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From:   Huacai Chen <chenhuacai@kernel.org>
Date:   Thu, 24 Aug 2023 10:47:30 +0800
Message-ID: <CAAhV-H7o+SK__ySWQRkbB4B-LTBFS-ryosqH_r53ZrZ+W1ZE-A@mail.gmail.com>
Subject: Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Z qiang <qiang.zhang1211@gmail.com>, paulmck@kernel.org,
        Huacai Chen <chenhuacai@loongson.cn>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        John Stultz <jstultz@google.com>,
        Stephen Boyd <sboyd@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, Binbin Zhou <zhoubinbin@loongson.cn>
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Hi, Thomas,

On Thu, Aug 24, 2023 at 6:03=E2=80=AFAM Thomas Gleixner <tglx@linutronix.de=
> wrote:
>
> On Thu, Aug 17 2023 at 16:06, Huacai Chen wrote:
> > On Thu, Aug 17, 2023 at 3:27=E2=80=AFAM Joel Fernandes <joel@joelfernan=
des.org> wrote:
> >> > If  do_update_jiffies_64() cannot be used in NMI context,
> >>
> >> Can you not make the jiffies update conditional on whether it is
> >> called within NMI context?
>
> Which solves what? If KGDB has a breakpoint in the jiffies lock held
> region then you still dead lock.
>
> >> I dislike that..
> > Is this acceptable?
> >
> > void rcu_cpu_stall_reset(void)
> > {
> >         unsigned long delta;
> >
> >         delta =3D nsecs_to_jiffies(ktime_get_ns() - ktime_get_coarse_ns=
());
> >
> >         WRITE_ONCE(rcu_state.jiffies_stall,
> >                    jiffies + delta + rcu_jiffies_till_stall_check());
> > }
> >
> > This can update jiffies_stall without updating jiffies (but has the
> > same effect).
>
> Now you traded the potential dead lock on jiffies lock for a potential
> live lock vs. tk_core.seq. Not really an improvement, right?
>
> The only way you can do the above is something like the incomplete and
> uncompiled below. NMI safe and therefore livelock proof time interfaces
> exist for a reason.
>
> Thanks,
>
>         tglx
> ---
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -51,6 +51,13 @@ struct tick_sched *tick_get_tick_sched(i
>   */
>  static ktime_t last_jiffies_update;
>
> +unsigned long tick_estimate_stale_jiffies(void)
> +{
> +       ktime_t delta =3D ktime_get_mono_fast_ns() - READ_ONCE(last_jiffi=
es_update);
> +
> +       return delta < 0 ? 0 : div_s64(delta, TICK_NSEC);
> +}
> +
>  /*
>   * Must be called with interrupts disabled !
>   */
Thank you for your advice, now the latest proposal is here [1], this
is very similar to your diff, please take a look.

[1] https://lore.kernel.org/rcu/CAAhV-H5mePbbF8Y3t-JfV+PNP8BEcjKtX4UokzL_vD=
zyw+2BRg@mail.gmail.com/T/#t

Huacai
>
>