tracing/user_events: Use bits vs bytes for enabled status page data

User processes may require many events and when they do the cache
performance of a byte index status check is less ideal than a bit index.
The previous event limit per-page was 4096, the new limit is 32,768.

This change adds a bitwise index to the user_reg struct. Programs check
that the bit at status_bit has a bit set within the status page(s).

Link: https://lkml.kernel.org/r/20220728233309.1896-6-beaub@linux.microsoft.com
Link: https://lore.kernel.org/all/2059213643.196683.1648499088753.JavaMail.zimbra@efficios.com/

Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 18 insertions, 7 deletions

diff --git a/samples/user_events/example.c b/samples/user_events/example.c
index 4f5778e441c0..d06dc24156ec 100644
--- a/samples/user_events/example.c
+++ b/samples/user_events/example.c
@@ -12,13 +12,21 @@
 #include <fcntl.h>
 #include <stdio.h>
 #include <unistd.h>
+#include <asm/bitsperlong.h>
+#include <endian.h>
 #include <linux/user_events.h>
 
+#if __BITS_PER_LONG == 64
+#define endian_swap(x) htole64(x)
+#else
+#define endian_swap(x) htole32(x)
+#endif
+
 /* Assumes debugfs is mounted */
 const char *data_file = "/sys/kernel/debug/tracing/user_events_data";
 const char *status_file = "/sys/kernel/debug/tracing/user_events_status";
 
-static int event_status(char **status)
+static int event_status(long **status)
 {
 	int fd = open(status_file, O_RDONLY);
 
@@ -33,7 +41,8 @@ static int event_status(char **status)
 	return 0;
 }
 
-static int event_reg(int fd, const char *command, int *status, int *write)
+static int event_reg(int fd, const char *command, long *index, long *mask,
+		     int *write)
 {
 	struct user_reg reg = {0};
 
@@ -43,7 +52,8 @@ static int event_reg(int fd, const char *command, int *status, int *write)
 	if (ioctl(fd, DIAG_IOCSREG, &reg) == -1)
 		return -1;
 
-	*status = reg.status_index;
+	*index = reg.status_bit / __BITS_PER_LONG;
+	*mask = endian_swap(1L << (reg.status_bit % __BITS_PER_LONG));
 	*write = reg.write_index;
 
 	return 0;
@@ -51,8 +61,9 @@ static int event_reg(int fd, const char *command, int *status, int *write)
 
 int main(int argc, char **argv)
 {
-	int data_fd, status, write;
-	char *status_page;
+	int data_fd, write;
+	long index, mask;
+	long *status_page;
 	struct iovec io[2];
 	__u32 count = 0;
 
@@ -61,7 +72,7 @@ int main(int argc, char **argv)
 
 	data_fd = open(data_file, O_RDWR);
 
-	if (event_reg(data_fd, "test u32 count", &status, &write) == -1)
+	if (event_reg(data_fd, "test u32 count", &index, &mask, &write) == -1)
 		return errno;
 
 	/* Setup iovec */
@@ -75,7 +86,7 @@ ask:
 	getchar();
 
 	/* Check if anyone is listening */
-	if (status_page[status]) {
+	if (status_page[index] & mask) {
 		/* Yep, trace out our data */
 		writev(data_fd, (const struct iovec *)io, 2);