Re: [PATCH v3] log: setlogmask(0) can actually result in a system call, don't use it

[David Gibson <david@gibson.dropbear.id.au>]

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On Sun, Feb 11, 2024 at 01:38:39PM +0100, Stefano Brivio wrote:
> Before commit 32d07f5e59f2 ("passt, pasta: Completely avoid dynamic
> memory allocation"), we didn't store the current log mask in a
> variable, and we fetched it using setlogmask(0) wherever needed.
> 
> But after that commit, we can use our log_mask copy instead. And we
> should: with recent glibc versions, setlogmask(0) actually results in
> a system call, which causes a substantial overhead with high transfer
> rates: we use setlogmask(0) even to decide we don't want to print
> debug messages.
> 
> Now that we rely on log_mask in early stages, before setlogmask() is
> called, we need to initialise that variable to the special LOG_EMERG
> mask value right away: define LOG_EARLY to make this clearer, and,
> while at it, group conditions in vlogmsg() into something more terse.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
> v3: Include changes to passt.c I accidentally didn't commit in v2
> 
> v2: Initialise log_mask so that prints before the first
>     __setlogmask() call still work.
> 
>  log.c   | 18 ++++++++++--------
>  passt.c |  7 ++-----
>  2 files changed, 12 insertions(+), 13 deletions(-)
> 
> diff --git a/log.c b/log.c
> index 4a70e29..eafaca2 100644
> --- a/log.c
> +++ b/log.c
> @@ -30,9 +30,12 @@
>  #include "util.h"
>  #include "passt.h"
>  
> +/* LOG_EARLY means we don't know yet: log everything. LOG_EMERG is unused */
> +#define LOG_EARLY		LOG_MASK(LOG_EMERG)
> +
>  static int	log_sock = -1;		/* Optional socket to system logger */
>  static char	log_ident[BUFSIZ];	/* Identifier string for openlog() */
> -static int	log_mask;		/* Current log priority mask */
> +static int	log_mask = LOG_EARLY;	/* Current log priority mask */
>  static int	log_opt;		/* Options for openlog() */
>  
>  static int	log_file = -1;		/* Optional log file descriptor */
> @@ -45,34 +48,33 @@ static time_t	log_start;		/* Start timestamp */
>  int		log_trace;		/* --trace mode enabled */
>  int		log_to_stdout;		/* Print to stdout instead of stderr */
>  
> -#define BEFORE_DAEMON		(setlogmask(0) == LOG_MASK(LOG_EMERG))
> -
>  void vlogmsg(int pri, const char *format, va_list ap)
>  {
> +	bool debug_print = (log_mask & LOG_MASK(LOG_DEBUG)) && log_file == -1;
> +	bool early_print = LOG_PRI(log_mask) == LOG_EARLY;
>  	FILE *out = log_to_stdout ? stdout : stderr;
>  	struct timespec tp;
>  
> -	if (setlogmask(0) & LOG_MASK(LOG_DEBUG) && log_file == -1) {
> +	if (debug_print) {
>  		clock_gettime(CLOCK_REALTIME, &tp);
>  		fprintf(out, "%lli.%04lli: ",
>  			(long long int)tp.tv_sec - log_start,
>  			(long long int)tp.tv_nsec / (100L * 1000));
>  	}
>  
> -	if ((LOG_MASK(LOG_PRI(pri)) & log_mask) || BEFORE_DAEMON) {
> +	if ((log_mask & LOG_MASK(LOG_PRI(pri))) || early_print) {
>  		va_list ap2;
>  
>  		va_copy(ap2, ap); /* Don't clobber ap, we need it again */
>  		if (log_file != -1)
>  			logfile_write(pri, format, ap2);
> -		else if (!(setlogmask(0) & LOG_MASK(LOG_DEBUG)))
> +		else if (!(log_mask & LOG_MASK(LOG_DEBUG)))
>  			passt_vsyslog(pri, format, ap2);
>  
>  		va_end(ap2);
>  	}
>  
> -	if ((setlogmask(0) & LOG_MASK(LOG_DEBUG) && log_file == -1) ||
> -	    (BEFORE_DAEMON && !(log_opt & LOG_PERROR))) {
> +	if (debug_print || (early_print && !(log_opt & LOG_PERROR))) {
>  		(void)vfprintf(out, format, ap);
>  		if (format[strlen(format)] != '\n')
>  			fprintf(out, "\n");
> diff --git a/passt.c b/passt.c
> index 44d3a0b..aaa8e58 100644
> --- a/passt.c
> +++ b/passt.c
> @@ -251,9 +251,6 @@ int main(int argc, char **argv)
>  
>  	__openlog(log_name, 0, LOG_DAEMON);
>  
> -	/* Meaning we don't know yet: log everything. LOG_EMERG is unused */
> -	__setlogmask(LOG_MASK(LOG_EMERG));
> -
>  	c.epollfd = epoll_create1(EPOLL_CLOEXEC);
>  	if (c.epollfd == -1) {
>  		perror("epoll_create1");
> @@ -322,8 +319,8 @@ int main(int argc, char **argv)
>  	if (isolate_prefork(&c))
>  		die("Failed to sandbox process, exiting");
>  
> -	/* Once the log mask is not LOG_EMERG, we will no longer
> -	 * log to stderr if there was a log file specified.
> +	/* Once the log mask is not LOG_EARLY, we will no longer log to stderr
> +	 * if there was a log file specified.
>  	 */
>  	if (c.debug)
>  		__setlogmask(LOG_UPTO(LOG_DEBUG));

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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