function-graph: ignore times across schedule
Impact: more accurate timings
The current method of function graph tracing does not take into
account the time spent when a task is not running. This shows functions
that call schedule have increased costs:
3) + 18.664 us | }
------------------------------------------
3) <idle>-0 => kblockd-123
------------------------------------------
3) | finish_task_switch() {
3) 1.441 us | _spin_unlock_irq();
3) 3.966 us | }
3) ! 2959.433 us | }
3) ! 2961.465 us | }
This patch uses the tracepoint in the scheduling context switch to
account for time that has elapsed while a task is scheduled out.
Now we see:
------------------------------------------
3) <idle>-0 => edac-po-1067
------------------------------------------
3) | finish_task_switch() {
3) 0.685 us | _spin_unlock_irq();
3) 2.331 us | }
3) + 41.439 us | }
3) + 42.663 us | }
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 89cd308..471e36d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1409,6 +1409,8 @@
int curr_ret_stack;
/* Stack of return addresses for return function tracing */
struct ftrace_ret_stack *ret_stack;
+ /* time stamp for last schedule */
+ unsigned long long ftrace_timestamp;
/*
* Number of functions that haven't been traced
* because of depth overrun.
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index c81a759..0b90364 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -29,6 +29,8 @@
#include <linux/list.h>
#include <linux/hash.h>
+#include <trace/sched.h>
+
#include <asm/ftrace.h>
#include "trace.h"
@@ -2590,6 +2592,31 @@
return ret;
}
+static void
+ftrace_graph_probe_sched_switch(struct rq *__rq, struct task_struct *prev,
+ struct task_struct *next)
+{
+ unsigned long long timestamp;
+ int index;
+
+ timestamp = trace_clock_local();
+
+ prev->ftrace_timestamp = timestamp;
+
+ /* only process tasks that we timestamped */
+ if (!next->ftrace_timestamp)
+ return;
+
+ /*
+ * Update all the counters in next to make up for the
+ * time next was sleeping.
+ */
+ timestamp -= next->ftrace_timestamp;
+
+ for (index = next->curr_ret_stack; index >= 0; index--)
+ next->ret_stack[index].calltime += timestamp;
+}
+
/* Allocate a return stack for each task */
static int start_graph_tracing(void)
{
@@ -2611,6 +2638,13 @@
ret = alloc_retstack_tasklist(ret_stack_list);
} while (ret == -EAGAIN);
+ if (!ret) {
+ ret = register_trace_sched_switch(ftrace_graph_probe_sched_switch);
+ if (ret)
+ pr_info("ftrace_graph: Couldn't activate tracepoint"
+ " probe to kernel_sched_switch\n");
+ }
+
kfree(ret_stack_list);
return ret;
}
@@ -2674,6 +2708,7 @@
mutex_lock(&ftrace_lock);
atomic_dec(&ftrace_graph_active);
+ unregister_trace_sched_switch(ftrace_graph_probe_sched_switch);
ftrace_graph_return = (trace_func_graph_ret_t)ftrace_stub;
ftrace_graph_entry = ftrace_graph_entry_stub;
ftrace_shutdown(FTRACE_STOP_FUNC_RET);
@@ -2694,6 +2729,7 @@
t->curr_ret_stack = -1;
atomic_set(&t->tracing_graph_pause, 0);
atomic_set(&t->trace_overrun, 0);
+ t->ftrace_timestamp = 0;
} else
t->ret_stack = NULL;
}