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@xxxxxxxxxx> --- 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 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
