linux-kernel - Re: frequent lockups in 3.18rc4

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Date:	Tue, 2 Dec 2014 21:22:45 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Dâniel Fraga <fragabr@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: frequent lockups in 3.18rc4

On Wed, Dec 03, 2014 at 10:03:56AM +0800, Lai Jiangshan wrote:
> On 12/03/2014 12:58 AM, Dâniel Fraga wrote:
> > On Tue, 2 Dec 2014 16:40:37 +0800
> > Lai Jiangshan <laijs@...fujitsu.com> wrote:
> > 
> >> It is needed at lest for testing.
> >>
> >> CONFIG_TREE_PREEMPT_RCU=y with CONFIG_PREEMPT=n is needed for testing too.
> >>
> >> Please enable them (or enable them under CONFIG_RCU_TRACE=y)
> > 
> > 	Lai, sorry but I didn't understand. Do you mean both of them
> > enabled? Because how can CONFIG_TREE_PREEMPT_RCU be enabled without
> > CONFIG_PREEMPT ?
> 
> 
> Sorry, I replied to Paul, and my reply was off-topic, it has nothing
> related your reports. Sorry again.
> 
> I think we need two combinations for testing (not mainline, but I think
> they (combinations) should be enabled for test farms).
> 
> So I hope Paul enable them (combinations).
> 
> combination1: CONFIG_TREE_PREEMPT_RCU=n & CONFIG_PREEMPT=y
> combination2: CONFIG_TREE_PREEMPT_RCU=y & CONFIG_PREEMPT=n
> 
> The core code should work correctly in these combinations.
> I agree with Paul that these combinations should not be enabled in production,
> So my request is: enable these combinations under CONFIG_RCU_TRACE
> or CONFIG_TREE_RCU_TRACE.
> 
> For myself, I always edit the Kconfig directly, thus it is not a problem
> for me.  But there is no way for test farms to test these combinations.

OK, I'll bite...

How have these two combinations helped you in your testing?

The reason I ask is that I am actually trying to -decrease- the RCU
configurations, not increase them.  Added configurations need to have
strong justification, for example, the kernel-tracing/patching need
for tasks_rcu.

							Thanx, Paul

> Thanks,
> Lai
> 
> > 
> > 	If you mean both enabled, I already reported a call trace with
> > both enabled:
> > 
> > https://bugzilla.kernel.org/show_bug.cgi?id=85941
> > 
> > 	Please see my previous answer to Linus and Paul too.
> > 
> > 	Regarding CONFIG_RCU_TRACE, do you mean
> > "CONFIG_TREE_RCU_TRACE"? I couldn't find CONFIG_RCU_TRACE.
> > 
> > 	Thanks.
> > 
> 

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