[ovs-dev] [PATCH v2 4/4] Prevent excessive contention of the logging mutex

anton.ivanov at cambridgegreys.com anton.ivanov at cambridgegreys.com
Fri Feb 14 17:54:29 UTC 2020


From: Anton Ivanov <anton.ivanov at cambridgegreys.com>

The log levels for a module which specify "should I log or should
I go" can be checked witout a mutex.

We need to grab a mutex only once we know we are likely to be
logging in the first place.

This mutex is highly contended - this code is hit from 2300+
places around OVS - every time you encounter a VLOG statement.

Signed-off-by: Anton Ivanov <anton.ivanov at cambridgegreys.com>
---
 lib/vlog.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/lib/vlog.c b/lib/vlog.c
index 559943d87..be28b59a1 100644
--- a/lib/vlog.c
+++ b/lib/vlog.c
@@ -1088,10 +1088,19 @@ vlog_valist(const struct vlog_module *module, enum vlog_level level,
 {
     bool log_to_console = module->levels[VLF_CONSOLE] >= level;
     bool log_to_syslog = module->levels[VLF_SYSLOG] >= level;
-    bool log_to_file;
+    bool log_to_file = module->levels[VLF_FILE] >= level;
+
+    /* Quickly exit without contending the log mutex if the module log 
+     * levels do not request any logging to start off with.
+     */
+
+    if (!(log_to_console || log_to_syslog || log_to_file)) {
+        return;
+    }
 
     ovs_mutex_lock(&log_file_mutex);
-    log_to_file = module->levels[VLF_FILE] >= level && log_fd >= 0;
+    /* check only log_fd under mutex */
+    log_to_file &= (log_fd >= 0);
     ovs_mutex_unlock(&log_file_mutex);
     if (log_to_console || log_to_syslog || log_to_file) {
         int save_errno = errno;
-- 
2.20.1



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