linux-kernel - Re: [PATCH v2 1/4] sched/task: Add the put_task_struct_atomic

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Date:   Mon, 30 Jan 2023 15:20:03 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Wander Lairson Costa <wander@...hat.com>
Cc:     Oleg Nesterov <oleg@...hat.com>, 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>,
        "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 30/01/23 11:58, Wander Lairson Costa wrote:
> On Mon, Jan 30, 2023 at 11:47 AM Valentin Schneider <vschneid@...hat.com> wrote:
>>
>> On 30/01/23 08:49, Wander Lairson Costa wrote:
>> > On Fri, Jan 27, 2023 at 12:55 PM Valentin Schneider <vschneid@...hat.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.
>

True, that'd be an improvement.

>> >> 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 would wager anything fork-heavy with short-lived tasks, say loops of
short hackbench runs, I belive stress-ng also has a fork test case.