>
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On Mon, Jan 30, 2023 at 11:47 AM Valentin Schneider <vschneid@redhat.com> wrote:
>
> On 30/01/23 08:49, Wander Lairson Costa wrote:
> > On Fri, Jan 27, 2023 at 12:55 PM Valentin Schneider <vschneid@redhat.com> wrote:
> >>
> >> On 23/01/23 14:24, Wander Lairson Costa wrote:
> >> > 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.
> >>
> >> My worry here is that it's easy to miss problematic callgraphs, and it's
> >> potentially easy for new ones to creep in. Having a solution within
> >> put_task_struct() itself would prevent that.
> >>
> >
> > We could add a WARN_ON statement in put_task_struct() to detect such cases.
> >
>
> Anyone running their kernel with DEBUG_ATOMIC_SLEEP should be able to
> detect misuse, but it doesn't change that some callgraphs will only
> materialize under certain hardware/configuration combos.
>

If we put a WARN_ON in put_task_struct(), we catch cases where the
reference count didn't reach zero.

> >> Another thing, if you look at release_task_stack(), it either caches the
> >> outgoing stack for later use, or frees it via RCU (regardless of
> >> PREEMPT_RT). Perhaps we could follow that and just always punt the freeing
> >> of the task struct to RCU?
> >>
> >
> > That's a point. Do you mean doing that even for !PREEMPT_RT?
>
> Could be worth a try?

Sure. But I would do it only for PREEMPT_RT.

> I think because of the cache thing the task stack is
> a bit less aggressive wrt RCU callback processing, but at a quick glance I
> don't see any fundamental reason why the task_struct itself can't be given
> the same treatment.
>

Any idea about tests to catch performance regressions?

I