Re: [PATCH v4 2/2] watchdog: add watchdog_exclude sysctl to assist nohz

[Ulrich Obergfell <uobergfe@redhat.com>]

Chris,

I'd like to comment on the following proposed change:

+int proc_dowatchdog_exclude(struct ctl_table *table, int write,
+                            void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+        int err;
+
+        mutex_lock(&watchdog_proc_mutex);
+        err = proc_do_large_bitmap(table, write, buffer, lenp, ppos);
+        if (!err && write && watchdog_user_enabled) {
+                watchdog_disable_all_cpus();
+                watchdog_enable_all_cpus(false);
+        }
+        mutex_unlock(&watchdog_proc_mutex);
+        return err;
+}

The watchdog mechanism is enabled if watchdog_user_enabled and watchdog_thresh
are both non-zero. Hence, I think the if-statement in the above snippet of code
should look like this:

         if (!err && write && watchdog_user_enabled && watchdog_thresh)

Please see proc_dowatchdog() which checks the content of both variables before
it calls watchdog_enable_all_cpus():

  https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/kernel/watchdog.c?id=refs/tags/v4.0-rc6#n682


For completeness, I'd also like to point out that if the patch series at
https://lkml.org/lkml/2015/2/5/626 gets accepted upstream, the if-statement
will have to be adjusted. I think it should then look like this:

         if (!err && write && watchdog_enabled && watchdog_thresh) {
                 watchdog_disable_all_cpus();
                 watchdog_enable_all_cpus();
         }

Please see proc_watchdog_update() here which is similar to the above.

  https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/tree/kernel/watchdog.c?id=refs/tags/next-20150402#n710


Regards,

Uli