Re: [PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()

[Huacai Chen]

Hi, Joel,

On Tue, Aug 22, 2023 at 11:34 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Aug 22, 2023 at 12:02:48PM +0800, Huacai Chen wrote:
> > The KGDB initial breakpoint gets an rcu stall warning after commit
> > a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall detection in
> > rcu_cpu_stall_reset()").
> >
> > [   53.452051] rcu: INFO: rcu_preempt self-detected stall on CPU
> [...]
> >
> > [1] https://lore.kernel.org/rcu/20230814020045.51950-1-chenhuacai@xxxxxxxxxxx/T/#t
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall detection in rcu_cpu_stall_reset()")
> > Reported-off-by: Binbin Zhou <zhoubinbin@xxxxxxxxxxx>
> > Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> > ---
> > V2: Use NMI safe functions.
> > V3: Add comments to explain why.
> >
> >  kernel/rcu/tree_stall.h | 17 ++++++++++++++++-
> >  1 file changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> > index b10b8349bb2a..e4e53113d062 100644
> > --- a/kernel/rcu/tree_stall.h
> > +++ b/kernel/rcu/tree_stall.h
> > @@ -150,11 +150,26 @@ static void panic_on_rcu_stall(void)
> >   * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
> >   *
> >   * The caller must disable hard irqs.
> > + *
> > + * The jiffies updating may be delayed for a very long time due to tickless and
> > + * irq disabled, especially in the KGDB case, so we need to add the delayed time
> > + * (delta) to rcu_state.jiffies_stall.
> > + *
> > + * This function may be called in NMI context, so we cannot use ktime_get_ns()
> > + * and ktime_get_coarse_ns(). Instead, we use their inaccurate but safe friends
> > + * ktime_get_mono_fast_ns() and ktime_get_seconds() which will cause rcu_state.
> > + * jiffies_stall to be a little large than expected (harmless and safer).
> >   */
> >  void rcu_cpu_stall_reset(void)
> >  {
> > +     u64 curr, last, delta;
> > +
> > +     curr = ktime_get_mono_fast_ns();
> > +     last = ktime_get_seconds() * NSEC_PER_SEC;
> > +     delta = nsecs_to_jiffies(curr - last);
> > +
> >       WRITE_ONCE(rcu_state.jiffies_stall,
> > -                jiffies + rcu_jiffies_till_stall_check());
> > +                jiffies + delta + rcu_jiffies_till_stall_check());
> >  }
>
> I prefer the following diff on top of your patch to take advantage of UBSAN
> detecting overflows.
>
> If you take my diff, feel free to add:
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>
> ---8<-----------------------
>
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 5e9e4779bdf1..3398cf2d19c5 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -162,14 +162,15 @@ static void panic_on_rcu_stall(void)
>   */
>  void rcu_cpu_stall_reset(void)
>  {
> -       u64 curr, last, delta;
> +       ktime_t last, delta_ns;
> +       u64 delta_jiff;
>
> -       curr = ktime_get_mono_fast_ns();
>         last = ktime_get_seconds() * NSEC_PER_SEC;
> -       delta = nsecs_to_jiffies(curr - last);
> +       delta_ns = ktime_sub(ktime_get_mono_fast_ns(), last);
Though ktime_t is the same as s64/u64 now, but I think we'd better to
not mix them. Then, ktime_get() and ktime_get_coarse() return ktime_t;
ktime_get_ns(), ktime_get_coarse_ns() and ktime_get_mono_fast_ns()
return s64/u64 (means nanoseconds); ktime_get_seconds() returns
seconds but ktime_get_seconds() * NSEC_PER_SEC is also nanoseconds.
So, the type definition in your diff is not suitable,
ktime_sub(ktime_get_mono_fast_ns(), last) is not suitable too.

Huacai

> +       delta_jiff = nsecs_to_jiffies(delta_ns);
>
>         WRITE_ONCE(rcu_state.jiffies_stall,
> -                  jiffies + delta + rcu_jiffies_till_stall_check());
> +                  jiffies + delta_jiff + rcu_jiffies_till_stall_check());
>  }
>
>  //////////////////////////////////////////////////////////////////////////////