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From:   Wander Lairson Costa <wander@redhat.com>
Date:   Fri, 20 Jan 2023 17:14:58 -0300
Message-ID: <CAAq0SUmt7HmyZes1ujA_TvA0gW9b7fX5moXywAtcpmzAiyfn2Q@mail.gmail.com>
Subject: Re: [PATCH v2 0/4] Fix put_task_struct() calls under PREEMPT_RT
To:     Valentin Schneider <vschneid@redhat.com>
Cc:     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>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Oleg Nesterov <oleg@redhat.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>
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On Fri, Jan 20, 2023 at 2:45 PM Valentin Schneider <vschneid@redhat.com> wrote:
>
> On 20/01/23 at 12:02, Wander Lairson Costa wrote:
> > put_task_struct() decrements a usage counter and calls
> > __put_task_struct() if the counter reaches zero.
> >
> > __put_task_struct() indirectly acquires a spinlock, which is a sleeping
> > lock under PREEMPT_RT. Therefore, we can't call put_task_struct() in an
> > atomic context in RT kernels.
> >
> > This patch series introduces put_task_struct_atomic_safe(), which defers
> > the call to __put_task_struct() to a process context when compiled with
> > PREEMPT_RT.
> >
> > It also fixes known problematic call sites.
> >
>
> Browsing around put_task_struct() callsites gives me the impression there
> are more problematic call sites lurking around, which makes me wonder:
> should we make the PREEMPT_RT put_task_struct() *always* be done via
> call_rcu()?
>

I thought about going on this route, but I was concerned about the
performance side effects this approach could bring. Another idea I had
was to check at runtime if we are in a preemptible context. Again,
this would have a (minor?) performance penalty.

> The task's stack is actually always freed that way in put_task_stack(), cf.
>
>   e540bf3162e8 ("fork: Only cache the VMAP stack in finish_task_switch()")
>
> > Changelog:
> > ==========
> >
> > v2:
> >  * Add the put_task_struct_atomic_safe() function that is responsible for
> >    handling the conditions to call put_task_struct().
> >  * Replace put_task_struct() by put_task_struct_atomic_safe() in known
> >    atomic call sites.
> >
> > Wander Lairson Costa (4):
> >   sched/task: Add the put_task_struct_atomic_safe function
> >   sched/deadline: fix inactive_task_timer splat
> >   sched/rt: use put_task_struct_atomic_safe() to avoid potential splat
> >   sched/core: use put_task_struct_atomic_safe() to avoid potential splat
> >
> >  include/linux/sched/task.h | 21 +++++++++++++++++++++
> >  kernel/fork.c              |  8 ++++++++
> >  kernel/sched/core.c        |  2 +-
> >  kernel/sched/deadline.c    |  2 +-
> >  kernel/sched/rt.c          |  4 ++--
> >  5 files changed, 33 insertions(+), 4 deletions(-)
> >
> > --
> > 2.39.0
>