rcu-tasks: Use accurate runstart time for RCU Tasks boot-time testing

Currently, test_rcu_tasks_callback() reads from the jiffies counter only
once when this function is invoked.  This introduces inaccuracies because
of the latencies induced by the synchronize_rcu_tasks*() invocations.
This commit therefore re-reads the jiffies counter at the beginning
of each test, thus avoiding penalizing later tests for the latencies
induced by earlier tests.

Therefore, this commit at the start of each RCU Tasks test, re-fetch the
jiffies time as the runstart time.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -1815,23 +1815,21 @@ static void test_rcu_tasks_callback(struct rcu_head *rhp)
 
 static void rcu_tasks_initiate_self_tests(void)
 {
-	unsigned long j = jiffies;
-
 	pr_info("Running RCU-tasks wait API self tests\n");
 #ifdef CONFIG_TASKS_RCU
-	tests[0].runstart = j;
+	tests[0].runstart = jiffies;
 	synchronize_rcu_tasks();
 	call_rcu_tasks(&tests[0].rh, test_rcu_tasks_callback);
 #endif
 
 #ifdef CONFIG_TASKS_RUDE_RCU
-	tests[1].runstart = j;
+	tests[1].runstart = jiffies;
 	synchronize_rcu_tasks_rude();
 	call_rcu_tasks_rude(&tests[1].rh, test_rcu_tasks_callback);
 #endif
 
 #ifdef CONFIG_TASKS_TRACE_RCU
-	tests[2].runstart = j;
+	tests[2].runstart = jiffies;
 	synchronize_rcu_tasks_trace();
 	call_rcu_tasks_trace(&tests[2].rh, test_rcu_tasks_callback);
 #endif