Commit 18bfee32 authored by Christophe Leroy's avatar Christophe Leroy Committed by Steven Rostedt (Google)
Browse files

ftrace: Make ftrace_graph_is_dead() a static branch 

ftrace_graph_is_dead() is used on hot paths, it just reads a variable
in memory and is not worth suffering function call constraints.

For instance, at entry of prepare_ftrace_return(), inlining it avoids
saving prepare_ftrace_return() parameters to stack and restoring them
after calling ftrace_graph_is_dead().

While at it using a static branch is even more performant and is
rather well adapted considering that the returned value will almost
never change.

Inline ftrace_graph_is_dead() and replace 'kill_ftrace_graph' bool
by a static branch.

The performance improvement is noticeable.

Link: https://lkml.kernel.org/r/e0411a6a0ed3eafff0ad2bc9cd4b0e202b4617df.1648623570.git.christophe.leroy@csgroup.eu



Signed-off-by: default avatarChristophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
parent fcbf591c

include/linux/ftrace.h

+15 −1
Original line number Diff line number Diff line
@@ -9,6 +9,7 @@ 


#include <linux/trace_recursion.h>

#include <linux/trace_clock.h>

#include <linux/jump_label.h>

#include <linux/kallsyms.h>

#include <linux/linkage.h>

#include <linux/bitops.h>
@@ -1015,7 +1016,20 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx, 
extern int register_ftrace_graph(struct fgraph_ops *ops);

extern void unregister_ftrace_graph(struct fgraph_ops *ops);



extern bool ftrace_graph_is_dead(void);

/**

 * ftrace_graph_is_dead - returns true if ftrace_graph_stop() was called

 *

 * ftrace_graph_stop() is called when a severe error is detected in

 * the function graph tracing. This function is called by the critical

 * paths of function graph to keep those paths from doing any more harm.

 */

DECLARE_STATIC_KEY_FALSE(kill_ftrace_graph);



static inline bool ftrace_graph_is_dead(void)

{

	return static_branch_unlikely(&kill_ftrace_graph);

}



extern void ftrace_graph_stop(void);



/* The current handlers in use */

kernel/trace/fgraph.c

+3 −14
Original line number Diff line number Diff line
@@ -7,6 +7,7 @@ 
 *

 * Highly modified by Steven Rostedt (VMware).

 */

#include <linux/jump_label.h>

#include <linux/suspend.h>

#include <linux/ftrace.h>

#include <linux/slab.h>
@@ -23,24 +24,12 @@ 
#define ASSIGN_OPS_HASH(opsname, val)

#endif



static bool kill_ftrace_graph;

DEFINE_STATIC_KEY_FALSE(kill_ftrace_graph);

int ftrace_graph_active;



/* Both enabled by default (can be cleared by function_graph tracer flags */

static bool fgraph_sleep_time = true;



/**

 * ftrace_graph_is_dead - returns true if ftrace_graph_stop() was called

 *

 * ftrace_graph_stop() is called when a severe error is detected in

 * the function graph tracing. This function is called by the critical

 * paths of function graph to keep those paths from doing any more harm.

 */

bool ftrace_graph_is_dead(void)

{

	return kill_ftrace_graph;

}



/**

 * ftrace_graph_stop - set to permanently disable function graph tracing

 *
@@ -51,7 +40,7 @@ bool ftrace_graph_is_dead(void) 
 */

void ftrace_graph_stop(void)

{

	kill_ftrace_graph = true;

	static_branch_enable(&kill_ftrace_graph);

}



/* Add a function return address to the trace stack on thread info.*/