linux-kernel - [bug report] sched: stop_machine() usage causes load balancer to misbehave

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <48E55FE5.40108@nortel.com>
Date:	Thu, 02 Oct 2008 17:57:25 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Ingo Molnar <mingo@...e.hu>,
	Linux kernel <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: [bug report] sched: stop_machine() usage causes load balancer to
 misbehave

I mentioned before that ftrace (specifically the ftraced daemon) seems 
to be interfering with the load balancer.  After some experimenting, it 
appears that any regular calls to stop_machine() will end up confusing 
the load balancer.

As an experiment, I disabled ftraced (which would normally result in 
correct load balancing) but added a single kernel thread which simply 
runs the following loop, where "chrisd2" is a dummy function.

while(1) {
	set_current_state(TASK_INTERRUPTIBLE);
	schedule_timeout(HZ);
	stop_machine(chrisd2, NULL, NULL);
}

With the modified kernel, my testcase shows that the load balancer 
doesn't balance--all tasks remain on one cpu while the other one stays idle.

Most of the users of stop_machine() (kprobes on s390, cpu hotplug, 
module load/unload, numa_zonelist_order, etc.) don't seem to be called 
on a regular basis.  Only ftrace behaves this way, which is why it 
appeared to be the source of the problem.

I haven't tracked down the specific reasons for the misbehaviour, but it 
seems undesirable.

Anyone have any ideas what might be causing this?  Is it a problem with 
the load balancer, or an unavoidable consequence of what stop_machine() 
is doing?

Thanks,

Chris
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/