Hi, I recently installed knot dns for a very small FreeBSD server. I noticed that it uses a surprising amount of CPU, even when there is no load: about 0.25%. That's not huge, but it seems unnecessarily high when my QPS is less than 0.01.
After some profiling, I came to the conclusion that this is caused by call_rcu_wait using futex_async to repeatedly wait. Since there is no futex on FreeBSD (without the Linux compatibility layer), this effectively turns into a permanent busy waiting loop. I think futex_noasync can be used here instead. call_rcu_wait is only supposed to be called from call_rcu_thread, never from a signal context. call_rcu calls get_call_rcu_data, which may call get_default_call_rcu_data, which calls pthread_mutex_lock through call_rcu_lock. Therefore, call_rcu is not async-signal-safe already. Also, I think it only makes sense to use call_rcu around a RCU write, which contradicts the README saying that only RCU reads are allowed in signal handlers. I applied "sed -i -e 's/futex_async/futex_noasync/' src/urcu-call-rcu-impl.h" and knot seems to work correctly with only 0.01% CPU now. I also ran tests/unit and tests/regression with default and signal backends and all completed successfully. I think that the other two usages of futex_async are also a little suspicious, but I didn't look too closely. Thanks, Alex. _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
