linux-kernel - Re: [RFC] cpuset update_cgroup_cpus

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Date:	Mon, 15 Oct 2007 19:34:39 -0700
From:	Paul Jackson <pj@....com>
To:	Paul Menage <menage@...gle.com>
Cc:	rientjes@...gle.com, nickpiggin@...oo.com.au,
	a.p.zijlstra@...llo.nl, balbir@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, clg@...ibm.com,
	ebiederm@...ssion.com, containers@...ts.osdl.org, serue@...ibm.com,
	svaidy@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	xemul@...nvz.org
Subject: Re: [RFC] cpuset update_cgroup_cpus_allowed

> currently against an older kernel 

ah .. which older kernel?

I tried it against the broken out 2.6.23-rc8-mm2 patch set,
inserting it before the task-containersv11-* patches, but
that blew up on me - three rejected hunks.

Any chance of getting this against a current cgroup (aka
container) kernel?

Could you use the diff --show-c-function option when composing
patches - they're easier to read that way - thanks.

+	if (!retval) {
+		cpus_allowed = cpuset_cpus_allowed(p);
+		if (!cpus_subset(new_mask, cpus_allowed)) {
+			/*
+			 * We must have raced with a concurrent cpuset
+			 * update. Just reset the cpus_allowed to the
+			 * cpuset's cpus_allowed
+			 */
+			new_mask = cpus_allowed;

This narrows the race, perhaps sufficiently, but I don't see that it
guarantees closure.  Memory accesses to two different locations are not
guaranteed to be ordered across nodes, as best I recall.  The second
line above, that rereads the cpuset cpus_allowed, could get an old
value, in essence.

	cpuset update task		sched_setaffinity task
	------------------		----------------------

	A. write cpuset	[Q]		V. read cpuset [Q]
	B. read task [P]		W. check ok
	C. write task [P]		X. write task [P]
					Y. reread cpuset [Q]
					Z. check ok again

Two memory locations:
	[P] the cpus_allowed mask in the task_struct of the
		task doing the sched_setaffinity call.
	[Q] the cpus_allowed mask in the cpuset of the cpuset
		to which the sched_setaffinity task is attached.

Even though, from the perspective of location [P], both B. and C.
happened before X., still from the perspective of location [Q] the
rereading in Y. could return the value the cpuset cpus_allowed had
before the write in A.  This could result in a task running with
a cpus_allowed that was totally outside its cpusets cpus_allowed.

I will grant that this is a narrow window.  I won't loose much sleep
over it.

>  - uses a priority heap to pick the processes to act on, based on start time

This adds a fair bit of code and complexity, relative to my patch.
This I do loose more sleep over.  There has to be a compelling
reason for doing this.

The point that David raises, regarding the interaction of this with
hotplug, seems to be a compelling reason for doing -something-
different than my patch proposal.

I don't know yet if it compels us to this much code, however.

Any chance you could provide a patch that works against cgroups?

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@....com> 1.925.600.0401
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