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

From: Randy Dunlap <[email-masked]>
Date: Sun, 27 Apr 2014 10:27:43 -0700

On 04/26/14 23:26, 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.
> 
> 							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]>

Still on uml on x86_64 defconfig:

  LD      init/built-in.o
init/version.o: In function `rcu_sysrq_start':
include/linux/rcupdate.h:258: multiple definition of `rcu_sysrq_start'
init/main.o:include/linux/rcupdate.h:258: first defined here
init/version.o: In function `rcu_sysrq_end':
include/linux/rcupdate.h:261: multiple definition of `rcu_sysrq_end'
init/main.o:include/linux/rcupdate.h:261: first defined here
init/mounts.o: In function `rcu_sysrq_start':
include/linux/rcupdate.h:258: multiple definition of `rcu_sysrq_start'
init/main.o:include/linux/rcupdate.h:258: first defined here
init/mounts.o: In function `rcu_sysrq_end':
include/linux/rcupdate.h:261: multiple definition of `rcu_sysrq_end'
init/main.o:include/linux/rcupdate.h:261: first defined here
init/noinitramfs.o: In function `rcu_sysrq_start':
include/linux/rcupdate.h:258: multiple definition of `rcu_sysrq_start'
init/main.o:include/linux/rcupdate.h:258: first defined here
init/noinitramfs.o: In function `rcu_sysrq_end':
include/linux/rcupdate.h:261: multiple definition of `rcu_sysrq_end'
init/main.o:include/linux/rcupdate.h:261: first defined here
init/init_task.o: In function `rcu_sysrq_start':
include/linux/rcupdate.h:258: multiple definition of `rcu_sysrq_start'
init/main.o:include/linux/rcupdate.h:258: first defined here
init/init_task.o: In function `rcu_sysrq_end':
include/linux/rcupdate.h:261: multiple definition of `rcu_sysrq_end'
init/main.o:include/linux/rcupdate.h:261: first defined here
make[2]: *** [init/built-in.o] Error 1

> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index a6f2664a1b77..3f53c96b3b77 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 */
> +void rcu_sysrq_start(void)
> +{
> +}
> +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;
> 

-- 
~Randy