Re: Hang ast / pipelk / piperd

From: Paul Floyd <>
Date: Mon, 30 May 2022 19:35:05 UTC

On 5/30/22 14:15, Mark Johnston wrote:

> "procstat -kk <valgrind PID>" might help to reveal what's going on,
> since it sounds like the hand/livelock is happening somewhere in the
> kernel.

Not knowing much about the kernel, my guess is that this is related to

commit 4808bab7fa6c3ec49b49476b8326d7a0250a03fa
Author: Alexander Motin <>
Date:   Tue Sep 21 18:14:22 2021 -0400

     sched_ule(4): Improve long-term load balancer.

and this bit of ast code

	 * Check for ASTs atomically with returning.  Disabling CPU
	 * interrupts provides sufficient locking even in the SMP case,
	 * since we will be informed of any new ASTs by an IPI.
	movq	PCPU(CURTHREAD),%rax
	je	doreti_exit
	movq	%rsp,%rdi	/* pass a pointer to the trapframe */
	call	ast
	jmp	doreti_ast

The above commit seems to be migrating loaded threads to another CPU.

My test system is a VirtualBox amd64 FreeBSD 13.1 with one CPU running 
on a 13.0 host.

I just tried restarting the VM with 2 CPUs and the testcase seems to be 
a lot better - it's been running in a loop for 10 minutes whereas 
previously it would hang at least 1 in 5 times.