Re: [PATCH v3] watchdog: add watchdog_cpumask sysctl to assist nohz

Chris Metcalf
2015-04-02T18:49:23Z

On 04/02/2015 02:33 PM, Peter Zijlstra wrote:
> On Thu, Apr 02, 2015 at 02:16:09PM -0400, Chris Metcalf wrote:
>> On 04/02/2015 02:06 PM, Peter Zijlstra wrote:
>>> On Thu, Apr 02, 2015 at 01:39:28PM -0400, cmetcalf@ezchip.com wrote:
>>>> @@ -431,6 +434,10 @@ static void watchdog_enable(unsigned int cpu)
>>>> hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>>>> hrtimer->function = watchdog_timer_fn;
>>>> + /* Exit if the cpu is not allowed for watchdog. */
>>>> + if (!cpumask_test_cpu(cpu, watchdog_mask))
>>>> + do_exit(0);
>>>> +
>>> Ick, that doesn't look right for smpboot threads.
>> I didn't see a better way to make this happen without adding
>> a bunch of infrastructure to the smpboot thread mechanism
>> to use a cpumask other than for_each_online_cpu(). The exit
>> seems benign in my testing, but I agree it's not the cleanest
>> way to express what we're trying to do here.
>>
>> Perhaps something like an optional cpumask_t pointer in
>> struct smp_hotplug_thread, which if present specifies the
>> cpus to run on, and otherwise we stick with cpu_online_mask?
> What's wrong with just leaving the thread be but making sure it'll never
> actually do anything?

I think a common case for nohz_full systems is that you'll
have a whole lot of watchdog threads that never do anything.
Our TILEGx-72 systems are often run with one housekeeping
core and the rest doing userspace nohz_full driver work. So
not creating the threads seems tidier - it keeps 71 threads out
of the "ps" listing :-)

Here's a quick sketch of the delta from my previous patch to
one with a new smp_hotplug_thread.cpumask field. If folks
are OK with modifying the smpboot threads like this, I think
it probably is a cleaner approach:

diff --git a/include/linux/smpboot.h b/include/linux/smpboot.h
index 13e929679550..f28519612ee3 100644
--- a/include/linux/smpboot.h
+++ b/include/linux/smpboot.h
@@ -27,6 +27,7 @@ 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: Optional cpumask, specifying what cores to run on.
* @selfparking: Thread is not parked by the park function.
* @thread_comm: The base name of the thread
*/
@@ -41,6 +42,7 @@ struct smp_hotplug_thread {
void (*park)(unsigned int cpu);
void (*unpark)(unsigned int cpu);
void (*pre_unpark)(unsigned int cpu);
+ cpumask_t *cpumask;
bool selfparking;
const char *thread_comm;
};
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 40190f28db35..be503c2ddb5f 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -172,6 +172,9 @@ __smpboot_create_thread(struct smp_hotplug_thread *ht, unsigned int cpu)
if (tsk)
return 0;

+ if (ht->cpumask && !cpumask_test_cpu(cpu, ht->cpumask))
+ return 0;
+
td = kzalloc_node(sizeof(*td), GFP_KERNEL, cpu_to_node(cpu));
if (!td)
return -ENOMEM;
@@ -220,9 +223,11 @@ static void smpboot_unpark_thread(struct smp_hotplug_thread *ht, unsigned int cp
{
struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);

- if (ht->pre_unpark)
- ht->pre_unpark(cpu);
- kthread_unpark(tsk);
+ if (tsk) {
+ if (ht->pre_unpark)
+ ht->pre_unpark(cpu);
+ kthread_unpark(tsk);
+ }
}

void smpboot_unpark_threads(unsigned int cpu)
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 2140c2d81dc9..681e5648e093 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -434,10 +434,6 @@ static void watchdog_enable(unsigned int cpu)
hrtimer_init(hrtimer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
hrtimer->function = watchdog_timer_fn;

- /* Exit if the cpu is not allowed for watchdog. */
- if (!cpumask_test_cpu(cpu, watchdog_mask))
- do_exit(0);
-
/* Enable the perf event */
watchdog_nmi_enable(cpu);

@@ -588,6 +584,7 @@ static struct smp_hotplug_thread watchdog_threads = {
.cleanup = watchdog_cleanup,
.park = watchdog_disable,
.unpark = watchdog_enable,
+ .cpumask = watchdog_mask,
};

static void restart_watchdog_hrtimer(void *info)

--
Chris Metcalf, EZChip Semiconductor