log: setlogmask(0) can actually result in a system call, don't use it

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>

1 files changed, 2 insertions, 5 deletions

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));