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

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Date:   Tue, 29 Nov 2016 23:02:12 -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 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?

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