Re: [PATCH v8 2/3] watchdog: add watchdog_cpumask sysctl to assist nohz

[Chris Metcalf]

On 04/16/2015 06:46 AM, Ulrich Obergfell wrote:
> if a user changes watchdog parameters in /proc/sys/kernel, the watchdog threads
> are not stopped and restarted in all cases. Parameters can also be changed 'on
> the fly', for example like 'watchdog_thresh' in the following flow of execution:
>
>    proc_watchdog_thresh
>      proc_watchdog_update
>        if (watchdog_enabled && watchdog_thresh)
>            watchdog_enable_all_cpus
>              if (!watchdog_running) {
>                  // watchdog threads are already running so we don't get here
>              } else {
>                  update_watchdog_all_cpus
>                    for_each_online_cpu <-----------------------------.
>                      update_watchdog                                 |
>                        watchdog_nmi_disable                          |
>                        watchdog_nmi_enable                           |
>              }                                                       |
>                                                                      |
> I think we would not want to call watchdog_nmi_enable() for each_online_  CPU,
> but rather for each CPU that has an_unparked_  watchdog thread (i.e. where the
> watchdog mechanism is actually enabled).

How about something like this?  I'll fold it into v9 of the patchset.
Thanks!

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 0c5a37cdbedd..a4e1c9a2e769 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -61,6 +61,10 @@ static cpumask_var_t watchdog_cpumask_for_smpboot;
 static cpumask_var_t watchdog_cpumask;
 unsigned long *watchdog_cpumask_bits;
 
+/* Helper for online, unparked cpus. */
+#define for_each_watchdog_cpu(cpu) \
+	for_each_cpu_and((cpu), cpu_online_mask, watchdog_cpumask)
+
 static int __read_mostly watchdog_running;
 static u64 __read_mostly sample_period;
 
@@ -209,7 +213,7 @@ void touch_all_softlockup_watchdogs(void)
 	 * do we care if a 0 races with a timestamp?
 	 * all it means is the softlock check starts one cycle later
 	 */
-	for_each_online_cpu(cpu)
+	for_each_watchdog_cpu(cpu)
 		per_cpu(watchdog_touch_ts, cpu) = 0;
 }
 
@@ -616,7 +620,7 @@ void watchdog_nmi_enable_all(void)
 		return;
 
 	get_online_cpus();
-	for_each_online_cpu(cpu)
+	for_each_watchdog_cpu(cpu)
 		watchdog_nmi_enable(cpu);
 	put_online_cpus();
 }
@@ -629,7 +633,7 @@ void watchdog_nmi_disable_all(void)
 		return;
 
 	get_online_cpus();
-	for_each_online_cpu(cpu)
+	for_each_watchdog_cpu(cpu)
 		watchdog_nmi_disable(cpu);
 	put_online_cpus();
 }
@@ -688,7 +692,7 @@ static void update_watchdog_all_cpus(void)
 	int cpu;
 
 	get_online_cpus();
-	for_each_online_cpu(cpu)
+	for_each_watchdog_cpu(cpu)
 		update_watchdog(cpu);
 	put_online_cpus();
 }

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

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