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

[Paul E. McKenney]

On Thu, Aug 24, 2023 at 01:09:42PM +0000, Joel Fernandes wrote:
> On Thu, Aug 24, 2023 at 04:40:42AM -0700, Paul E. McKenney wrote:
> > On Thu, Aug 24, 2023 at 10:50:41AM +0800, Huacai Chen wrote:
> > > 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.
> > 
> > I am not seeing the ULONG_MAX/2, so could you please point me to that
> > original code?
> > 
> > The advantage of ULONG_MAX/4 over ULONG_MAX/2 is that the time_after()
> > and time_before() macros have ULONG_MAX/4 slop in either direction
> > before giving you the wrong answer.  You can get nearly the same result
> > using ULONG_MAX/2, but it requires a bit more care.  And even on 32-bit
> > HZ=1000 systems, ULONG_MAX/4 gets you more than 12 days of gdb session
> > or jiffies-update delay before you start getting false positives.
> > 
> > Then things can be reset after (say) 3 calls to rcu_gp_fqs() and
> > also the current reset at the beginning of a grace period, which
> > is in record_gp_stall_check_time().
> 
> I like Paul's suggestion a lot except that if someone sets a breakpoint right
> when the jiffies is being reset, so then we have to come back to doing
> Thomas's suggestion.

Please note that ULONG_MAX / 4 allows for jiffies not having been reset
for more than 10 days on 32-bit systems and for many millions of years
on 64-bit systems.  ;-)

> So maybe a combination of Paul's and Thomas's suggestions (of using
> last_jiffies_update with the NMI-safe timestamp read) may work.

I am absolutely not a fan of reworking all of the RCU CPU stall-warning
code to use some other timebase, at least not without a very important
reason to do so.  Nothing mentioned in this thread even comes close to
that level of importance.

> > It would be better if RCU could get notified at both ends of the debug
> > session, but given gdb commands such as "next", along with Thomas's
> > point about gdb breakpoints being pretty much anywhere, this might or
> > might not be so helpful in real life.  But worth looking into.
> 
> True, I was curious if rcu_cpu_stall_reset() can be called on a tickless
> kernel as well before jiffies gets a chance to update, in which case I think
> your suggestion of biasing the stall time and later resetting it would help a
> lot for such situations.

What code path can possibly invoke rcu_cpu_stall_reset() after an
extended full-system nohz_full time period without first doing at least
one context switch on the CPU that invokes rcu_cpu_stall_reset()?

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> 
> > 							Thanx, Paul
> > 
> > > 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 !
> > > > >   */
> > > > >
> > > > >