Re: frequent lockups in 3.18rc4

[Chris Mason]

On Fri, Dec 19, 2014 at 6:22 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> 
wrote:
> On Fri, 19 Dec 2014, Chris Mason wrote:
> >  On Fri, Dec 19, 2014 at 11:15:21AM -0800, Linus Torvalds wrote:
> >  > Here's another pattern. In your latest thing, every single time 
> > that
> >  > CPU1 is waiting for some other CPU to pick up the IPI, we have 
> > CPU0
> >  > doing this:
> >  >
> >  > [24998.060963] NMI backtrace for cpu 0
> >  > [24998.061989] CPU: 0 PID: 2940 Comm: trinity-c150 Not tainted 
> > 3.18.0+ #108
> >  > [24998.064073] task: ffff8801bf3536b0 ti: ffff880197e0c000 
> > task.ti:
> >  > ffff880197e0c000
> >  > [24998.065137] RIP: 0010:[<ffffffff8103e006>]  
> > [<ffffffff8103e006>]
> >  > read_hpet+0x16/0x20
> >  > [24998.083577]  [<ffffffff810e0d3e>] ktime_get+0x3e/0xa0
> >  > [24998.084450]  [<ffffffff810e9cd3>] tick_sched_timer+0x23/0x160
> >  > [24998.085315]  [<ffffffff810daf96>] __run_hrtimer+0x76/0x1f0
> >  > [24998.086173]  [<ffffffff810e9cb0>] ? tick_init_highres+0x20/0x20
> >  > [24998.087025]  [<ffffffff810db2e7>] hrtimer_interrupt+0x107/0x260
> >  > [24998.087877]  [<ffffffff81031a4b>] 
> > local_apic_timer_interrupt+0x3b/0x70
> >  > [24998.088732]  [<ffffffff8179bca5>] 
> > smp_apic_timer_interrupt+0x45/0x60
> >  > [24998.089583]  [<ffffffff8179a0df>] 
> > apic_timer_interrupt+0x6f/0x80
> >  > [24998.090435]  <EOI>
> >  > [24998.091279]  [<ffffffff810da66e>] ? __remove_hrtimer+0x4e/0xa0
> >  > [24998.092118]  [<ffffffff812c7c7a>] ? ipcget+0x8a/0x1e0
> >  > [24998.092951]  [<ffffffff812c7c6c>] ? ipcget+0x7c/0x1e0
> >  > [24998.093779]  [<ffffffff812c8d6d>] SyS_msgget+0x4d/0x70
> >  >
> >  > and I think that's the smoking gun. The reason CPU0 isn't picking 
> > up
> >  > any IPI's is because it is in some endless loop around 
> > read_hpet().
> >  >
> >  > There is even time information in the register dump:
> >  >
> >  >  RAX: 0000000061fece8a RBX: 0000000000510792 RCX: 0000000000000000
> >  >  RAX: 0000000079e588fc RBX: 0000000000511d6e RCX: 0000000000000000
> >  >  RAX: 0000000091ca7f65 RBX: 0000000000513346 RCX: 0000000000000000
> >  >  RAX: 00000000a9afbd0d RBX: 000000000051491e RCX: 0000000000000000
> >  >  RAX: 00000000cbd1340c RBX: 000000000051684a RCX: 0000000000000000
> >  >  RAX: 00000000fb9d303f RBX: 00000000005193fc RCX: 0000000000000000
> >  >  RAX: 000000002b67efe4 RBX: 000000000051c224 RCX: 0000000000000004
> >  >
> >  > That RAX value is the value we just read from the HPET, and RBX 
> > seems
> >  > to be monotonically increasing too, so it's likely the sequence
> >  > counter in ktime_get().
> >  >
> >  > So it's not stuck *inside* read_hpet(), and it's almost certainly 
> > not
> >  > the loop over the sequence counter in ktime_get() either (it's not
> >  > increasing *that* quickly). But some basically infinite 
> > __run_hrtimer
> >  > thing or something?
> >
> >  Really interesting.
> >
> >  So, we're calling __run_hrtimer in a loop:
> >
> >                  while ((node = timerqueue_getnext(&base->active))) {
> >  				...
> >  				__run_hrtimer(timer, &basenow);
> >  				...
> >  		}
> >
> >  The easy question is how often does trinity call nanosleep?
> >
> >  Looking at __run_hrtimer(), it drops the lock and runs the function 
> > and then
> >  takes the lock again, maybe enqueueing us again right away.
> >
> >  timer->state is supposed to protect us from other CPUs jumping in 
> > and doing
> >  something else with the timer, but it feels racey wrt 
> > remove_hrtimer().
> >  Something like this, but I'm not sure how often 
> > __hrtimer_start_range_ns gets
> >  called
> >
> >  CPU 0						CPU 1
> >  __run_hrtimer()
> >      timer->state = HRTIMER_STATE_CALLBACK
> >      removed from list
> >      unlock cpu_base->lock
> >      restrt = fn(timer)
> >      						__hrtimer_start_range_ns()
> >  						base = lock_hrtimer_base()
> >  						ret = remove_hrtimer()
> >  						    finds timer->state = HRTIMER_STATE_CALLBACK
> >  						    does nothing
> >  						new_base = switch_hrtimer_base()
> >  						    now we're on a different base, different lock
> >      lock(cpu_base->lock)
> >      enqueue the timer
> >      						enqueue the timer
>
> But at the very end this would be detected by the runtime check of the
> hrtimer interrupt, which does not trigger. And it would trigger at
> some point as ALL cpus including CPU0 in that trace dump make
> progress.

I'll admit that at some point we should be hitting one of the WARN or 
BUG_ON, but it's possible to thread that needle and corrupt the timer 
list, without hitting a warning (CPU 1 in my example has to enqueue 
last).  Once the rbtree is hosed, it can go forever.  Probably not the 
bug we're looking for, but still suspect in general.

-chris



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