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Subject[RFC PATCH] tracing: add sched_prio_update
Date
This tracepoint allows to keep track of all priority changes of a task.
It outputs the scheduling policy, the nice value, the rt_priority and
the deadline-related attributes (dl_runtime, dl_deadline and dl_period).

It is emitted in the code path of the sched_setscheduler, sched_setattr,
sched_setparam, nice and the fork system calls. For fork, it is emitted
after the sched_process_fork tracepoint for timeline consistency and
because the PID is not yet set when sched_fork() is called.

This allows the analysis of real-time scheduling delays based on the
configured scheduling priorities and policies, which cannot be performed
with the current instrumentation in sched_switch. Also, instead of
exposing the internal kernel prio field, this tracepoint only outputs
the user-visible priority attributes.

The effective priority of running threads can also be temporarily
changed in the PI code, but a dedicated tracepoint is already in place
to cover this case.

Here are a few output examples:
After fork of a normal task:
sched_prio_update: comm=bash pid=2104, policy=SCHED_NORMAL, nice=0,
rt_priority=0, dl_runtime=0, dl_deadline=0, dl_period=0

renice -n 10 of a normal task:
sched_prio_update: comm=sleep pid=2130, policy=SCHED_NORMAL, nice=10,
rt_priority=0, dl_runtime=0, dl_deadline=0, dl_period=0

SCHED_FIFO 60:
sched_prio_update: comm=chrt pid=2105, policy=SCHED_FIFO, nice=0,
rt_priority=60, dl_runtime=0, dl_deadline=0, dl_period=0

SCHED_RR 60:
sched_prio_update: comm=chrt pid=2109, policy=SCHED_RR, nice=0,
rt_priority=60, dl_runtime=0, dl_deadline=0, dl_period=0

SCHED_DEADLINE:
sched_prio_update: comm=b pid=2110, policy=SCHED_DEADLING, nice=0,
rt_priority=0, dl_runtime=10000000, dl_deadline=30000000,
dl_period=30000000

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
---
include/trace/events/sched.h | 68 ++++++++++++++++++++++++++++++++++++++++++++
kernel/fork.c | 1 +
kernel/sched/core.c | 3 ++
3 files changed, 72 insertions(+)

diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index 9b90c57..fcb0f29 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -8,6 +8,34 @@
#include <linux/tracepoint.h>
#include <linux/binfmts.h>

+#define SCHEDULING_POLICY \
+ EM( SCHED_NORMAL, "SCHED_NORMAL") \
+ EM( SCHED_FIFO, "SCHED_FIFO") \
+ EM( SCHED_RR, "SCHED_RR") \
+ EM( SCHED_BATCH, "SCHED_BATCH") \
+ EM( SCHED_IDLE, "SCHED_IDLE") \
+ EMe(SCHED_DEADLINE, "SCHED_DEADLINE")
+
+/*
+ * First define the enums in the above macros to be exported to userspace
+ * via TRACE_DEFINE_ENUM().
+ */
+#undef EM
+#undef EMe
+#define EM(a, b) TRACE_DEFINE_ENUM(a);
+#define EMe(a, b) TRACE_DEFINE_ENUM(a);
+
+SCHEDULING_POLICY
+
+/*
+ * Now redefine the EM() and EMe() macros to map the enums to the strings
+ * that will be printed in the output.
+ */
+#undef EM
+#undef EMe
+#define EM(a, b) {a, b},
+#define EMe(a, b) {a, b}
+
/*
* Tracepoint for calling kthread_stop, performed to end a kthread:
*/
@@ -562,6 +590,46 @@ TRACE_EVENT(sched_wake_idle_without_ipi,

TP_printk("cpu=%d", __entry->cpu)
);
+
+/*
+ * Tracepoint for showing scheduling priority changes.
+ */
+TRACE_EVENT(sched_prio_update,
+
+ TP_PROTO(struct task_struct *tsk),
+
+ TP_ARGS(tsk),
+
+ TP_STRUCT__entry(
+ __array( char, comm, TASK_COMM_LEN )
+ __field( pid_t, pid )
+ __field( unsigned int, policy )
+ __field( int, nice )
+ __field( unsigned int, rt_priority )
+ __field( u64, dl_runtime )
+ __field( u64, dl_deadline )
+ __field( u64, dl_period )
+ ),
+
+ TP_fast_assign(
+ memcpy(__entry->comm, tsk->comm, TASK_COMM_LEN);
+ __entry->pid = tsk->pid;
+ __entry->policy = tsk->policy;
+ __entry->nice = task_nice(tsk);
+ __entry->rt_priority = tsk->rt_priority;
+ __entry->dl_runtime = tsk->dl.dl_runtime;
+ __entry->dl_deadline = tsk->dl.dl_deadline;
+ __entry->dl_period = tsk->dl.dl_period;
+ ),
+
+ TP_printk("comm=%s pid=%d, policy=%s, nice=%d, rt_priority=%u, "
+ "dl_runtime=%Lu, dl_deadline=%Lu, dl_period=%Lu",
+ __entry->comm, __entry->pid,
+ __print_symbolic(__entry->policy, SCHEDULING_POLICY),
+ __entry->nice, __entry->rt_priority,
+ __entry->dl_runtime, __entry->dl_deadline,
+ __entry->dl_period)
+);
#endif /* _TRACE_SCHED_H */

/* This part must be outside protection */
diff --git a/kernel/fork.c b/kernel/fork.c
index 7926993..ac4294a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1773,6 +1773,7 @@ long _do_fork(unsigned long clone_flags,
struct pid *pid;

trace_sched_process_fork(current, p);
+ trace_sched_prio_update(p);

pid = get_task_pid(p, PIDTYPE_PID);
nr = pid_vnr(pid);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ce83e39..c729425 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3708,6 +3708,7 @@ void set_user_nice(struct task_struct *p, long nice)
resched_curr(rq);
}
out_unlock:
+ trace_sched_prio_update(p);
task_rq_unlock(rq, p, &rf);
}
EXPORT_SYMBOL(set_user_nice);
@@ -3912,6 +3913,8 @@ static void __setscheduler(struct rq *rq, struct task_struct *p,
p->sched_class = &rt_sched_class;
else
p->sched_class = &fair_sched_class;
+
+ trace_sched_prio_update(p);
}

static void
--
1.9.1
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