Re: [PATCH v10 1/3] smpboot: allow excluding cpus from the smpboot threads

[Chris Metcalf <cmetcalf@ezchip.com>]

On 05/01/2015 05:23 PM, Frederic Weisbecker wrote:
> On Fri, May 01, 2015 at 03:57:51PM -0400, Chris Metcalf wrote:
>> On 05/01/2015 04:53 AM, Frederic Weisbecker wrote:
>>>> +	/* Unpark any threads that were voluntarily parked. */
>>>> +	for_each_cpu_not(cpu, &ht->cpumask) {
>>>> +		if (cpu_online(cpu)) {
>>>> +			struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);
>>>> +			if (tsk)
>>>> +				kthread_unpark(tsk);
>>> I'm still not clear why we are doing that. kthread_stop() should be able
>>> to handle parked kthreads, otherwise it needs to be fixed.
>> Checking without the unpark, it's actually only a problem with nohz_full.
>> In a system without nohz_full, the kthreads are able to stop even when
>> they are parked; it's only in the nohz_full case that things wedge.
>> For example, booting with only cpu 0 as a housekeeping core (and
>> therefore all watchdogs 1-35 on my 36-core tilegx are parked), and
>> immediately doing "echo 0 > /proc/sys/kernel/watchdog", I see
>> (via SysRq ^O-l) the first parked watchdog, on cpu 1, hung with:
>>
>>    frame 0: 0xfffffff7000f2928 lock_hrtimer_base+0xb8/0xc0
>>    frame 1: 0xfffffff7000f2a28 hrtimer_try_to_cancel+0x40/0x170
>>    frame 2: 0xfffffff7000f2a28 hrtimer_try_to_cancel+0x40/0x170
>>    frame 3: 0xfffffff7000f2b98 hrtimer_cancel+0x40/0x68
>>    frame 4: 0xfffffff70014cce0 watchdog_disable+0x50/0x70
>>    frame 5: 0xfffffff70008c2d0 smpboot_thread_fn+0x350/0x438
>>    frame 6: 0xfffffff700084b28 kthread+0x160/0x178

I finally had some time to look into this issue some more.

With PROVE_LOCKING enabled (after a fix I'll send to LKML shortly), we
get no warnings, and ^O-d to print locks shows:

Showing all locks held in the system:
3 locks held by watchdog/1/15:
  #0:  (&(&hp->lock)->rlock){-.....}, at: [<fffffff700620740>] hvc_poll+0xb8/0x4b8
  #1:  (rcu_read_lock){......}, at: [<fffffff70061d710>] __handle_sysrq+0x0/0x440
  #2:  (tasklist_lock){.+.+..}, at: [<fffffff7000d7310>] debug_show_all_locks+0xc0/0x350
3 locks held by sh/1732:
  #0:  (sb_writers#4){.+.+.+}, at: [<fffffff70022f6b8>] vfs_write+0x268/0x2c0
  #1:  (watchdog_proc_mutex){+.+.+.}, at: [<fffffff70016f368>] proc_watchdog_common+0x78/0x1c8
  #2:  (smpboot_threads_lock){+.+.+.}, at: [<fffffff700093558>] smpboot_unregister_percpu_thread+0x48/0x88

All the watchdog/1/15 locks are attributable to the fact that it's running
on the same core that ended up handling the "^O-d" request from SysRq.

The sh process from which I ran the echo eventually shows up as "blocked for
more than 120 seconds" and pretty much where you'd expect it to be, waiting
on a completion in kthread_stop() at kthread.c:473.

I instrumented lock_hrtimer_base(), and timer->base is null, and never gets
set non-null, so the loop spins forever.  Perhaps something in nohz is preventing
the timer->base from being set?

I'm happy to keep debugging this but I'm not really clear on what could
be going wrong here.  Any ideas?

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com