sched, trace: Fix prev_state output in sched_switch tracepoint

commit 3f5fe9fef5b2 ("sched/debug: Fix task state recording/printout")
tried to fix the problem introduced by a previous commit efb40f588b43
("sched/tracing: Fix trace_sched_switch task-state printing"). However
the prev_state output in sched_switch is still broken.

task_state_index() uses fls() which considers the LSB as 1. Left
shifting 1 by this value gives an incorrect mapping to the task state.
Fix this by decrementing the value returned by __get_task_state()
before shifting.

Link: http://lkml.kernel.org/r/1540882473-1103-1-git-send-email-pkondeti@codeaurora.org

Cc: stable@vger.kernel.org
Fixes: 3f5fe9fef5b2 ("sched/debug: Fix task state recording/printout")
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/trace/events/sched.h

@@ -107,6 +107,8 @@ DEFINE_EVENT(sched_wakeup_template, sched_wakeup_new,
 #ifdef CREATE_TRACE_POINTS
 static inline long __trace_sched_switch_state(bool preempt, struct task_struct *p)
 {
+	unsigned int state;
+
 #ifdef CONFIG_SCHED_DEBUG
 	BUG_ON(p != current);
 #endif /* CONFIG_SCHED_DEBUG */
@@ -118,7 +120,15 @@ static inline long __trace_sched_switch_state(bool preempt, struct task_struct *
 	if (preempt)
 		return TASK_REPORT_MAX;
 
-	return 1 << task_state_index(p);
+	/*
+	 * task_state_index() uses fls() and returns a value from 0-8 range.
+	 * Decrement it by 1 (except TASK_RUNNING state i.e 0) before using
+	 * it for left shift operation to get the correct task->state
+	 * mapping.
+	 */
+	state = task_state_index(p);
+
+	return state ? (1 << (state - 1)) : state;
 }
 #endif /* CREATE_TRACE_POINTS */