linux-kernel - Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB kthread wakeups

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Message-ID: <20140822173752.GA25807@grmbl.mre>
Date:	Fri, 22 Aug 2014 23:07:52 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, riel@...hat.com, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	sbw@....edu
Subject: Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB
 kthread wakeups

On (Fri) 22 Aug 2014 [22:44:05], Amit Shah wrote:
> Hm, found it:
> 
> The stall happens in do_initcalls().
> 
> pm_sysrq_init() is the function that causes the hang.  When I #if 0
> the line
> 
>     register_sysrq_key('o', &sysrq_poweroff_op);
> 
> in pm_sysrq_init(), the boot proceeds normally.
> 
> Now what this is, and what relation this has to rcu and that patch in
> particular is next...

... and enabling the following debug options makes the bug disappear:

CONFIG_DEBUG_OBJECTS=y
CONFIG_DEBUG_OBJECTS_SELFTEST=y
CONFIG_DEBUG_OBJECTS_FREE=y
CONFIG_DEBUG_OBJECTS_TIMERS=y
CONFIG_DEBUG_OBJECTS_WORK=y
CONFIG_DEBUG_OBJECTS_RCU_HEAD=y
CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER=y
CONFIG_DEBUG_OBJECTS_ENABLE_DEFAULT=1

Anyway, so it looks like a race somewhere in the schedule_work_on()
chain.  Not sure how to capture the debug messages there w/o disabling
these debug options.  I'll keep trying, though.


		Amit
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