[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20121127161639.GF2474@linux.vnet.ibm.com>
Date: Tue, 27 Nov 2012 08:16:39 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: Li Zhong <zhong@...ux.vnet.ibm.com>,
linux-next list <linux-next@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>, sasha.levin@...cle.com,
gleb@...hat.com, avi@...hat.com
Subject: Re: [RFC PATCH] Fix abnormal rcu dynticks_nesting values related to
async page fault
On Tue, Nov 27, 2012 at 04:39:59PM +0100, Frederic Weisbecker wrote:
> 2012/11/27 Li Zhong <zhong@...ux.vnet.ibm.com>:
> > I noticed some warnings complaining about dynticks_nesting value, like
> >
> > [ 267.545032] ------------[ cut here ]------------
> > [ 267.545032] WARNING: at kernel/rcutree.c:382 rcu_eqs_enter+0xab/0xc0()
> > [ 267.545032] Hardware name: Bochs
> > [ 267.545032] Modules linked in:
> > [ 267.545032] Pid: 0, comm: swapper/2 Not tainted 3.7.0-rc5-next-20121115 #8
> > [ 267.545032] Call Trace:
> > [ 267.545032] [<ffffffff8104714f>] warn_slowpath_common+0x7f/0xc0
> > [ 267.545032] [<ffffffff810471aa>] warn_slowpath_null+0x1a/0x20
> > [ 267.545032] [<ffffffff810e607b>] rcu_eqs_enter+0xab/0xc0
> > [ 267.545032] [<ffffffff810e60bb>] rcu_idle_enter+0x2b/0x70
> > [ 267.545032] [<ffffffff8100d44f>] cpu_idle+0x6f/0x100
> > [ 267.545032] [<ffffffff814bf055>] start_secondary+0x205/0x20c
> > [ 267.545032] ---[ end trace 924ae80da035028d ]---
> >
> > After enabling rcu-dyntick tracing, I got following abnormal
> > dynticks_nesting values (13fffffffffffff, ff00000000000001,etc):
> > ...
> > 1 <idle>-0 [002] dN.2 18739.518567: rcu_dyntick: End 0 140000000000000 rcu_idle_exit
> > 2 sshd-696 [002] d..1 18739.518675: rcu_dyntick: ++= 140000000000000 140000000000001 rcu_irq_enter - apf (not present)
> >
> > 3 <idle>-0 [002] d..2 18739.518705: rcu_dyntick: Start 140000000000001 0 rcu_idle_enter
> > 4 <idle>-0 [002] d..2 18739.521252: rcu_dyntick: End 0 1 rcu_irq_enter - apf (page ready)
> > 5 <idle>-0 [002] dN.2 18739.521261: rcu_dyntick: Start 1 0 rcu_irq_exit - apf (page ready)
> > 6 <idle>-0 [002] dN.2 18739.521263: rcu_dyntick: End 0 140000000000000 rcu_idle_exit
> >
> > 7 sshd-696 [002] d..1 18739.521299: rcu_dyntick: --= 140000000000000 13fffffffffffff rcu_irq_exit - apf (not present)
>
> Calling rcu_irq_exit() without a matching rcu_irq_enter() after the
> last rcu_idle_exit() is illegal, isn't it?
It is OK to call rcu_irq_exit() without a matching rcu_irq_enter() -only-
if you have also called rcu_idle_exit() since the last rcu_idle_enter().
There will be a similar rule for rcu_user_exit().
More generally, it is OK to call rcu_irq_exit() without a matching
rcu_irq_enter() only if RCU believes that the CPU you are running on is
non-idle. On 32-bit systems, you are only allowed a few tens of million
such unmatched rcu_irq_enter() calls in a given RCU-non-idle region.
All courtesy of RCU's need to tolerate architectures that enter
interrupt handlers without ever leaving them and vice versa. ;-)
Thanx, Paul
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists