Subject: Re: mmotm 2014-04-24-13-07 uploaded

From: Randy Dunlap <[email-masked]>
Date: Mon, 28 Apr 2014 10:17:08 -0700

On 04/28/14 06:55, Paul E. McKenney wrote:
> On Sat, Apr 26, 2014 at 11:26:50PM -0700, Paul E. McKenney wrote:
>> On Fri, Apr 25, 2014 at 01:52:30PM -0700, Randy Dunlap wrote:
>>> On 04/25/14 13:44, Randy Dunlap wrote:
>>>> On 04/25/14 07:04, Rik van Riel wrote:
>>>>> On Thu, 24 Apr 2014 15:40:29 -0700
>>>>> Randy Dunlap <[email-masked]> wrote:
>>>>>
>>>>>
>>>>>> uml on x86_64 defconfig:
>>>>>>
>>>>>> drivers/built-in.o: In function `__handle_sysrq':
>>>>>> drivers/tty/sysrq.c:514: undefined reference to `rcu_sysrq_start'
>>>>>> drivers/tty/sysrq.c:558: undefined reference to `rcu_sysrq_end'
>>>>>
>>>>> Randy, does the patch below fix it?
>>>
>>> CONFIG_TINY_RCU=y
>>>
>>> tiny.c does not provide these functions...  Paul...
>>>
>>>> Hm, no, it does not.  I'll look into it.
>>>>
>>>>> Mike, this patch should also address your concerns.
>>>>>
>>>>> Andrew, this can be a -fix patch for the sysctl-rcu patch,
>>>>> assuming it fixes things :)
>>>>>
>>>>> Signed-off-by: Rik van Riel <[email-masked]
>>>>> ---
>>>>>  drivers/tty/sysrq.c | 1 +
>>>>>  kernel/rcu/update.c | 3 ++-
>>>>>  2 files changed, 3 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
>>>>> index dd53af9..0867433 100644
>>>>> --- a/drivers/tty/sysrq.c
>>>>> +++ b/drivers/tty/sysrq.c
>>>>> @@ -46,6 +46,7 @@
>>>>>  #include <linux/jiffies.h>
>>>>>  #include <linux/syscalls.h>
>>>>>  #include <linux/of.h>
>>>>> +#include <linux/rcupdate.h>
>>>>>  
>>>>>  #include <asm/ptrace.h>
>>>>>  #include <asm/irq_regs.h>
>>>>> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
>>>>> index 2ac3289..d22309c 100644
>>>>> --- a/kernel/rcu/update.c
>>>>> +++ b/kernel/rcu/update.c
>>>>> @@ -322,7 +322,8 @@ int rcu_jiffies_till_stall_check(void)
>>>>>  
>>>>>  void rcu_sysrq_start(void)
>>>>>  {
>>>>> -	rcu_cpu_stall_suppress = 2;
>>>>> +	if (!rcu_cpu_stall_suppress)
>>>>> +		rcu_cpu_stall_suppress = 2;
>>>>>  }
>>>>>  
>>>>>  void rcu_sysrq_end(void)
>>
>> You would think that I would have learned by now that RCU requires a bit
>> more care...  Anyway, patch containing only the APIs that -should- work
>> is below.  Currently testing it on the usual set of configs.
> 
> Right...  Adding a "static inline" would help, wouldn't it?  Updated
> patch below.
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> rcu: Provide API to suppress stall warnings while sysrc runs
> 
> Some sysrq handlers can run for a long time, because they dump a lot
> of data onto a serial console. Having RCU stall warnings pop up in
> the middle of them only makes the problem worse.
> 
> This commit provides rcu_sysrq_start() and rcu_sysrq_end() APIs to
> temporarily suppress RCU CPU stall warnings while a sysrq request is
> handled.
> 
> Signed-off-by: Rik van Riel <[email-masked]>
> [ paulmck: Fix TINY_RCU build error. ]
> Signed-off-by: Paul E. McKenney <[email-masked]>
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index a6f2664a1b77..ca6fe55913b7 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -248,6 +248,18 @@ void rcu_idle_exit(void);
>  void rcu_irq_enter(void);
>  void rcu_irq_exit(void);
>  
> +#ifdef CONFIG_RCU_STALL_COMMON
> +void rcu_sysrq_start(void);
> +void rcu_sysrq_end(void);
> +#else /* #ifdef CONFIG_RCU_STALL_COMMON */
> +static inline void rcu_sysrq_start(void)
> +{
> +}
> +static inline void rcu_sysrq_end(void)
> +{
> +}
> +#endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
> +
>  #ifdef CONFIG_RCU_USER_QS
>  void rcu_user_enter(void);
>  void rcu_user_exit(void);
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index ed7a0d72562c..a2aeb4df0f60 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -320,6 +320,18 @@ int rcu_jiffies_till_stall_check(void)
>  	return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
>  }
>  
> +void rcu_sysrq_start(void)
> +{
> +	if (!rcu_cpu_stall_suppress)
> +		rcu_cpu_stall_suppress = 2;
> +}
> +
> +void rcu_sysrq_end(void)
> +{
> +	if (rcu_cpu_stall_suppress == 2)
> +		rcu_cpu_stall_suppress = 0;
> +}
> +
>  static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
>  {
>  	rcu_cpu_stall_suppress = 1;
> 
> --

uml defconfig on x86_64:

  CC      arch/um/kernel/asm-offsets.s
In file included from include/linux/sem.h:5:0,
                 from include/linux/sched.h:35,
                 from arch/x86/um/shared/sysdep/kernel-offsets.h:2,
                 from arch/um/kernel/asm-offsets.c:1:
include/linux/rcupdate.h:257:20: error: static declaration of 'rcu_sysrq_start' follows non-static declaration
include/linux/rcupdate.h:250:6: note: previous declaration of 'rcu_sysrq_start' was here
include/linux/rcupdate.h:260:20: error: static declaration of 'rcu_sysrq_end' follows non-static declaration
include/linux/rcupdate.h:251:6: note: previous declaration of 'rcu_sysrq_end' was here
make[2]: *** [arch/um/kernel/asm-offsets.s] Error 1

Please test your patch(es).

-- 
~Randy