linux-kernel - Re: next: Commit 'mm: Prevent __alloc_pages_nodemask() RCU CPU stall ...' causing hang on sparc32 qemu

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Message-ID: <929f6b29-461a-6e94-fcfd-710c3da789e9@roeck-us.net>
Date:   Wed, 30 Nov 2016 02:52:11 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     paulmck@...ux.vnet.ibm.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        sparclinux@...r.kernel.org, davem@...emloft.net
Subject: Re: next: Commit 'mm: Prevent __alloc_pages_nodemask() RCU CPU stall
 ...' causing hang on sparc32 qemu

On 11/29/2016 11:02 PM, Paul E. McKenney wrote:
> On Tue, Nov 29, 2016 at 08:32:51PM -0800, Guenter Roeck wrote:
>> On 11/29/2016 05:28 PM, Paul E. McKenney wrote:
>>> On Tue, Nov 29, 2016 at 01:23:08PM -0800, Guenter Roeck wrote:
>>>> Hi Paul,
>>>>
>>>> most of my qemu tests for sparc32 targets started to fail in next-20161129.
>>>> The problem is only seen in SMP builds; non-SMP builds are fine.
>>>> Bisect points to commit 2d66cccd73436 ("mm: Prevent __alloc_pages_nodemask()
>>>> RCU CPU stall warnings"); reverting that commit fixes the problem.
>>>>
>>>> Test scripts are available at:
>>>> 	https://github.com/groeck/linux-build-test/tree/master/rootfs/sparc
>>>> Test results are at:
>>>> 	https://github.com/groeck/linux-build-test/tree/master/rootfs/sparc
>>>>
>>>> Bisect log is attached.
>>>>
>>>> Please let me know if there is anything I can do to help tracking down the
>>>> problem.
>>>
>>> Apologies!!!  Does the patch below help?
>>>
>> No, sorry, it doesn't make a difference.
>
> Interesting...  Could you please send me the build failure messages?
>

There is no failure message; it just hangs until I abort the qemu session.

http://kerneltests.org/builders/qemu-sparc-next/builds/532/steps/qemubuildcommand/logs/stdio

Guenter

> 							Thanx, Paul
>
>> Guenter
>>
>>> 							Thanx, Paul
>>>
>>> ------------------------------------------------------------------------
>>>
>>> commit 97708e737e2a55fed4bdbc005bf05ea909df6b73
>>> Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>>> Date:   Tue Nov 29 11:06:05 2016 -0800
>>>
>>>    rcu: Allow boot-time use of cond_resched_rcu_qs()
>>>
>>>    The cond_resched_rcu_qs() macro is used to force RCU quiescent states into
>>>    long-running in-kernel loops.  However, some of these loops can execute
>>>    during early boot when interrupts are disabled, and during which time
>>>    it is therefore illegal to enter the scheduler.  This commit therefore
>>>    makes cond_resched_rcu_qs() be a no-op during early boot.
>>>
>>>    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>>>
>>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
>>> index 525ca34603b7..b6944cc19a07 100644
>>> --- a/include/linux/rcupdate.h
>>> +++ b/include/linux/rcupdate.h
>>> @@ -423,7 +423,7 @@ extern struct srcu_struct tasks_rcu_exit_srcu;
>>>  */
>>> #define cond_resched_rcu_qs() \
>>> do { \
>>> -	if (!cond_resched()) \
>>> +	if (!is_idle_task(current) && !cond_resched()) \
>>> 		rcu_note_voluntary_context_switch(current); \
>>> } while (0)
>>>
>>> diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
>>> index 7232d199a81c..20f5990deeee 100644
>>> --- a/include/linux/rcutiny.h
>>> +++ b/include/linux/rcutiny.h
>>> @@ -228,6 +228,7 @@ static inline void exit_rcu(void)
>>> extern int rcu_scheduler_active __read_mostly;
>>> void rcu_scheduler_starting(void);
>>> #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
>>> +#define rcu_scheduler_active false
>>> static inline void rcu_scheduler_starting(void)
>>> {
>>> }
>>>
>>>
>>
>
>