Re: [BUG] kernel freezes with latest tree

[Peter Zijlstra]

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..

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