[RFC PATCH 09/14] powerpc/ftrace: Use a hash table for tracking ftrace stubs

[Naveen N. Rao]

In preparation for having to deal with large number of ftrace stubs in
support of ftrace direct calls, convert existing stubs to use a hash
table. The hash table is key'ed off the target address for the stubs
since there could be multiple stubs for the same target to cover the
full kernel text.

Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
---
 arch/powerpc/kernel/trace/ftrace.c | 75 +++++++++++++-----------------
 1 file changed, 33 insertions(+), 42 deletions(-)

diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c
index 14b39f7797d455..7ddb6e4b527c39 100644
--- a/arch/powerpc/kernel/trace/ftrace.c
+++ b/arch/powerpc/kernel/trace/ftrace.c
@@ -13,6 +13,7 @@
 
 #define pr_fmt(fmt) "ftrace-powerpc: " fmt
 
+#include <linux/hashtable.h>
 #include <linux/spinlock.h>
 #include <linux/hardirq.h>
 #include <linux/uaccess.h>
@@ -32,14 +33,12 @@
 
 #ifdef CONFIG_DYNAMIC_FTRACE
 
-/*
- * We generally only have a single long_branch tramp and at most 2 or 3 plt
- * tramps generated. But, we don't use the plt tramps currently. We also allot
- * 2 tramps after .text and .init.text. So, we only end up with around 3 usable
- * tramps in total. Set aside 8 just to be sure.
- */
-#define	NUM_FTRACE_TRAMPS	8
-static unsigned long ftrace_tramps[NUM_FTRACE_TRAMPS];
+static DEFINE_HASHTABLE(ppc_ftrace_stubs, 8);
+struct ppc_ftrace_stub_data {
+	unsigned long addr;
+	unsigned long target;
+	struct hlist_node hentry;
+};
 
 static struct ppc_inst
 ftrace_call_replace(unsigned long ip, unsigned long addr, int link)
@@ -288,36 +287,31 @@ __ftrace_make_nop(struct module *mod,
 #endif /* PPC64 */
 #endif /* CONFIG_MODULES */
 
-static unsigned long find_ftrace_tramp(unsigned long ip)
+static unsigned long find_ftrace_tramp(unsigned long ip, unsigned long target)
 {
-	int i;
+	struct ppc_ftrace_stub_data *stub;
 	struct ppc_inst instr;
 
-	/*
-	 * We have the compiler generated long_branch tramps at the end
-	 * and we prefer those
-	 */
-	for (i = NUM_FTRACE_TRAMPS - 1; i >= 0; i--)
-		if (!ftrace_tramps[i])
-			continue;
-		else if (create_branch(&instr, (void *)ip,
-				       ftrace_tramps[i], 0) == 0)
-			return ftrace_tramps[i];
+	hash_for_each_possible(ppc_ftrace_stubs, stub, hentry, target)
+		if (stub->target == target && !create_branch(&instr, (void *)ip, stub->addr, 0))
+			return stub->addr;
 
 	return 0;
 }
 
-static int add_ftrace_tramp(unsigned long tramp)
+static int add_ftrace_tramp(unsigned long tramp, unsigned long target)
 {
-	int i;
+	struct ppc_ftrace_stub_data *stub;
 
-	for (i = 0; i < NUM_FTRACE_TRAMPS; i++)
-		if (!ftrace_tramps[i]) {
-			ftrace_tramps[i] = tramp;
-			return 0;
-		}
+	stub = kmalloc(sizeof(*stub), GFP_KERNEL);
+	if (!stub)
+		return -1;
 
-	return -1;
+	stub->addr = tramp;
+	stub->target = target;
+	hash_add(ppc_ftrace_stubs, &stub->hentry, target);
+
+	return 0;
 }
 
 /*
@@ -328,16 +322,14 @@ static int add_ftrace_tramp(unsigned long tramp)
  */
 static int setup_mcount_compiler_tramp(unsigned long tramp)
 {
-	int i;
 	struct ppc_inst op;
-	unsigned long ptr;
 	struct ppc_inst instr;
+	struct ppc_ftrace_stub_data *stub;
+	unsigned long ptr, ftrace_target = ppc_global_function_entry((void *)FTRACE_REGS_ADDR);
 
 	/* Is this a known long jump tramp? */
-	for (i = 0; i < NUM_FTRACE_TRAMPS; i++)
-		if (!ftrace_tramps[i])
-			break;
-		else if (ftrace_tramps[i] == tramp)
+	hash_for_each_possible(ppc_ftrace_stubs, stub, hentry, ftrace_target)
+		if (stub->target == ftrace_target && stub->addr == tramp)
 			return 0;
 
 	/* New trampoline -- read where this goes */
@@ -361,19 +353,18 @@ static int setup_mcount_compiler_tramp(unsigned long tramp)
 	}
 
 	/* Let's re-write the tramp to go to ftrace_[regs_]caller */
-	ptr = ppc_global_function_entry((void *)FTRACE_REGS_ADDR);
-	if (create_branch(&instr, (void *)tramp, ptr, 0)) {
+	if (create_branch(&instr, (void *)tramp, ftrace_target, 0)) {
 		pr_debug("%ps is not reachable from existing mcount tramp\n",
-				(void *)ptr);
+				(void *)ftrace_target);
 		return -1;
 	}
 
-	if (patch_branch((struct ppc_inst *)tramp, ptr, 0)) {
+	if (patch_branch((struct ppc_inst *)tramp, ftrace_target, 0)) {
 		pr_debug("REL24 out of range!\n");
 		return -1;
 	}
 
-	if (add_ftrace_tramp(tramp)) {
+	if (add_ftrace_tramp(tramp, ftrace_target)) {
 		pr_debug("No tramp locations left\n");
 		return -1;
 	}
@@ -405,7 +396,7 @@ static int __ftrace_make_nop_kernel(struct dyn_ftrace *rec, unsigned long addr)
 
 	if (setup_mcount_compiler_tramp(tramp)) {
 		/* Are other trampolines reachable? */
-		if (!find_ftrace_tramp(ip)) {
+		if (!find_ftrace_tramp(ip, FTRACE_REGS_ADDR)) {
 			pr_err("No ftrace trampolines reachable from %ps\n",
 					(void *)ip);
 			return -EINVAL;
@@ -646,7 +637,7 @@ static int __ftrace_make_call_kernel(struct dyn_ftrace *rec, unsigned long addr)
 		return -EINVAL;
 	}
 
-	tramp = find_ftrace_tramp((unsigned long)ip);
+	tramp = find_ftrace_tramp((unsigned long)ip, FTRACE_REGS_ADDR);
 	if (!tramp) {
 		pr_err("No ftrace trampolines reachable from %ps\n", ip);
 		return -EINVAL;
@@ -894,7 +885,7 @@ int __init ftrace_dyn_arch_init(void)
 		memcpy(tramp[i], stub_insns, sizeof(stub_insns));
 		tramp[i][1] |= PPC_HA(reladdr);
 		tramp[i][2] |= PPC_LO(reladdr);
-		add_ftrace_tramp((unsigned long)tramp[i]);
+		add_ftrace_tramp((unsigned long)tramp[i], addr);
 	}
 
 	return 0;
-- 
2.25.4