From:   Valentin Schneider <vschneid@redhat.com>
To:     Wander Lairson Costa <wander@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>
Subject: Re: [PATCH v2 1/4] sched/task: Add the put_task_struct_atomic_safe
 function
In-Reply-To: <CAAq0SUnUH6DEjwEs2RxRCtkTU121JXpdsV_rZky1d0Bo04=fiQ@mail.gmail.com>
References: <20230120150246.20797-1-wander@redhat.com>
 <20230120150246.20797-2-wander@redhat.com>
 <20230123163002.GB6268@redhat.com>
 <CAAq0SUk1vfNDuzGbXNftgW4wq4PC_EzMhpq4E=RBQNkOB3f4YQ@mail.gmail.com>
 <xhsmhbkmkdla4.mognet@vschneid.remote.csb>
 <CAAq0SUnUH6DEjwEs2RxRCtkTU121JXpdsV_rZky1d0Bo04=fiQ@mail.gmail.com>
Date:   Mon, 30 Jan 2023 14:46:54 +0000
Message-ID: <xhsmh357sdqqp.mognet@vschneid.remote.csb>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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.

>> 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? 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.