[PATCH v3 5/6] tracing/user_events: Use bits vs bytes for enabled status page data

From: Beau Belgrave
Date: Thu Jul 28 2022 - 19:33:43 EST

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://lore.kernel.org/all/2059213643.196683.1648499088753.JavaMail.zimbra@xxxxxxxxxxxx/

Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Signed-off-by: Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx>
---
include/linux/user_events.h | 15 +---
kernel/trace/trace_events_user.c | 75 +++++++++++++++++--
samples/user_events/example.c | 25 +++++--
.../selftests/user_events/ftrace_test.c | 47 ++++++++++--
.../testing/selftests/user_events/perf_test.c | 11 ++-
5 files changed, 135 insertions(+), 38 deletions(-)

diff --git a/include/linux/user_events.h b/include/linux/user_events.h
index 736e05603463..592a3fbed98e 100644
--- a/include/linux/user_events.h
+++ b/include/linux/user_events.h
@@ -20,15 +20,6 @@
#define USER_EVENTS_SYSTEM "user_events"
#define USER_EVENTS_PREFIX "u:"

-/* Bits 0-6 are for known probe types, Bit 7 is for unknown probes */
-#define EVENT_BIT_FTRACE 0
-#define EVENT_BIT_PERF 1
-#define EVENT_BIT_OTHER 7
-
-#define EVENT_STATUS_FTRACE (1 << EVENT_BIT_FTRACE)
-#define EVENT_STATUS_PERF (1 << EVENT_BIT_PERF)
-#define EVENT_STATUS_OTHER (1 << EVENT_BIT_OTHER)
-
/* Create dynamic location entry within a 32-bit value */
#define DYN_LOC(offset, size) ((size) << 16 | (offset))

@@ -45,12 +36,12 @@ struct user_reg {
/* Input: Pointer to string with event name, description and flags */
__u64 name_args;

- /* Output: Byte index of the event within the status page */
- __u32 status_index;
+ /* Output: Bitwise index of the event within the status page */
+ __u32 status_bit;

/* Output: Index of the event to use when writing data */
__u32 write_index;
-};
+} __attribute__((__packed__));

#define DIAG_IOC_MAGIC '*'

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 2bcae7abfa81..2c0a6ec75548 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -40,17 +40,44 @@
*/
#define MAX_PAGE_ORDER 0
#define MAX_PAGES (1 << MAX_PAGE_ORDER)
-#define MAX_EVENTS (MAX_PAGES * PAGE_SIZE)
+#define MAX_BYTES (MAX_PAGES * PAGE_SIZE)
+#define MAX_EVENTS (MAX_BYTES * 8)

/* Limit how long of an event name plus args within the subsystem. */
#define MAX_EVENT_DESC 512
#define EVENT_NAME(user_event) ((user_event)->tracepoint.name)
#define MAX_FIELD_ARRAY_SIZE 1024

+/*
+ * The MAP_STATUS_* macros are used for taking a index and determining the
+ * appropriate byte and the bit in the byte to set/reset for an event.
+ *
+ * The lower 3 bits of the index decide which bit to set.
+ * The remaining upper bits of the index decide which byte to use for the bit.
+ *
+ * This is used when an event has a probe attached/removed to reflect live
+ * status of the event wanting tracing or not to user-programs via shared
+ * memory maps.
+ */
+#define MAP_STATUS_BYTE(index) ((index) >> 3)
+#define MAP_STATUS_MASK(index) BIT((index) & 7)
+
+/*
+ * Internal bits (kernel side only) to keep track of connected probes:
+ * These are used when status is requested in text form about an event. These
+ * bits are compared against an internal byte on the event to determine which
+ * probes to print out to the user.
+ *
+ * These do not reflect the mapped bytes between the user and kernel space.
+ */
+#define EVENT_STATUS_FTRACE BIT(0)
+#define EVENT_STATUS_PERF BIT(1)
+#define EVENT_STATUS_OTHER BIT(7)
+
static char *register_page_data;

static DEFINE_MUTEX(reg_mutex);
-static DEFINE_HASHTABLE(register_table, 4);
+static DEFINE_HASHTABLE(register_table, 8);
static DECLARE_BITMAP(page_bitmap, MAX_EVENTS);

/*
@@ -72,6 +99,7 @@ struct user_event {
int index;
int flags;
int min_size;
+ char status;
};

/*
@@ -106,6 +134,22 @@ static u32 user_event_key(char *name)
return jhash(name, strlen(name), 0);
}

+static __always_inline
+void user_event_register_set(struct user_event *user)
+{
+ int i = user->index;
+
+ register_page_data[MAP_STATUS_BYTE(i)] |= MAP_STATUS_MASK(i);
+}
+
+static __always_inline
+void user_event_register_clear(struct user_event *user)
+{
+ int i = user->index;
+
+ register_page_data[MAP_STATUS_BYTE(i)] &= ~MAP_STATUS_MASK(i);
+}
+
static __always_inline __must_check
bool user_event_last_ref(struct user_event *user)
{
@@ -648,7 +692,7 @@ static int destroy_user_event(struct user_event *user)

dyn_event_remove(&user->devent);

- register_page_data[user->index] = 0;
+ user_event_register_clear(user);
clear_bit(user->index, page_bitmap);
hash_del(&user->node);

@@ -827,7 +871,12 @@ static void update_reg_page_for(struct user_event *user)
rcu_read_unlock_sched();
}

- register_page_data[user->index] = status;
+ if (status)
+ user_event_register_set(user);
+ else
+ user_event_register_clear(user);
+
+ user->status = status;
}

/*
@@ -1332,7 +1381,17 @@ static long user_reg_get(struct user_reg __user *ureg, struct user_reg *kreg)
if (size > PAGE_SIZE)
return -E2BIG;

- return copy_struct_from_user(kreg, sizeof(*kreg), ureg, size);
+ if (size < offsetofend(struct user_reg, write_index))
+ return -EINVAL;
+
+ ret = copy_struct_from_user(kreg, sizeof(*kreg), ureg, size);
+
+ if (ret)
+ return ret;
+
+ kreg->size = size;
+
+ return 0;
}

/*
@@ -1376,7 +1435,7 @@ static long user_events_ioctl_reg(struct file *file, unsigned long uarg)
return ret;

put_user((u32)ret, &ureg->write_index);
- put_user(user->index, &ureg->status_index);
+ put_user(user->index, &ureg->status_bit);

return 0;
}
@@ -1485,7 +1544,7 @@ static int user_status_mmap(struct file *file, struct vm_area_struct *vma)
{
unsigned long size = vma->vm_end - vma->vm_start;

- if (size != MAX_EVENTS)
+ if (size != MAX_BYTES)
return -EINVAL;

return remap_pfn_range(vma, vma->vm_start,
@@ -1520,7 +1579,7 @@ static int user_seq_show(struct seq_file *m, void *p)
mutex_lock(&reg_mutex);

hash_for_each(register_table, i, user, node) {
- status = register_page_data[user->index];
+ status = user->status;
flags = user->flags;

seq_printf(m, "%d:%s", user->index, EVENT_NAME(user));
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 @@ int main(int argc, char **argv)
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);

diff --git a/tools/testing/selftests/user_events/ftrace_test.c b/tools/testing/selftests/user_events/ftrace_test.c
index a80fb5ef61d5..404a2713dcae 100644
--- a/tools/testing/selftests/user_events/ftrace_test.c
+++ b/tools/testing/selftests/user_events/ftrace_test.c
@@ -22,6 +22,11 @@ const char *enable_file = "/sys/kernel/debug/tracing/events/user_events/__test_e
const char *trace_file = "/sys/kernel/debug/tracing/trace";
const char *fmt_file = "/sys/kernel/debug/tracing/events/user_events/__test_event/format";

+static inline int status_check(char *status_page, int status_bit)
+{
+ return status_page[status_bit >> 3] & (1 << (status_bit & 7));
+}
+
static int trace_bytes(void)
{
int fd = open(trace_file, O_RDONLY);
@@ -197,12 +202,12 @@ TEST_F(user, register_events) {
/* Register should work */
ASSERT_EQ(0, ioctl(self->data_fd, DIAG_IOCSREG, &reg));
ASSERT_EQ(0, reg.write_index);
- ASSERT_NE(0, reg.status_index);
+ ASSERT_NE(0, reg.status_bit);

/* Multiple registers should result in same index */
ASSERT_EQ(0, ioctl(self->data_fd, DIAG_IOCSREG, &reg));
ASSERT_EQ(0, reg.write_index);
- ASSERT_NE(0, reg.status_index);
+ ASSERT_NE(0, reg.status_bit);

/* Ensure disabled */
self->enable_fd = open(enable_file, O_RDWR);
@@ -212,15 +217,15 @@ TEST_F(user, register_events) {
/* MMAP should work and be zero'd */
ASSERT_NE(MAP_FAILED, status_page);
ASSERT_NE(NULL, status_page);
- ASSERT_EQ(0, status_page[reg.status_index]);
+ ASSERT_EQ(0, status_check(status_page, reg.status_bit));

/* Enable event and ensure bits updated in status */
ASSERT_NE(-1, write(self->enable_fd, "1", sizeof("1")))
- ASSERT_EQ(EVENT_STATUS_FTRACE, status_page[reg.status_index]);
+ ASSERT_NE(0, status_check(status_page, reg.status_bit));

/* Disable event and ensure bits updated in status */
ASSERT_NE(-1, write(self->enable_fd, "0", sizeof("0")))
- ASSERT_EQ(0, status_page[reg.status_index]);
+ ASSERT_EQ(0, status_check(status_page, reg.status_bit));

/* File still open should return -EBUSY for delete */
ASSERT_EQ(-1, ioctl(self->data_fd, DIAG_IOCSDEL, "__test_event"));
@@ -240,6 +245,8 @@ TEST_F(user, write_events) {
struct iovec io[3];
__u32 field1, field2;
int before = 0, after = 0;
+ int page_size = sysconf(_SC_PAGESIZE);
+ char *status_page;

reg.size = sizeof(reg);
reg.name_args = (__u64)"__test_event u32 field1; u32 field2";
@@ -254,10 +261,18 @@ TEST_F(user, write_events) {
io[2].iov_base = &field2;
io[2].iov_len = sizeof(field2);

+ status_page = mmap(NULL, page_size, PROT_READ, MAP_SHARED,
+ self->status_fd, 0);
+
/* Register should work */
ASSERT_EQ(0, ioctl(self->data_fd, DIAG_IOCSREG, &reg));
ASSERT_EQ(0, reg.write_index);
- ASSERT_NE(0, reg.status_index);
+ ASSERT_NE(0, reg.status_bit);
+
+ /* MMAP should work and be zero'd */
+ ASSERT_NE(MAP_FAILED, status_page);
+ ASSERT_NE(NULL, status_page);
+ ASSERT_EQ(0, status_check(status_page, reg.status_bit));

/* Write should fail on invalid slot with ENOENT */
io[0].iov_base = &field2;
@@ -271,6 +286,9 @@ TEST_F(user, write_events) {
self->enable_fd = open(enable_file, O_RDWR);
ASSERT_NE(-1, write(self->enable_fd, "1", sizeof("1")))

+ /* Event should now be enabled */
+ ASSERT_NE(0, status_check(status_page, reg.status_bit));
+
/* Write should make it out to ftrace buffers */
before = trace_bytes();
ASSERT_NE(-1, writev(self->data_fd, (const struct iovec *)io, 3));
@@ -298,7 +316,7 @@ TEST_F(user, write_fault) {
/* Register should work */
ASSERT_EQ(0, ioctl(self->data_fd, DIAG_IOCSREG, &reg));
ASSERT_EQ(0, reg.write_index);
- ASSERT_NE(0, reg.status_index);
+ ASSERT_NE(0, reg.status_bit);

/* Write should work normally */
ASSERT_NE(-1, writev(self->data_fd, (const struct iovec *)io, 2));
@@ -315,6 +333,11 @@ TEST_F(user, write_validator) {
int loc, bytes;
char data[8];
int before = 0, after = 0;
+ int page_size = sysconf(_SC_PAGESIZE);
+ char *status_page;
+
+ status_page = mmap(NULL, page_size, PROT_READ, MAP_SHARED,
+ self->status_fd, 0);

reg.size = sizeof(reg);
reg.name_args = (__u64)"__test_event __rel_loc char[] data";
@@ -322,7 +345,12 @@ TEST_F(user, write_validator) {
/* Register should work */
ASSERT_EQ(0, ioctl(self->data_fd, DIAG_IOCSREG, &reg));
ASSERT_EQ(0, reg.write_index);
- ASSERT_NE(0, reg.status_index);
+ ASSERT_NE(0, reg.status_bit);
+
+ /* MMAP should work and be zero'd */
+ ASSERT_NE(MAP_FAILED, status_page);
+ ASSERT_NE(NULL, status_page);
+ ASSERT_EQ(0, status_check(status_page, reg.status_bit));

io[0].iov_base = &reg.write_index;
io[0].iov_len = sizeof(reg.write_index);
@@ -340,6 +368,9 @@ TEST_F(user, write_validator) {
self->enable_fd = open(enable_file, O_RDWR);
ASSERT_NE(-1, write(self->enable_fd, "1", sizeof("1")))

+ /* Event should now be enabled */
+ ASSERT_NE(0, status_check(status_page, reg.status_bit));
+
/* Full in-bounds write should work */
before = trace_bytes();
loc = DYN_LOC(0, bytes);
diff --git a/tools/testing/selftests/user_events/perf_test.c b/tools/testing/selftests/user_events/perf_test.c
index 26851d51d6bb..8b4c7879d5a7 100644
--- a/tools/testing/selftests/user_events/perf_test.c
+++ b/tools/testing/selftests/user_events/perf_test.c
@@ -35,6 +35,11 @@ static long perf_event_open(struct perf_event_attr *pe, pid_t pid,
return syscall(__NR_perf_event_open, pe, pid, cpu, group_fd, flags);
}

+static inline int status_check(char *status_page, int status_bit)
+{
+ return status_page[status_bit >> 3] & (1 << (status_bit & 7));
+}
+
static int get_id(void)
{
FILE *fp = fopen(id_file, "r");
@@ -120,8 +125,8 @@ TEST_F(user, perf_write) {
/* Register should work */
ASSERT_EQ(0, ioctl(self->data_fd, DIAG_IOCSREG, &reg));
ASSERT_EQ(0, reg.write_index);
- ASSERT_NE(0, reg.status_index);
- ASSERT_EQ(0, status_page[reg.status_index]);
+ ASSERT_NE(0, reg.status_bit);
+ ASSERT_EQ(0, status_check(status_page, reg.status_bit));

/* Id should be there */
id = get_id();
@@ -144,7 +149,7 @@ TEST_F(user, perf_write) {
ASSERT_NE(MAP_FAILED, perf_page);

/* Status should be updated */
- ASSERT_EQ(EVENT_STATUS_PERF, status_page[reg.status_index]);
+ ASSERT_NE(0, status_check(status_page, reg.status_bit));

event.index = reg.write_index;
event.field1 = 0xc001;
--
2.25.1