[PATCH] x86/ftrace: fix boot time slowdown

Mike Rapoport
2024-11-24T14:07:17Z

From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>

Steven Rostedt reported slowdown by over 30ms caused by commit 9bfc4824fd48
("x86/module: prepare module loading for ROX allocations of text")

Before:

# cat /sys/kernel/tracing/dyn_ftrace_total_info
57695 pages:231 groups: 9
ftrace boot update time = 14733459 (ns)
ftrace module total update time = 449016 (ns)

After:

# cat /sys/kernel/tracing/dyn_ftrace_total_info
57708 pages:231 groups: 9
ftrace boot update time = 47195374 (ns)
ftrace module total update time = 592080 (ns)

The slowdown happened because initial patching of kernel code for ftrace
was switched from text_poke_early() to text_poke() to accommodate ftrace
updates of module text allocated as ROX.

Restore the use of text_poke_early() for boot time patching of the kernel
code.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Closes: https://lore.kernel.org/all/20241118132501.4eddb46c@gandalf.local.home
Fixes: 9bfc4824fd48 ("x86/module: prepare module loading for ROX allocations of text")
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
arch/x86/kernel/ftrace.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 4dd0ad6c94d6..9e50288abbaa 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -111,7 +111,7 @@ static int ftrace_verify_code(unsigned long ip, const char *old_code)
*/
static int __ref
ftrace_modify_code_direct(unsigned long ip, const char *old_code,
- const char *new_code)
+ const char *new_code, struct module *mod)
{
int ret = ftrace_verify_code(ip, old_code);
if (ret)
@@ -120,6 +120,8 @@ ftrace_modify_code_direct(unsigned long ip, const char *old_code,
/* replace the text with the new text */
if (ftrace_poke_late) {
text_poke_queue((void *)ip, new_code, MCOUNT_INSN_SIZE, NULL);
+ } else if (!mod) {
+ text_poke_early((void *)ip, new_code, MCOUNT_INSN_SIZE);
} else {
mutex_lock(&text_mutex);
text_poke((void *)ip, new_code, MCOUNT_INSN_SIZE);
@@ -145,7 +147,7 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec, unsigned long ad
* just modify the code directly.
*/
if (addr == MCOUNT_ADDR)
- return ftrace_modify_code_direct(ip, old, new);
+ return ftrace_modify_code_direct(ip, old, new, mod);

/*
* x86 overrides ftrace_replace_code -- this function will never be used
@@ -164,7 +166,7 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
new = ftrace_call_replace(ip, addr);

/* Should only be called when module is loaded */
- return ftrace_modify_code_direct(rec->ip, old, new);
+ return ftrace_modify_code_direct(rec->ip, old, new, NULL);
}

/*

base-commit: 9f16d5e6f220661f73b36a4be1b21575651d8833
--
2.45.2