clocksource: Reduce the default clocksource_watchdog() retries to 2

With the previous patch, there is an extra watchdog read in each retry.
Now the total number of clocksource reads is increased to 4 per iteration.
In order to avoid increasing the clock skew check overhead, the default
maximum number of retries is reduced from 3 to 2 to maintain the same 12
clocksource reads in the worst case.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -603,8 +603,8 @@
 	clocksource.max_cswd_read_retries= [KNL]
 			Number of clocksource_watchdog() retries due to
 			external delays before the clock will be marked
-			unstable.  Defaults to three retries, that is,
-			four attempts to read the clock under test.
+			unstable.  Defaults to two retries, that is,
+			three attempts to read the clock under test.
 
 	clocksource.verify_n_cpus= [KNL]
 			Limit the number of CPUs checked for clocksources

kernel/time/clocksource.c

@@ -199,7 +199,7 @@ void clocksource_mark_unstable(struct clocksource *cs)
 	spin_unlock_irqrestore(&watchdog_lock, flags);
 }
 
-ulong max_cswd_read_retries = 3;
+ulong max_cswd_read_retries = 2;
 module_param(max_cswd_read_retries, ulong, 0644);
 EXPORT_SYMBOL_GPL(max_cswd_read_retries);
 static int verify_n_cpus = 8;