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

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Date:   Wed, 30 Nov 2016 04:03:33 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Guenter Roeck <linux@...ck-us.net>
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 Wed, Nov 30, 2016 at 02:52:11AM -0800, Guenter Roeck wrote:
> 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.

And I have dropped this patch.  Michal Hocko showed me the error of
my ways with this patch.

							Thanx, Paul

> >>>>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)
> >>>{
> >>>}
> >>>
> >>>
> >>
> >
> >
>