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SubjectRe: [RFC] cpuset update_cgroup_cpus_allowed
On 10/15/07, Paul Jackson <pj@sgi.com> wrote:
> > currently against an older kernel
>
> ah .. which older kernel?

2.6.18, but I can do a version against 2.6.23-mm1.

> + 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.

But cpuset_cpus_allowed() synchronizes on callback_mutex. So I assert
this race isn't an issue.

>
> 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.

My plan was to hide this inside cgroup_iter_* so that users didn't
have to hold the cssgroup_lock across the entire iteration.

>
> 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?
>

Will do - I justed wanted to get this quickly out to show the idea
that I was working on.

Paul
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