Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()

[Huacai Chen]

Hi, Thomas,

On Thu, Aug 24, 2023 at 6:03 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Thu, Aug 17 2023 at 16:06, Huacai Chen wrote:
> > On Thu, Aug 17, 2023 at 3:27 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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.
>
> 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 !
>   */
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_vDzyw+2BRg@xxxxxxxxxxxxxx/T/#t

Huacai
>
>