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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: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Christophe Leroy 2022-03-30 09:00:19 +02:00 committed by Steven Rostedt (Google)
parent fcbf591ced
commit 18bfee3216
2 changed files with 18 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
include/linux/ftrace.h
View File

@ -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 */

17
kernel/trace/fgraph.c
View File

@ -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.*/