Re: [Qemu-devel] [PATCH 1/3] log: improve performance of qemu_log and qemu_log_mask if disabled

["Alex Bennée" <alex.bennee@linaro.org>]

Denis V. Lunev <den@openvz.org> writes:

> The patch is intended to avoid to perform any operation including
> calculation of log function arguments when the log is not enabled due to
> various reasons.
>
> Functions qemu_log and qemu_log_mask are replaced with variadic macros.
> Unfortunately the code is not C99 compatible and we can not use
> portable __VA_ARGS__ way. There are a lot of warnings in the other
> places with --std=c99. Thus the only way to achive the result is to use
> args.. GCC extension.
>
> Format checking performed by compiler will not suffer by this patch. It
> will be done inside in fprintf arguments checking.
>
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
> CC: Markus Armbruster <armbru@redhat.com>
> CC: Luiz Capitulino <lcapitulino@redhat.com>
> CC: Eric Blake <eblake@redhat.com>
> CC: Peter Maydell <peter.maydell@linaro.org>
> ---
>  include/qemu/log.h | 17 ++++++++++++++---
>  qemu-log.c         | 21 ---------------------
>  2 files changed, 14 insertions(+), 24 deletions(-)
>
> diff --git a/include/qemu/log.h b/include/qemu/log.h
> index f880e66..57b8c96 100644
> --- a/include/qemu/log.h
> +++ b/include/qemu/log.h
> @@ -53,7 +53,13 @@ static inline bool qemu_loglevel_mask(int mask)
>  
>  /* main logging function
>   */
> -void GCC_FMT_ATTR(1, 2) qemu_log(const char *fmt, ...);
> +#define qemu_log(args...)                   \
> +    do {                                    \
> +        if (!qemu_log_enabled()) {          \
> +            break;                          \
> +        }                                   \
> +        fprintf(qemu_logfile, args);        \
> +    } while (0)
>

I've had one of Peter's patches in my logging improvements queue for a
while although it uses a slightly different form which I prefer:

-/* log only if a bit is set on the current loglevel mask
+/* log only if a bit is set on the current loglevel mask:
+ * @mask: bit to check in the mask
+ * @fmt: printf-style format string
+ * @args: optional arguments for format string
  */
-void GCC_FMT_ATTR(2, 3) qemu_log_mask(int mask, const char *fmt, ...);
-
+#define qemu_log_mask(MASK, FMT, ...)                   \
+    do {                                                \
+        if (unlikely(qemu_loglevel_mask(MASK))) {       \
+            qemu_log(FMT, ## __VA_ARGS__);              \
+        }                                               \

See the message:

qemu-log: Avoid function call for disabled qemu_log_mask logging

>  /* vfprintf-like logging function
>   */
> @@ -67,8 +73,13 @@ qemu_log_vprintf(const char *fmt, va_list va)
>  
>  /* log only if a bit is set on the current loglevel mask
>   */
> -void GCC_FMT_ATTR(2, 3) qemu_log_mask(int mask, const char *fmt, ...);
> -
> +#define qemu_log_mask(mask, args...)        \
> +    do {                                    \
> +        if (!qemu_loglevel_mask(mask)) {    \
> +            break;                          \
> +        }                                   \
> +        qemu_log(args);                     \
> +    } while (0)
>  
>  /* Special cases: */
>  
> diff --git a/qemu-log.c b/qemu-log.c
> index 13f3813..e6d2b3f 100644
> --- a/qemu-log.c
> +++ b/qemu-log.c
> @@ -25,27 +25,6 @@ FILE *qemu_logfile;
>  int qemu_loglevel;
>  static int log_append = 0;
>  
> -void qemu_log(const char *fmt, ...)
> -{
> -    va_list ap;
> -
> -    va_start(ap, fmt);
> -    if (qemu_logfile) {
> -        vfprintf(qemu_logfile, fmt, ap);
> -    }
> -    va_end(ap);
> -}
> -
> -void qemu_log_mask(int mask, const char *fmt, ...)
> -{
> -    va_list ap;
> -
> -    va_start(ap, fmt);
> -    if ((qemu_loglevel & mask) && qemu_logfile) {
> -        vfprintf(qemu_logfile, fmt, ap);
> -    }
> -    va_end(ap);
> -}
>  
>  /* enable or disable low levels log */
>  void do_qemu_set_log(int log_flags, bool use_own_buffers)

-- 
Alex Bennée