[PATCH] TRACING: FTRACE: Use xarray structure for ftrace syscalls

[Hassan Naveed]

From: Hassan Naveed <hnaveed@xxxxxxxxxxxx>

Currently, a lot of memory is wasted for architectures like MIPS when
init_ftrace_syscalls() allocates the array for syscalls using kcalloc.
This is because syscalls numbers start from 4000, 5000 or 6000 and
array elements up to that point are unused.
Fix this by using a data structure more suited to storing sparsely
populated arrays. The XARRAY data structure, implemented using radix
trees, is much more memory efficient for storing the syscalls in
question.

Signed-off-by: Hassan Naveed <hnaveed@xxxxxxxxxxxx>
---
 kernel/trace/trace_syscalls.c | 17 ++++-------------
 1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index f93a56d2db27..1fee710be874 100644
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -7,6 +7,7 @@
 #include <linux/module.h>	/* for MODULE_NAME_LEN via KSYM_SYMBOL_LEN */
 #include <linux/ftrace.h>
 #include <linux/perf_event.h>
+#include <linux/xarray.h>
 #include <asm/syscall.h>
 
 #include "trace_output.h"
@@ -30,7 +31,7 @@ syscall_get_enter_fields(struct trace_event_call *call)
 extern struct syscall_metadata *__start_syscalls_metadata[];
 extern struct syscall_metadata *__stop_syscalls_metadata[];
 
-static struct syscall_metadata **syscalls_metadata;
+static DEFINE_XARRAY(syscalls_metadata);
 
 #ifndef ARCH_HAS_SYSCALL_MATCH_SYM_NAME
 static inline bool arch_syscall_match_sym_name(const char *sym, const char *name)
@@ -101,10 +102,7 @@ find_syscall_meta(unsigned long syscall)
 
 static struct syscall_metadata *syscall_nr_to_meta(int nr)
 {
-	if (!syscalls_metadata || nr >= NR_syscalls || nr < 0)
-		return NULL;
-
-	return syscalls_metadata[nr];
+	return xa_load(&syscalls_metadata, (unsigned long)nr);
 }
 
 const char *get_syscall_name(int syscall)
@@ -535,13 +533,6 @@ void __init init_ftrace_syscalls(void)
 	unsigned long addr;
 	int i;
 
-	syscalls_metadata = kcalloc(NR_syscalls, sizeof(*syscalls_metadata),
-				    GFP_KERNEL);
-	if (!syscalls_metadata) {
-		WARN_ON(1);
-		return;
-	}
-
 	for (i = 0; i < NR_syscalls; i++) {
 		addr = arch_syscall_addr(i);
 		meta = find_syscall_meta(addr);
@@ -549,7 +540,7 @@ void __init init_ftrace_syscalls(void)
 			continue;
 
 		meta->syscall_nr = i;
-		syscalls_metadata[i] = meta;
+		xa_store(&syscalls_metadata, i, meta, GFP_KERNEL);
 	}
 }
 
-- 
2.17.1