Re: [PATCH v7 3/5] cpuset: Add a root-only cpus.isolated v2 control file
From: Waiman Long <longman@redhat.com>
Date: 2018-05-08 00:30:08
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On 05/02/2018 10:08 AM, Peter Zijlstra wrote:
On Thu, Apr 19, 2018 at 09:47:02AM -0400, Waiman Long wrote:quoted
diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt index c970bd7..8d89dc2 100644 --- a/Documentation/cgroup-v2.txt +++ b/Documentation/cgroup-v2.txt@@ -1484,6 +1484,31 @@ Cpuset Interface Files a subset of "cpuset.cpus". Its value will be affected by CPU hotplug events. + cpuset.cpus.isolated + A read-write multiple values file which exists on root cgroup + only. + + It lists the CPUs that have been withdrawn from the root cgroup + for load balancing. These CPUs can still be allocated to child + cpusets with load balancing enabled, if necessary. + + If a child cpuset contains only an exclusive set of CPUs that are + a subset of the isolated CPUs and with load balancing enabled, + these CPUs will be load balanced on a separate root domain from + the one in the root cgroup. + + Just putting the CPUs into "cpuset.cpus.isolated" will be + enough to disable load balancing on those CPUs as long as they + do not appear in a child cpuset with load balancing enabled. + Fine-grained control of cpu isolation can also be done by + putting these isolated CPUs into child cpusets with load + balancing disabled. + + The "cpuset.cpus.isolated" should be set up before child + cpusets are created. Once child cpusets are present, changes + to "cpuset.cpus.isolated" will not be allowed if the CPUs that + change their states are in any of the child cpusets. +So I see why you did this, but it is _really_ ugly and breaks the container invariant. Ideally we'd make the root group less special, not more special.
Yes, I am planning to make the root cgroup less special by putting a new isolation flag into all the non-root cgroup. The container invariant thing, however, is a bit hard to do. Do we really need a container root to behave exactly like the real root? I guess we can make that happen if we really want to, but it will certainly make the code more complex. So it is a trade-off about what is worth to do and what is not. Cheers, Longman