[RFC PATCH 1/3] sched, tracing: report task state in symbolic chars instead

[Ze Gao <zegao2021@gmail.com>]

Internal representations of task state are likely to be changed or ordered,
and reporting them to userspace without exporting them as part of API is not
a good choice, which can easily break a userspace observability tool as kernel
evolves. For example, perf suffers from this and still reports wrong states
by this patch.

OTOH, some masqueraded state like TASK_REPORT_IDLE and TASK_REPORT_MAX are
also reported inadvertently, which confuses things even more.

So report task state in symbolic char instead, which is self-explaining and
no further translation is needed.

Note for PREEMPT_ACTIVE, we introduce 'p' to report it and use the old
conventions for the rest.

Signed-off-by: Ze Gao <zegao@tencent.com>
---
 include/trace/events/sched.h | 41 +++++++++++++-----------------------
 1 file changed, 15 insertions(+), 26 deletions(-)

diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index fbb99a61f714..51102a7c0c2d 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -6,6 +6,7 @@
 #define _TRACE_SCHED_H
 
 #include <linux/kthread.h>
+#include <linux/sched.h>
 #include <linux/sched/numa_balancing.h>
 #include <linux/tracepoint.h>
 #include <linux/binfmts.h>
@@ -187,7 +188,7 @@ DEFINE_EVENT(sched_wakeup_template, sched_wakeup_new,
 	     TP_ARGS(p));
 
 #ifdef CREATE_TRACE_POINTS
-static inline long __trace_sched_switch_state(bool preempt,
+static inline const char __trace_sched_switch_state(bool preempt,
 					      unsigned int prev_state,
 					      struct task_struct *p)
 {
@@ -197,13 +198,6 @@ static inline long __trace_sched_switch_state(bool preempt,
 	BUG_ON(p != current);
 #endif /* CONFIG_SCHED_DEBUG */
 
-	/*
-	 * Preemption ignores task state, therefore preempted tasks are always
-	 * RUNNING (we will not have dequeued if state != RUNNING).
-	 */
-	if (preempt)
-		return TASK_REPORT_MAX;
-
 	/*
 	 * 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
@@ -212,7 +206,16 @@ static inline long __trace_sched_switch_state(bool preempt,
 	 */
 	state = __task_state_index(prev_state, p->exit_state);
 
-	return state ? (1 << (state - 1)) : state;
+	/*
+	 * Preemption ignores task state, therefore preempted tasks are always
+	 * RUNNING (we will not have dequeued if state != RUNNING).
+	 * Here, we use 'p' to denote this case and only for this case.
+	 */
+	if (preempt)
+		return 'p';
+
+
+	return task_index_to_char(state);
 }
 #endif /* CREATE_TRACE_POINTS */
 
@@ -232,7 +235,7 @@ TRACE_EVENT(sched_switch,
 		__array(	char,	prev_comm,	TASK_COMM_LEN	)
 		__field(	pid_t,	prev_pid			)
 		__field(	int,	prev_prio			)
-		__field(	long,	prev_state			)
+		__field(	char,	prev_state			)
 		__array(	char,	next_comm,	TASK_COMM_LEN	)
 		__field(	pid_t,	next_pid			)
 		__field(	int,	next_prio			)
@@ -249,22 +252,8 @@ TRACE_EVENT(sched_switch,
 		/* XXX SCHED_DEADLINE */
 	),
 
-	TP_printk("prev_comm=%s prev_pid=%d prev_prio=%d prev_state=%s%s ==> next_comm=%s next_pid=%d next_prio=%d",
-		__entry->prev_comm, __entry->prev_pid, __entry->prev_prio,
-
-		(__entry->prev_state & (TASK_REPORT_MAX - 1)) ?
-		  __print_flags(__entry->prev_state & (TASK_REPORT_MAX - 1), "|",
-				{ TASK_INTERRUPTIBLE, "S" },
-				{ TASK_UNINTERRUPTIBLE, "D" },
-				{ __TASK_STOPPED, "T" },
-				{ __TASK_TRACED, "t" },
-				{ EXIT_DEAD, "X" },
-				{ EXIT_ZOMBIE, "Z" },
-				{ TASK_PARKED, "P" },
-				{ TASK_DEAD, "I" }) :
-		  "R",
-
-		__entry->prev_state & TASK_REPORT_MAX ? "+" : "",
+	TP_printk("prev_comm=%s prev_pid=%d prev_prio=%d prev_state=%c ==> next_comm=%s next_pid=%d next_prio=%d",
+		__entry->prev_comm, __entry->prev_pid, __entry->prev_prio, __entry->prev_state,
 		__entry->next_comm, __entry->next_pid, __entry->next_prio)
 );
 
-- 
2.40.1