linux-kernel - Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall

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Message-ID: <03fe7084-0509-45fa-87ee-8f8705a221a6@paulmck-laptop>
Date:   Wed, 23 Aug 2023 15:41:51 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Huacai Chen <chenhuacai@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Z qiang <qiang.zhang1211@...il.com>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        John Stultz <jstultz@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Binbin Zhou <zhoubinbin@...ngson.cn>
Subject: Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()

On Thu, Aug 24, 2023 at 12:03:25AM +0200, Thomas Gleixner wrote:
> On Thu, Aug 17 2023 at 16:06, Huacai Chen wrote:
> > On Thu, Aug 17, 2023 at 3:27 AM Joel Fernandes <joel@...lfernandes.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 = 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.

Just for completeness, another approach, with its own advantages
and disadvantage, is to add something like ULONG_MAX/4 to
rcu_state.jiffies_stall, but also set a counter indicating that this
has been done.  Then RCU's force-quiescent processing could decrement
that counter (if non-zero) and reset rcu_state.jiffies_stall when it
does reach zero.

Setting the counter to three should cover most cases, but "live by the
heuristic, die by the heuristic".  ;-)

It would be good to have some indication when gdb exited, but things
like the gdb "next" command can make that "interesting" when applied to
a long-running function.

 							Thanx, Paul

> 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 = ktime_get_mono_fast_ns() - READ_ONCE(last_jiffies_update);
> +
> +	return delta < 0 ? 0 : div_s64(delta, TICK_NSEC);
> +}
> +
>  /*
>   * Must be called with interrupts disabled !
>   */
> 
>