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

[Huacai Chen]

Hi, Paul,

On Thu, Aug 24, 2023 at 6:41 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> 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@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.
>
> 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.
The original code is adding ULONG_MAX/2, so adding ULONG_MAX/4 may
make no much difference? The simplest way is adding 300*HZ, but Joel
dislikes that.

Huacai

>
>                                                         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 !
> >   */
> >
> >