Subject: Re: [RFC PATCH 2/3] powerpc/ftrace: Override ftrace_location_lookup() for MPROFILE_KERNEL

From: "Naveen N. Rao" <[email-masked]>
Date: Thu, 10 Feb 2022 13:58:29 +0000

Steven Rostedt wrote:
> On Wed, 09 Feb 2022 17:50:09 +0000
> "Naveen N. Rao" <[email-masked]> wrote:
> 
>> However, I think we will not be able to use a fixed range.  I would like 
>> to reserve instructions from function entry till the branch to 
>> _mcount(), and it can be two or four instructions depending on whether a 
>> function has a global entry point. For this, I am considering adding a 
>> field in 'struct dyn_arch_ftrace', and a hook in ftrace_process_locs() 
>> to initialize the same. I may need to override ftrace_cmp_recs().
> 
> Be careful about adding anything to dyn_arch_ftrace. powerpc already adds
> the pointer to the module. Anything you add to that gets multiplied by
> thousands of times (which takes up memory).
> 
> At boot up you may see something like:
> 
>   ftrace: allocating 45363 entries in 178 pages
> 
> That's 45,363 dyn_arch_ftrace structures. And each module loads their own
> as well. To see how many total you have after boot up:
> 
> 
>   # cat /sys/kernel/tracing/dyn_ftrace_total_info 
> 55974 pages:295 groups: 89
> 
> That's from the same kernel. Another 10,000 entries were created by modules.
> (This was for x86_64)
> 
> What you may be able to do, is to add a way to look at the already saved
> kallsyms, which keeps track of the function entry and exit to know how to
> map an address back to the function.
> 
>    kallsyms_lookup(addr, NULL, &offset, NULL, NULL);
> 
> Should give you the offset of addr from the start of the function.

Good point. I should be able to overload the existing field for this 
purpose. Is something like the below ok?

---
 arch/powerpc/include/asm/ftrace.h  | 13 ++++++
 arch/powerpc/kernel/trace/ftrace.c | 73 ++++++++++++++++++++++++++----
 kernel/trace/ftrace.c              |  2 +
 3 files changed, 78 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
index debe8c4f706260..96d6e26cee86af 100644
--- a/arch/powerpc/include/asm/ftrace.h
+++ b/arch/powerpc/include/asm/ftrace.h
@@ -59,6 +59,19 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
 struct dyn_arch_ftrace {
 	struct module *mod;
 };
+
+struct dyn_ftrace;
+struct module *ftrace_mod_addr_get(struct dyn_ftrace *rec);
+void ftrace_mod_addr_set(struct dyn_ftrace *rec, struct module *mod);
+
+#ifdef CONFIG_MPROFILE_KERNEL
+int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
+#define ftrace_init_nop ftrace_init_nop
+
+int ftrace_cmp_recs(const void *a, const void *b);
+#define ftrace_cmp_recs ftrace_cmp_recs
+#endif
+
 #endif /* __ASSEMBLY__ */
 
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c
index 80b6285769f27c..d9b6faa4c98a8c 100644
--- a/arch/powerpc/kernel/trace/ftrace.c
+++ b/arch/powerpc/kernel/trace/ftrace.c
@@ -428,21 +428,21 @@ int ftrace_make_nop(struct module *mod,
 	 * We should either already have a pointer to the module
 	 * or it has been passed in.
 	 */
-	if (!rec->arch.mod) {
+	if (!ftrace_mod_addr_get(rec)) {
 		if (!mod) {
 			pr_err("No module loaded addr=%lx\n", addr);
 			return -EFAULT;
 		}
-		rec->arch.mod = mod;
+		ftrace_mod_addr_set(rec, mod);
 	} else if (mod) {
-		if (mod != rec->arch.mod) {
+		if (mod != ftrace_mod_addr_get(rec)) {
 			pr_err("Record mod %p not equal to passed in mod %p\n",
-			       rec->arch.mod, mod);
+			       ftrace_mod_addr_get(rec), mod);
 			return -EINVAL;
 		}
 		/* nothing to do if mod == rec->arch.mod */
 	} else
-		mod = rec->arch.mod;
+		mod = ftrace_mod_addr_get(rec);
 
 	return __ftrace_make_nop(mod, rec, addr);
 #else
@@ -451,6 +451,59 @@ int ftrace_make_nop(struct module *mod,
 #endif /* CONFIG_MODULES */
 }
 
+#ifdef CONFIG_MPROFILE_KERNEL
+struct module *ftrace_mod_addr_get(struct dyn_ftrace *rec)
+{
+	return (struct module *)((unsigned long)rec->arch.mod & ~0x1);
+}
+
+void ftrace_mod_addr_set(struct dyn_ftrace *rec, struct module *mod)
+{
+	rec->arch.mod = (struct module *)(((unsigned long)rec->arch.mod & 0x1) | (unsigned long)mod);
+}
+
+bool ftrace_location_has_gep(const struct dyn_ftrace *rec)
+{
+	return !!((unsigned long)rec->arch.mod & 0x1);
+}
+
+int ftrace_cmp_recs(const void *a, const void *b)
+{
+	const struct dyn_ftrace *key = a;
+	const struct dyn_ftrace *rec = b;
+	int offset = ftrace_location_has_gep(rec) ? 12 : 4;
+
+	if (key->flags < rec->ip - offset)
+		return -1;
+	if (key->ip >= rec->ip + MCOUNT_INSN_SIZE)
+		return 1;
+	return 0;
+}
+
+int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
+{
+	unsigned long offset;
+
+	if (!kallsyms_lookup_size_offset(rec->ip, NULL, &offset) || (offset != 12 && offset != 4)) {
+		/* TODO: implement logic to deduce lep/gep from code */
+	} else if (offset == 12) {
+		ftrace_mod_addr_set(rec, (struct module *)1);
+	}
+
+	return ftrace_make_nop(mod, rec, MCOUNT_ADDR);
+}
+#else
+struct module *ftrace_mod_addr_get(struct dyn_ftrace *rec)
+{
+	return rec->arch.mod;
+}
+
+void ftrace_mod_addr_set(struct dyn_ftrace *rec, struct module * mod)
+{
+	rec->arch.mod = mod;
+}
+#endif /* CONFIG_MPROFILE_KERNEL */
+
 #ifdef CONFIG_MODULES
 #ifdef CONFIG_PPC64
 /*
@@ -494,7 +547,7 @@ __ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 	ppc_inst_t instr;
 	void *ip = (void *)rec->ip;
 	unsigned long entry, ptr, tramp;
-	struct module *mod = rec->arch.mod;
+	struct module *mod = ftrace_mod_addr_get(rec);
 
 	/* read where this goes */
 	if (copy_inst_from_kernel_nofault(op, ip))
@@ -561,7 +614,7 @@ __ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 	int err;
 	ppc_inst_t op;
 	u32 *ip = (u32 *)rec->ip;
-	struct module *mod = rec->arch.mod;
+	struct module *mod = ftrace_mod_addr_get(rec);
 	unsigned long tramp;
 
 	/* read where this goes */
@@ -678,7 +731,7 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 	 * Being that we are converting from nop, it had better
 	 * already have a module defined.
 	 */
-	if (!rec->arch.mod) {
+	if (!ftrace_mod_addr_get(rec)) {
 		pr_err("No module loaded\n");
 		return -EINVAL;
 	}
@@ -699,7 +752,7 @@ __ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
 	ppc_inst_t op;
 	unsigned long ip = rec->ip;
 	unsigned long entry, ptr, tramp;
-	struct module *mod = rec->arch.mod;
+	struct module *mod = ftrace_mod_addr_get(rec);
 
 	/* If we never set up ftrace trampolines, then bail */
 	if (!mod->arch.tramp || !mod->arch.tramp_regs) {
@@ -814,7 +867,7 @@ int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
 	/*
 	 * Out of range jumps are called from modules.
 	 */
-	if (!rec->arch.mod) {
+	if (!ftrace_mod_addr_get(rec)) {
 		pr_err("No module loaded\n");
 		return -EINVAL;
 	}
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index f9feb197b2daaf..68f20cf34b0c47 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -1510,6 +1510,7 @@ ftrace_ops_test(struct ftrace_ops *ops, unsigned long ip, void *regs)
 	}

+#ifndef ftrace_cmp_recs
 static int ftrace_cmp_recs(const void *a, const void *b)
 {
 	const struct dyn_ftrace *key = a;
@@ -1521,6 +1522,7 @@ static int ftrace_cmp_recs(const void *a, const void *b)
 		return 1;
 	return 0;
 }
+#endif
 
 static struct dyn_ftrace *lookup_rec(unsigned long start, unsigned long end)
 {

Thanks,
Naveen