On Mon, Jun 10, 2024 at 04:03:56PM GMT, Steven Rostedt wrote:

On Mon, 10 Jun 2024 14:08:15 +0530
Naveen N Rao [off-list ref] wrote:

quoted

Pointer to struct module is only relevant for ftrace records belonging
to kernel modules. Having this field in dyn_arch_ftrace wastes memory
for all ftrace records belonging to the kernel. Remove the same in
favour of looking up the module from the ftrace record address, similar
to other architectures.

Signed-off-by: Naveen N Rao <naveen@kernel.org>
---
 arch/powerpc/include/asm/ftrace.h        |  1 -
 arch/powerpc/kernel/trace/ftrace.c       | 47 ++++++++++-----
 arch/powerpc/kernel/trace/ftrace_64_pg.c | 73 +++++++++++-------------
 3 files changed, 64 insertions(+), 57 deletions(-)

diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
index 107fc5a48456..201f9d15430a 100644
--- a/arch/powerpc/include/asm/ftrace.h
+++ b/arch/powerpc/include/asm/ftrace.h

@@ -26,7 +26,6 @@ unsigned long prepare_ftrace_return(unsigned long parent, unsigned long ip,
 struct module;
 struct dyn_ftrace;
 struct dyn_arch_ftrace {
-	struct module *mod;
 };

Nice. I always hated that extra field.

It was your complaint a while back that prompted this change :)

Though I introduce a different pointer here in the next patch. /me 
ducks.

quoted

 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS

diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c
index d8d6b4fd9a14..041be965485e 100644
--- a/arch/powerpc/kernel/trace/ftrace.c
+++ b/arch/powerpc/kernel/trace/ftrace.c

@@ -106,20 +106,36 @@ static unsigned long find_ftrace_tramp(unsigned long ip)
 	return 0;
 }
 
+static struct module *ftrace_lookup_module(struct dyn_ftrace *rec)
+{
+	struct module *mod = NULL;
+
+#ifdef CONFIG_MODULES
+	/*
+	 * NOTE: __module_text_address() must be called with preemption
+	 * disabled, but we can rely on ftrace_lock to ensure that 'mod'
+	 * retains its validity throughout the remainder of this code.
+	*/
+	preempt_disable();
+	mod = __module_text_address(rec->ip);
+	preempt_enable();
+
+	if (!mod)
+		pr_err("No module loaded at addr=%lx\n", rec->ip);
+#endif
+
+	return mod;
+}

It may look nicer to have:

#ifdef CONFIG_MODULES
static struct module *ftrace_lookup_module(struct dyn_ftrace *rec)
{
	struct module *mod = NULL;

	/*
	 * NOTE: __module_text_address() must be called with preemption
	 * disabled, but we can rely on ftrace_lock to ensure that 'mod'
	 * retains its validity throughout the remainder of this code.
	*/
	preempt_disable();
	mod = __module_text_address(rec->ip);
	preempt_enable();

	if (!mod)
		pr_err("No module loaded at addr=%lx\n", rec->ip);

	return mod;
}
#else
static inline struct module *ftrace_lookup_module(struct dyn_ftrace *rec)
{
	return NULL;
}
#endif

I wrote this, and then I thought it will be simpler to do the version I 
posted. I will move back to this since it looks to be the preferred way.

quoted

+
 static int ftrace_get_call_inst(struct dyn_ftrace *rec, unsigned long addr, ppc_inst_t *call_inst)
 {
 	unsigned long ip = rec->ip;
 	unsigned long stub;
+	struct module *mod;
 
 	if (is_offset_in_branch_range(addr - ip)) {
 		/* Within range */
 		stub = addr;
-#ifdef CONFIG_MODULES
-	} else if (rec->arch.mod) {
-		/* Module code would be going to one of the module stubs */
-		stub = (addr == (unsigned long)ftrace_caller ? rec->arch.mod->arch.tramp :
-							       rec->arch.mod->arch.tramp_regs);
-#endif
 	} else if (core_kernel_text(ip)) {
 		/* We would be branching to one of our ftrace stubs */
 		stub = find_ftrace_tramp(ip);

@@ -128,7 +144,16 @@ static int ftrace_get_call_inst(struct dyn_ftrace *rec, unsigned long addr, ppc_
 			return -EINVAL;
 		}
 	} else {
-		return -EINVAL;
+		mod = ftrace_lookup_module(rec);
+		if (mod) {
+#ifdef CONFIG_MODULES
+			/* Module code would be going to one of the module stubs */
+			stub = (addr == (unsigned long)ftrace_caller ? mod->arch.tramp :
+								       mod->arch.tramp_regs);
+#endif

You have CONFIG_MODULES here and in ftrace_lookup_module() above, which
would always return NULL. Could you combine the above to be done in
ftrace_lookup_module() so that there's no #ifdef CONFIG_MODULES here?

Yes, indeed. That will look cleaner.


Thanks,
Naveen