linux-kernel - Re: RCU idle CPU detection is broken in linux-next

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Message-ID: <20120925115917.GB2310@somewhere.redhat.com>
Date:	Tue, 25 Sep 2012 13:59:26 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: RCU idle CPU detection is broken in linux-next

On Mon, Sep 24, 2012 at 09:04:20PM -0700, Paul E. McKenney wrote:
> On Tue, Sep 25, 2012 at 01:41:18AM +0200, Frederic Weisbecker wrote:
> > >
> > > [  168.703017] ------------[ cut here ]------------
> > > [  168.708117] WARNING: at kernel/rcutree.c:502 rcu_eqs_exit_common+0x4a/0x3a0()
> > > [  168.710034] Pid: 7871, comm: trinity-child65 Tainted: G        W
> > > 3.6.0-rc6-next-20120924-sasha-00030-g71f256c #5
> > > [  168.710034] Call Trace:
> > > [  168.710034]  <IRQ>  [<ffffffff811c737a>] ? rcu_eqs_exit_common+0x4a/0x3a0
> > > [  168.710034]  [<ffffffff811078b6>] warn_slowpath_common+0x86/0xb0
> > > [  168.710034]  [<ffffffff811079a5>] warn_slowpath_null+0x15/0x20
> > > [  168.710034]  [<ffffffff811c737a>] rcu_eqs_exit_common+0x4a/0x3a0
> > > [  168.710034]  [<ffffffff811c79cc>] rcu_eqs_exit+0x9c/0xb0
> > > [  168.710034]  [<ffffffff811c7a4c>] rcu_user_exit+0x6c/0xd0
> > > [  168.710034]  [<ffffffff8106eb1f>] do_general_protection+0x1f/0x170
> > > [  168.710034]  [<ffffffff83a0e624>] ? restore_args+0x30/0x30
> > > [  168.710034]  [<ffffffff83a0e875>] general_protection+0x25/0x30
> > > [  168.710034]  [<ffffffff810a3f06>] ? native_read_msr_safe+0x6/0x20
> > > [  168.710034]  [<ffffffff81a0b34b>] __rdmsr_safe_on_cpu+0x2b/0x50
> > > [  168.710034]  [<ffffffff819ec971>] ? list_del+0x11/0x40
> > > [  168.710034]  [<ffffffff811886dc>]
> > > generic_smp_call_function_single_interrupt+0xec/0x120
> > > [  168.710034]  [<ffffffff81151147>] ? account_system_vtime+0xd7/0x140
> > > [  168.710034]  [<ffffffff81096f72>]
> > > smp_call_function_single_interrupt+0x22/0x40
> > > [  168.710034]  [<ffffffff83a0fe2f>] call_function_single_interrupt+0x6f/0x80
> > > [  168.710034]  <EOI>  [<ffffffff83a0e5f4>] ? retint_restore_args+0x13/0x13
> > > [  168.710034]  [<ffffffff811c7285>] ? rcu_user_enter+0x105/0x110
> > > [  168.710034]  [<ffffffff8107e06d>] syscall_trace_leave+0xfd/0x150
> > > [  168.710034]  [<ffffffff83a0f1ef>] int_check_syscall_exit_work+0x34/0x3d
> > > [  168.710034] ---[ end trace fd408dd21b70b87c ]---
> > >
> > > This is an exception inside an interrupt, and the interrupt
> > > interrupted RCU user mode.
> > > And we have that nesting:
> > >
> > > rcu_irq_enter(); <--- irq entry
> > > rcu_user_exit(); <--- exception entry
> > >
> > > And rcu_eqs_exit() doesn't handle that very well...
> > 
> > So either I should return immediately from rcu_user_exit() if
> > we are in an interrupt, or we make rcu_user_exit() able to nest
> > on rcu_irq_enter()   :)
> 
> Both of the two are eminently doable, with varying degrees of hackery.
> 
> What makes the most sense from an adaptive-idle viewpoint?

Given that we have:

rcu_irq_enter()
	rcu_user_exit()
	rcu_user_enter()
rcu_irq_exit()

And we already have rcu_user_exit_after_irq(), this starts to be confusing
if we allow that nesting. Although if we find a solution that, in the end,
merge rcu_user_exit() with rcu_user_exit_after_irq() and same for the enter version,
this would probably be a good thing. Provided this doesn't involve some more
complicated rdtp->dyntick_nesting trickies nor more overhead.

Otherwise we could avoid to call rcu_user_* when we are in an irq. When we'll have
the user_hooks layer, we can perhaps manage that from that place. For
now may be we can return after in_interrupt() in the rcu user apis.

Let's first ensure I diagnosed it well and we don't have other problems detected
by Sasha. I'm cooking a testing patch.
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