Re: [BUG] kernel freezes with latest tree

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mercredi 11 janvier 2012 à 10:04 +0100, Peter Zijlstra a écrit :
> On Tue, 2012-01-10 at 23:35 -0700, David Ahern wrote:
> > On 01/10/2012 04:44 PM, Linus Torvalds wrote:
> > > Anybody? Any ideas? Clearly there can be a merge problem that doesn't
> > > actually show as a real data conflict, just some semantic conflict,
> > > but I don't see what such issues would be brouht in by the scheduler
> > > merge anyway.
> > 
> > This is really easy to reproduce in a KVM hosted VM.
> > 
> > Using the gdb stub one cpu is spinning here:
> > 
> > (gdb) bt
> > #0  try_to_wake_up (p=0xf529b200, state=<optimized out>, wake_flags=1)
> >     at /mnt/sw/kernel-2.6.git/kernel/sched/core.c:1575
> > #1  0xc0470ab0 in default_wake_function (curr=<optimized out>,
> > mode=<optimized out>,
> >     wake_flags=<optimized out>, key=0xc3) at
> > /mnt/sw/kernel-2.6.git/kernel/sched/core.c:3364
> > 
> > So basically:
> >     while (p->on_cpu) {
> > ...
> >         cpu_relax();
> >     }
> > 
> > 
> > And the other vcpu is here:
> > 
> > #0  tg_load_down (tg=0xf55b9c00, data=<optimized out>) at
> > /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:3351
> > #1  0xc0470049 in walk_tg_tree_from (from=0xc0ba5400, down=0xc04753c0
> > <tg_load_down>, up=0xc046a3b0 <tg_nop>,
> >     data=0x0) at /mnt/sw/kernel-2.6.git/kernel/sched/core.c:664
> > #2  0xc04793f7 in walk_tg_tree (data=<optimized out>, up=<optimized
> > out>, down=0xc04753c0 <tg_load_down>)
> >     at /mnt/sw/kernel-2.6.git/kernel/sched/sched.h:175
> > #3  update_h_load (cpu=<optimized out>) at
> > /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:3361
> > #4  load_balance_fair (lb_flags=<synthetic pointer>,
> > idle=CPU_NEWLY_IDLE, sd=0xf5c30800, max_load_move=278,
> >     busiest=0xf6607d00, this_cpu=1, this_rq=0xf6707d00) at
> > /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:3374
> > #5  move_tasks (lb_flags=<synthetic pointer>, idle=CPU_NEWLY_IDLE,
> > sd=0xf5c30800, max_load_move=278,
> >     busiest=<optimized out>, this_cpu=1, this_rq=0xf6707d00)
> >     at /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:3444
> > #6  load_balance (this_cpu=1, this_rq=0xf6707d00, sd=0xf5c30800,
> > idle=CPU_NEWLY_IDLE, balance=0xf5217cb4)
> >     at /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:4496
> > #7  0xc0479be2 in idle_balance (this_cpu=1, this_rq=0xf6707d00)
> >     at /mnt/sw/kernel-2.6.git/kernel/sched/fair.c:4640
> > 
> > 
> > Based on the file in question (sched/fair.c) I took a stab at guessing
> > the commit: without a195f004 I was not able to lock it up. With the
> > patch the VM spins after a few hackbench iterations.
> > 
> > I don't have time for a proper bisect tonight. I can do that in the a.m.
> > if I am not totally off base here. Peter: any chance this commit could
> > explain the spinning cpus / system freeze?
> 
> It could, I certainly ran into similar issues while developing that
> patch. I ran into all sorts of weird stuff but had hoped I'd cured all
> of it.
> 
> If Eric can confirm this is indeed what is causing his pain, I'm fine
> with reverting it and having another go at it later. How easy is it to
> reproduce using your KVM thing?
> 
> I think simply replacing the one |= LBF_NEED_BREAK with a LBF_ABORT or
> removing that condition all-together should make the hang go away.
> 
> I'll try and figure out how it ends up in the infinite retry loop after
> my brain wakes up a bit more. Maybe adding a few more NEED_BREAK bits
> and making it a counter and overflowing it into ABORT might be good.
> 
> /me off to get breakfast and morning-juice..
> 


I let my machine run 3.2 kernel for 37 minutes with no hang, so at least
we know there is no hardware glitch.

$ uptime
 11:26:45 up 37 min,  2 users,  load average: 201.80, 206.06, 170.68

I'll try now latest tree with a195f004 reverted.

Thanks