[lttng-dev] [PATCH] Improve tracef/tracelog to use the stack for small strings
Mathieu Desnoyers
mathieu.desnoyers at efficios.com
Fri Jul 22 10:44:45 EDT 2022
----- On Jul 18, 2022, at 5:47 PM, Norbert Lange via lttng-dev lttng-dev at lists.lttng.org wrote:
> Support two common cases, one being that the resulting message is
> small enough to fit into a on-stack buffer.
> The seconds being the common 'printf("%s", "Message")' scheme.
>
> Unfortunately, iterating a va_list is destructive,
> so it has to be copied before calling vprintf.
>
> The implementation was moved to a separate file,
> used by both tracef.c and tracelog.c.
I agree with the overall approach. Some nits below,
>
> Signed-off-by: Norbert Lange <nolange79 at gmail.com>
> ---
> v2:
> - move define into src/common/tracer.h
> see https://lists.lttng.org/pipermail/lttng-dev/2021-May/029977.html
> - moved macro magic into common tracelog-internal.h header
> - rebased onto master
> ---
> src/common/tracer.h | 2 +
> src/lib/lttng-ust/tracef.c | 32 +++--------
> src/lib/lttng-ust/tracelog-internal.h | 83 +++++++++++++++++++++++++++
> src/lib/lttng-ust/tracelog.c | 40 +++----------
> 4 files changed, 102 insertions(+), 55 deletions(-)
> create mode 100644 src/lib/lttng-ust/tracelog-internal.h
>
> diff --git a/src/common/tracer.h b/src/common/tracer.h
> index 2affd6ab..8e18c9b5 100644
> --- a/src/common/tracer.h
> +++ b/src/common/tracer.h
> @@ -26,6 +26,8 @@
> #define LTTNG_RFLAG_EXTENDED RING_BUFFER_RFLAG_END
> #define LTTNG_RFLAG_END (LTTNG_RFLAG_EXTENDED << 1)
>
> +#define LTTNG_TRACE_PRINTF_BUFSIZE 512
> +
> /*
> * LTTng client type enumeration. Used by the consumer to map the
> * callbacks from its own address space.
> diff --git a/src/lib/lttng-ust/tracef.c b/src/lib/lttng-ust/tracef.c
> index c05c7811..92911e1d 100644
> --- a/src/lib/lttng-ust/tracef.c
> +++ b/src/lib/lttng-ust/tracef.c
> @@ -7,6 +7,7 @@
> #define _LGPL_SOURCE
> #include <stdio.h>
> #include "common/macros.h"
> +#include "common/tracer.h"
>
> /* The tracepoint definition is public, but the provider definition is hidden.
> */
> #define LTTNG_UST_TRACEPOINT_PROVIDER_HIDDEN_DEFINITION
> @@ -15,39 +16,22 @@
> #define LTTNG_UST_TRACEPOINT_DEFINE
> #include "lttng-ust-tracef-provider.h"
>
> -static inline
> -void lttng_ust___vtracef(const char *fmt, va_list ap)
> - __attribute__((always_inline, format(printf, 1, 0)));
> -static inline
> -void lttng_ust___vtracef(const char *fmt, va_list ap)
> -{
> - char *msg;
> - const int len = vasprintf(&msg, fmt, ap);
> -
> - /* len does not include the final \0 */
> - if (len < 0)
> - goto end;
> - lttng_ust_tracepoint_cb_lttng_ust_tracef___event(msg, len,
> - LTTNG_UST_CALLER_IP());
> - free(msg);
> -end:
> - return;
> -}
> +#include "tracelog-internal.h"
>
> void lttng_ust__vtracef(const char *fmt, va_list ap)
> __attribute__((format(printf, 1, 0)));
> void lttng_ust__vtracef(const char *fmt, va_list ap)
> {
> - lttng_ust___vtracef(fmt, ap);
> + LTTNG_UST_TRACELOG_VALIST(fmt, ap,
> + lttng_ust_tracepoint_cb_lttng_ust_tracef___event,
> + msg, len, LTTNG_UST_CALLER_IP());
> }
>
> void lttng_ust__tracef(const char *fmt, ...)
> __attribute__((format(printf, 1, 2)));
> void lttng_ust__tracef(const char *fmt, ...)
> {
> - va_list ap;
> -
> - va_start(ap, fmt);
> - lttng_ust___vtracef(fmt, ap);
> - va_end(ap);
> + LTTNG_UST_TRACELOG_VARARG(fmt,
> + lttng_ust_tracepoint_cb_lttng_ust_tracef___event,
> + msg, len, LTTNG_UST_CALLER_IP());
> }
> diff --git a/src/lib/lttng-ust/tracelog-internal.h
> b/src/lib/lttng-ust/tracelog-internal.h
> new file mode 100644
> index 00000000..291ff27c
> --- /dev/null
> +++ b/src/lib/lttng-ust/tracelog-internal.h
> @@ -0,0 +1,83 @@
> +/*
> + * SPDX-License-Identifier: MIT
> + *
> + * Copyright (C) 2013-2014 Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> + * Copyright (C) 2021 Norbert Lange <nolange79 at gmail.com>
> + *
> + * Shared helper macro for tracelog and tracef
Add "." at end of comment.
> + */
> +
As a coding style requirement, in case we ever use a macro in a conditional,
we always use the do { ... } while (0) wrapping to ensure we don't end up
with surprises with unexpected semicolons:
> +#define LTTNG_UST_TRACELOG_VARARG(fmt, callback, ...) \
do { \
> + va_list ap; \
> + char local_buf[LTTNG_TRACE_PRINTF_BUFSIZE]; \
> + int len; \
int ret;
size_t len = 0;
> + char *msg = local_buf; \
> + size_t buflen = sizeof(local_buf); \
> + char *alloc_buff = NULL; \
> +\
> + if (caa_unlikely(fmt[0] == '%' && fmt[1] == 's' && fmt[2] == '\0')) { \
> + va_start(ap, fmt); \
> + msg = va_arg(ap, char *); \
> + va_end(ap); \
> + len = strlen(msg); \
> + } else \
> + for(;;) { \
> + va_start(ap, fmt); \
> + len = vsnprintf(msg, buflen, fmt, ap); \
> + va_end(ap); \
> +\
> + if (caa_unlikely(len >= (int)sizeof(local_buf) && !alloc_buff)) { \
> + buflen = (size_t)len + 1U; \
> + len = -1; \
> + alloc_buff = (char *)malloc(buflen); \
> + msg = alloc_buff; \
> + if (!alloc_buff) \
> + break; \
> + } else \
> + break; \
> + } \
It's odd to have an infinite for (;;) loop for something that really has at
most two iterations, and never ever more: the first is an attempt with the
local buffer, and the second is using malloc'd memory. I would prefer that
we open-code the two iterations, even if it is a small repetition of code,
to make it crystal clear that we are not adding a never-ending loop in the
instrumentation code, even by mistake. I'd rather have, in the else leg:
/* On-stack buffer attempt */
va_start(ap, fmt);
ret = vsnprintf(msg, buflen, fmt, ap);
va_end(ap);
if (caa_unlikely(ret < 0))
goto end;
len = ret;
if (caa_unlikely(len >= sizeof(local_buf)) {
/* On-heap buffer */
buflen = len + 1;
alloc_buff = (char *)malloc(buflen);
if (!alloc_buff) {
goto end;
msg = alloc_buff;
va_start(ap, fmt);
ret = vsnprintf(msg, buflen, fmt, ap);
va_end(ap);
lttng_ust_runtime_bug_on(ret < 0 || (size_t)ret != buflen - 1);
len = ret;
}
> +\
> + /* len does not include the final \0 */ \
This conditional goes away because len is a size_t. Breaking from the
while (0) is what skips over tracing and freeing the buffer.
> + if (caa_likely(len >= 0)) { \
> + callback(__VA_ARGS__); \
> + } \
> + /* Dont call a potentially instrumented forbidden free needlessly. */ \
Dont -> Don't
end: (add end label to skip tracing but keep freeing the buffer. I prefer this
to a break of the while (0) to make sure we don't introduce memory leaks by
error in the future if the implementation is modified)
> + if (caa_unlikely(alloc_buff)) \
> + free(alloc_buff);
} while (0)
Same changes proposed above for the macro below.
Thanks,
Mathieu
> +
> +#define LTTNG_UST_TRACELOG_VALIST(fmt, ap, callback, ...) \
> + char local_buf[LTTNG_TRACE_PRINTF_BUFSIZE]; \
> + int len; \
> + char *msg = local_buf; \
> + size_t buflen = sizeof(local_buf); \
> + char *alloc_buff = NULL; \
> +\
> + if (caa_unlikely(fmt[0] == '%' && fmt[1] == 's' && fmt[2] == '\0')) { \
> + msg = va_arg(ap, char *); \
> + len = strlen(msg); \
> + } else \
> + for(;;) { \
> + va_list ap2; \
> +\
> + va_copy(ap2, ap); \
> + len = vsnprintf(msg, buflen, fmt, ap2); \
> + va_end(ap2); \
> +\
> + if (caa_unlikely(len >= (int)sizeof(local_buf) && !alloc_buff)) { \
> + buflen = (size_t)len + 1U; \
> + len = -1; \
> + alloc_buff = (char *)malloc(buflen); \
> + msg = alloc_buff; \
> + if (!alloc_buff) \
> + break; \
> + } else \
> + break; \
> + } \
> +\
> + /* len does not include the final \0 */ \
> + if (caa_likely(len >= 0)) { \
> + callback(__VA_ARGS__); \
> + } \
> + /* Dont call a potentially instrumented forbidden free needlessly. */ \
> + if (caa_unlikely(alloc_buff)) \
> + free(alloc_buff);
> diff --git a/src/lib/lttng-ust/tracelog.c b/src/lib/lttng-ust/tracelog.c
> index 8147d7a3..bd38032c 100644
> --- a/src/lib/lttng-ust/tracelog.c
> +++ b/src/lib/lttng-ust/tracelog.c
> @@ -7,6 +7,7 @@
> #define _LGPL_SOURCE
> #include <stdio.h>
> #include "common/macros.h"
> +#include "common/tracer.h"
>
> /* The tracepoint definition is public, but the provider definition is hidden.
> */
> #define LTTNG_UST_TRACEPOINT_PROVIDER_HIDDEN_DEFINITION
> @@ -15,32 +16,9 @@
> #define LTTNG_UST_TRACEPOINT_DEFINE
> #include "lttng-ust-tracelog-provider.h"
>
> +#include "tracelog-internal.h"
> +
> #define LTTNG_UST_TRACELOG_CB(level) \
> - static inline \
> - void lttng_ust___vtracelog_##level(const char *file, \
> - int line, const char *func, \
> - const char *fmt, va_list ap) \
> - __attribute__((always_inline, format(printf, 4, 0))); \
> - \
> - static inline \
> - void lttng_ust___vtracelog_##level(const char *file, \
> - int line, const char *func, \
> - const char *fmt, va_list ap) \
> - { \
> - char *msg; \
> - const int len = vasprintf(&msg, fmt, ap); \
> - \
> - /* len does not include the final \0 */ \
> - if (len < 0) \
> - goto end; \
> - lttng_ust_tracepoint_cb_lttng_ust_tracelog___##level(file, \
> - line, func, msg, len, \
> - LTTNG_UST_CALLER_IP()); \
> - free(msg); \
> - end: \
> - return; \
> - } \
> - \
> void lttng_ust__vtracelog_##level(const char *file, \
> int line, const char *func, \
> const char *fmt, va_list ap) \
> @@ -53,7 +31,9 @@
> int line, const char *func, \
> const char *fmt, va_list ap) \
> { \
> - lttng_ust___vtracelog_##level(file, line, func, fmt, ap); \
> + LTTNG_UST_TRACELOG_VALIST(fmt, ap, \
> + lttng_ust_tracepoint_cb_lttng_ust_tracelog___##level, \
> + file, line, func, msg, len, LTTNG_UST_CALLER_IP()); \
> } \
> \
> void lttng_ust__tracelog_##level(const char *file, \
> @@ -68,11 +48,9 @@
> int line, const char *func, \
> const char *fmt, ...) \
> { \
> - va_list ap; \
> - \
> - va_start(ap, fmt); \
> - lttng_ust___vtracelog_##level(file, line, func, fmt, ap); \
> - va_end(ap); \
> + LTTNG_UST_TRACELOG_VARARG(fmt, \
> + lttng_ust_tracepoint_cb_lttng_ust_tracelog___##level, \
> + file, line, func, msg, len, LTTNG_UST_CALLER_IP()); \
> }
>
> LTTNG_UST_TRACELOG_CB(LTTNG_UST_TRACEPOINT_LOGLEVEL_EMERG)
> --
> 2.35.1
>
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> lttng-dev mailing list
> lttng-dev at lists.lttng.org
> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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