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

[Alan Huang]

>>>>>>>>> 
>>>>>>>>> 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?

> 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