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

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Message-ID: <20140822125649.GI16198@grmbl.mre>
Date:	Fri, 22 Aug 2014 18:26:49 +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 [18:06:51], Amit Shah wrote:
> On (Fri) 22 Aug 2014 [17:54:53], Amit Shah wrote:
> > On (Mon) 18 Aug 2014 [21:01:49], Paul E. McKenney wrote:
> > 
> > > The odds are low over the next few days.  I am adding nastier rcutorture
> > > testing, however.  It would still be very good to get debug information
> > > from your setup.  One approach would be to convert the trace function
> > > calls into printk(), if that would help.
> > 
> > I added a few printks on the lines of the traces in cases where
> > rcu_nocb_poll was checked -- since that reproduces the hang.  Are the
> > following traces sufficient, or should I keep adding more printks?
> > 
> > In the case of rcu-trace-nopoll.txt, the messages stop after a while
> > (when the guest locks up hard).  That's when I kill the qemu process.
> 
> And this is bt from gdb when the endless 
> 
>   RCUDEBUG __call_rcu_nocb_enqueue 2146 rcu_preempt 0 WakeNot
> 
> messages are being spewed.
> 
> I can't time it, but hope it gives some indication along with the printks.

... and after the system 'locks up', this is the state it's in:

^C
Program received signal SIGINT, Interrupt.
native_safe_halt () at ./arch/x86/include/asm/irqflags.h:50
50		 }
(gdb) bt
#0  native_safe_halt () at ./arch/x86/include/asm/irqflags.h:50
#1  0xffffffff8100b9c1 in arch_safe_halt () at ./arch/x86/include/asm/paravirt.h:111
#2  default_idle () at arch/x86/kernel/process.c:311
#3  0xffffffff8100c107 in arch_cpu_idle () at arch/x86/kernel/process.c:302
#4  0xffffffff8106a25a in cpuidle_idle_call () at kernel/sched/idle.c:120
#5  cpu_idle_loop () at kernel/sched/idle.c:220
#6  cpu_startup_entry (state=<optimized out>) at kernel/sched/idle.c:268
#7  0xffffffff813e068b in rest_init () at init/main.c:418
#8  0xffffffff81a8cf5a in start_kernel () at init/main.c:680
#9  0xffffffff81a8c4ba in x86_64_start_reservations (real_mode_data=<optimized out>) at arch/x86/kernel/head64.c:193
#10 0xffffffff81a8c607 in x86_64_start_kernel (real_mode_data=0x13f90 <cpu_lock_stats+29184> <error: Cannot access memory at address 0x13f90>)
    at arch/x86/kernel/head64.c:182
#11 0x0000000000000000 in ?? ()


Wondering why it's doing this.  Am stepping through
cpu_startup_entry() to see if I get any clues.


		Amit
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