From: Willy Tarreau <w@1wt.eu>
Date: Wed, 10 Sep 2025 14:51:14 +0000 (+0200)
Subject: MEDIUM: resolvers: make the process_resolvers() task single-threaded
X-Git-Tag: v3.3-dev9~152
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=2ce5e0edcce03d648a443c1f84307e3bdc7a2710;p=thirdparty%2Fhaproxy.git

MEDIUM: resolvers: make the process_resolvers() task single-threaded

This task is sometimes caught triggering the watchdog while waiting for
the infamous resolvers lock, or the scheduler's wait queue lock in
task_queue(). Both are caused by its multi-threaded capability. The
task may indeed start on a thread that's different from the one that
is currently receiving a response and that holds the resolvers lock,
and when being queued back, it requires to lock the wait queue. Both
problems disappear when sticking it to a single thread. But for configs
running multiple resolvers sections, it would be suboptimal to run them
all on the same thread. In order to avoid this, we implement a counter
in the resolvers_finalize_config() section that rotates the thread for
each resolvers section.

This was sufficient to further improve the performance here, making the
CPU usage drop to about 7% (from 11 previously or 38 initially) and not
showing any resolvers lock contention anymore in perf top output.

The change was kept fairly minimal to permit a backport once enough
testing is conducted on it. It could address a significant part of
the trouble reported by Felipe in GH issue #3101.
---

diff --git a/src/resolvers.c b/src/resolvers.c
index 6daee3e14..8471c7344 100644
--- a/src/resolvers.c
+++ b/src/resolvers.c
@@ -2665,6 +2665,7 @@ static int resolvers_finalize_config(void)
 	const struct protocol *proto;
 	struct resolvers *resolvers;
 	struct proxy	     *px;
+	static int operating_thread = 0;
 	int err_code = 0;
 
 	enter_resolver_code();
@@ -2703,12 +2704,17 @@ static int resolvers_finalize_config(void)
 			}
 		}
 
-		/* Create the task associated to the resolvers section */
-		if ((t = task_new_anywhere()) == NULL) {
+		/* Create the task associated to the resolvers section.
+		 * We try to bind each resolvers section to a different thread
+		 * in order to avoid expensive multi-threading tasks and make
+		 * sure that the same thread deals with DNS I/O and scheduling.
+		 */
+		if ((t = task_new_on(operating_thread)) == NULL) {
 			ha_alert("resolvers '%s' : out of memory.\n", resolvers->id);
 			err_code |= (ERR_ALERT|ERR_ABORT);
 			goto err;
 		}
+		operating_thread = (operating_thread + 1) % global.nbthread;
 
 		/* Update task's parameters */
 		t->process   = process_resolvers;