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

[Huacai Chen]

Hi, Alan,

On Wed, Aug 16, 2023 at 11:57 PM Alan Huang <mmpgouride@xxxxxxxxx> wrote:
>
> >>>>>>>>>
> >>>>>>>>> Currently rcu_cpu_stall_reset() set rcu_state.jiffies_stall to one check
> >>>>>>>>> period later, i.e. jiffies + rcu_jiffies_till_stall_check(). But jiffies
> >>>>>>>>> is only updated in the timer interrupt, so when kgdb_cpu_enter() begins
> >>>>>>>>> to run there may already be nearly one rcu check period after jiffies.
> >>>>>>>>> Since all interrupts are disabled during kgdb_cpu_enter(), jiffies will
> >>>>>>>>> not be updated. When kgdb_cpu_enter() returns, rcu_state.jiffies_stall
> >>>>>>>>> maybe already gets timeout.
> >>>>>>>>>
> >>>>>>>>> We can set rcu_state.jiffies_stall to two rcu check periods later, e.g.
> >>>>>>>>> jiffies + (rcu_jiffies_till_stall_check() * 2) in rcu_cpu_stall_reset()
> >>>>>>>>> to avoid this problem. But this isn't a complete solution because kgdb
> >>>>>>>>> may take a very long time in irq disabled context.
> >>>>>>>>>
> >>>>>>>>> Instead, update jiffies at the beginning of rcu_cpu_stall_reset() can
> >>>>>>>>> solve all kinds of problems.
> >>>>>>>>
> >>>>>>>> Would it make sense for there to be a kgdb_cpu_exit()?  In that case,
> >>>>>>>> the stalls could simply be suppressed at the beginning of the debug
> >>>>>>>> session and re-enabled upon exit, as is currently done for sysrq output
> >>>>>>>> via rcu_sysrq_start() and rcu_sysrq_end().
> >>>>>>> Thank you for your advice, but that doesn't help. Because
> >>>>>>> rcu_sysrq_start() and rcu_sysrq_end() try to suppress the warnings
> >>>>>>> during sysrq, but kgdb already has no warnings during kgdb_cpu_enter()
> >>>>>>> since it is executed in irq disabled context. Instead, this patch
> >>>>>>> wants to suppress the warnings *after* kgdb_cpu_enter() due to a very
> >>>>>>> old jiffies value.
> >>>>>>>
> >>>>>>
> >>>>>> Hello, Huacai
> >>>>>>
> >>>>>> Is it possible to set  the rcu_cpu_stall_suppress is true in
> >>>>>> dbg_touch_watchdogs()
> >>>>>> and reset the rcu_cpu_stall_suppress at the beginning and end of the
> >>>>>> RCU grace period?
> >>>>> This is possible but not the best: 1, kgdb is not the only caller of
> >>>>> rcu_cpu_stall_reset(); 2, it is difficult to find the "end" to reset
> >>>>> rcu_cpu_stall_suppress.
> >>>>>
> >>>>
> >>>> You can replace rcu_state.jiffies_stall update by setting rcu_cpu_stall_suppress
> >>>> in rcu_cpu_stall_reset(),  and reset rcu_cpu_stall_suppress in rcu_gp_init() and
> >>>> rcu_gp_cleanup().
> >>> What's the advantage compared with updating jiffies? Updating jiffies
> >>> seems more straight forward.
> >>>
> >>
> >> In do_update_jiffies_64(), need to acquire jiffies_lock raw spinlock,
> >> like you said, kgdb is not the only caller of rcu_cpu_stall_reset(),
> >> the rcu_cpu_stall_reset() maybe invoke in NMI  (arch/x86/platform/uv/uv_nmi.c)
> > Reset rcu_cpu_stall_suppress in rcu_gp_init()/rcu_gp_cleanup() is
> > still not so good to me, because it does a useless operation in most
> > cases. Moreover, the rcu core is refactored again and again, something
> > may be changed in future.
> >
> > If  do_update_jiffies_64() cannot be used in NMI context, can we
>
> What about updating jiffies in dbg_touch_watchdogs or adding a wrapper which updates
> both jiffies and jiffies_stall?
This can solve the kgdb problem, but I found that most callers of
rcu_cpu_stall_reset() are in irq disabled context so they may meet
similar problems. Modifying rcu_cpu_stall_reset() can solve all of
them.

But due to the NMI issue, from my point of view, setting jiffies_stall
to jiffies + 300*HZ is the best solution now. :)

Huacai
>
> > consider my old method [1]?
> > https://lore.kernel.org/rcu/CAAhV-H7j9Y=VvRLm8thLw-EX1PGqBA9YfT4G1AN7ucYS=iP+DQ@xxxxxxxxxxxxxx/T/#t
> >
> > Of course we should set rcu_state.jiffies_stall large enough, so we
> > can do like this:
> >
> > void rcu_cpu_stall_reset(void)
> > {
> >  WRITE_ONCE(rcu_state.jiffies_stall,
> > -   jiffies + rcu_jiffies_till_stall_check());
> > +   jiffies + 300 * HZ);
> > }
> >
> > 300s is the largest timeout value, and I think 300s is enough here in practice.
> >
> > Huacai
> >
> >>
> >> Thanks
> >> Zqiang
> >>
> >>
> >>> Huacai
> >>>
> >>>>
> >>>> Thanks
> >>>> Zqiang
> >>>>
> >>>>>
> >>>>>> or set rcupdate.rcu_cpu_stall_suppress_at_boot=1 in bootargs can
> >>>>>> suppress RCU stall
> >>>>>> in booting.
> >>>>> This is also possible, but it suppresses all kinds of stall warnings,
> >>>>> which is not what we want.
> >>>>>
> >>>>> Huacai
> >>>>>>
> >>>>>>
> >>>>>> Thanks
> >>>>>> Zqiang
> >>>>>>
> >>>>>>
> >>>>>>>
> >>>>>>> Huacai
> >>>>>>>
> >>>>>>>>
> >>>>>>>>                                                        Thanx, Paul
> >>>>>>>>
> >>>>>>>>> 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>
> >>>>>>>>> ---
> >>>>>>>>> kernel/rcu/tree_stall.h | 1 +
> >>>>>>>>> 1 file changed, 1 insertion(+)
> >>>>>>>>>
> >>>>>>>>> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> >>>>>>>>> index b10b8349bb2a..1c7b540985bf 100644
> >>>>>>>>> --- a/kernel/rcu/tree_stall.h
> >>>>>>>>> +++ b/kernel/rcu/tree_stall.h
> >>>>>>>>> @@ -153,6 +153,7 @@ static void panic_on_rcu_stall(void)
> >>>>>>>>>  */
> >>>>>>>>> void rcu_cpu_stall_reset(void)
> >>>>>>>>> {
> >>>>>>>>> +     do_update_jiffies_64(ktime_get());
> >>>>>>>>>      WRITE_ONCE(rcu_state.jiffies_stall,
> >>>>>>>>>                 jiffies + rcu_jiffies_till_stall_check());
> >>>>>>>>> }
> >>>>>>>>> --
> >>>>>>>>> 2.39.3
>
>