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

[Chris Metcalf <cmetcalf@ezchip.com>]

On 05/01/2015 04:53 AM, Frederic Weisbecker wrote:
> On Thu, Apr 30, 2015 at 03:39:24PM -0400, Chris Metcalf wrote:
>> This change allows some cores to be excluded from running the
>> smp_hotplug_thread tasks.  The following commit to update
>> kernel/watchdog.c to use this functionality is the motivating
>> example, and more information on the motivation is provided there.
>>
>> A new smp_hotplug_thread field is introduced, "cpumask", which
>> is cpumask field managed by the smpboot subsystem that indicates whether
>> or not the given smp_hotplug_thread should run on that core; the
>> cpumask is checked when deciding whether to unpark the thread.
>>
>> To limit the cpumask to less than cpu_possible, you must call
>> smpboot_update_cpumask_percpu_thread() after registering.
>>
>> Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>
>> ---
>>   include/linux/smpboot.h |  5 +++++
>>   kernel/smpboot.c        | 55 ++++++++++++++++++++++++++++++++++++++++++++++++-
>>   2 files changed, 59 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/smpboot.h b/include/linux/smpboot.h
>> index d600afb21926..7c42153edfac 100644
>> --- a/include/linux/smpboot.h
>> +++ b/include/linux/smpboot.h
>> @@ -27,6 +27,8 @@ struct smpboot_thread_data;
>>    * @pre_unpark:		Optional unpark function, called before the thread is
>>    *			unparked (cpu online). This is not guaranteed to be
>>    *			called on the target cpu of the thread. Careful!
>> + * @cpumask:		Internal state.  To update which threads are unparked,
>> + *			call smpboot_update_cpumask_percpu_thread().
>>    * @selfparking:	Thread is not parked by the park function.
>>    * @thread_comm:	The base name of the thread
>>    */
>> @@ -41,11 +43,14 @@ struct smp_hotplug_thread {
>>   	void				(*park)(unsigned int cpu);
>>   	void				(*unpark)(unsigned int cpu);
>>   	void				(*pre_unpark)(unsigned int cpu);
>> +	struct cpumask			cpumask;
> I believe it should be allocated dynamically, otherwise it gets the size of NR_CPUS
> instead of nr_cpus_bits. It's not _that_ much space spared but think there should be
> several struct smp_hotplug_thread registered.

I'll submit a follow-up patch to do this.  I'm assuming this doesn't need to
be rolled as a v11, and can be a stand-alone patch, but I'll do it whichever
way Andrew prefers.

>> +	/* 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

The other cores are all idle.

I have no idea why lock_hrtimer_base() is hanging; perhaps the
hrtimer_cpu_base lock is taken by some other task that is now
scheduled out.

The config does not have NO_HZ_FULL_ALL or NO_HZ_FULL_SYSIDLE
set, and does have RCU_FAST_NO_HZ and RCU_NOCB_CPU_ALL.

I don't really know how to start debugging this, but I do know that
unparking the threads first avoids the issue :-)

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