On 11/16/21 12:54, Michal Koutný wrote:
On Mon, Nov 15, 2021 at 04:10:29PM -0500, Waiman Long longman@redhat.com wrote:
On Mon, Oct 18, 2021 at 10:36:18AM -0400, Waiman Long longman@redhat.com wrote:
- scheduler. Tasks in such a partition must be explicitly bound
- to each individual CPU.
[...]
It can be a problem when one is trying to move from one cgroup to another cgroup with non-overlapping cpus laterally. However, if a task is initially from a parent cgroup with affinity mask that include cpus in the isolated child cgroup, I believe it should be able to move to the isolated child cgroup without problem. Otherwise, it is a bug that needs to be fixed.
app_root cpuset.cpus=0-3 `- non_rt cpuset.cpus=0-1 cpuset.cpus.partition=member `- rt cpuset.cpus=2-3 cpuset.cpus.partition=isolated
The app_root would have cpuset.cpus.effective=0-1 so even the task in app_root can't sched_setaffinity() to cpus 2-3. But AFAICS, the migration calls set_cpus_allowed_ptr() anyway, so the task in the isolated partition needn't to bind explicitly with sched_setaffinity(). (It'd have two cpus available, so one more sched_setaffinity() or migration into a single-cpu list is desirable.)
All in all, I think the behavior is OK and the explicit binding of tasks in an isolated cpuset is optional (not a must as worded currently).
I think the wording may be confusing. What I meant is none of the requested cpu can be granted. So if there is at least one granted, the effective cpus won't be empty.
Ack.
You currently cannot make change to cpuset.cpus that violates the cpu exclusivity rule. The above constraints will not disallow you to make the change. They just affect the validity of the partition root.
Sibling exclusivity should be a validity condition regardless of whether transition is allowed or not. (At least it looks simpler to me.)
Changing a partition root to "member" is always allowed.If there are child partition roots underneath it, however,they will be forced to be switched back to "member" too andlose their partitions. So care must be taken to double checkfor this condition before disabling a partition root.(Or is this how delegation is intended?) However, AFAICS, parent still can't remove cpuset.cpus even when the child is a "member". Otherwise, I agree with the back-switch.
There are only 2 possibilities here. Either we force the child partitions to be become members or invalid partition root.
My point here was mostly about preempting the cpus (as a v2 specific feature). (I'm rather indifferent whether children turn into invalid roots or members.)
Below is my latest iterations of the cpuset.cpus.partition documentation. If there is no objection or other suggestion for improvement, I am going to send out another iteration of the patch series with the updated documentation.
Cheers, Longman
--------------------------------------------------------------
cpuset.cpus.partition A read-write single value file which exists on non-root cpuset-enabled cgroups. This flag is owned by the parent cgroup and is not delegatable.
It accepts only the following input values when written to.
======== ================================ "member" Non-root member of a partition "root" Partition root "isolated" Partition root without load balancing ======== ================================
The root cgroup is always a partition root and its state cannot be changed. All other non-root cgroups start out as "member".
When set to "root", the current cgroup is the root of a new partition or scheduling domain that comprises itself and all its descendants except those that are separate partition roots themselves and their descendants.
The value shown in "cpuset.cpus.effective" of a partition root is the CPUs that the parent partition root can dedicate to the new partition root. They are subtracted from "cpuset.cpus.effective" of the parent and may be different from "cpuset.cpus"
When set to "isolated", the CPUs in that partition root will be in an isolated state without any load balancing from the scheduler. Tasks placed in such a partition with multiple CPUs should be carefully distributed and bound to each of the individual CPUs for optimal performance.
A partition root ("root" or "isolated") can be in one of the two possible states - valid or invalid. An invalid partition root is in a degraded state where some state information are retained, but behaves more like a "member".
On read, the "cpuset.cpus.partition" file can show the following values.
====================== ============================== "member" Non-root member of a partition "root" Partition root "isolated" Partition root without load balancing "root invalid (<reason>)" Invalid partition root ====================== ==============================
In the case of an invalid partition root, a descriptive string on why the partition is invalid is included within parentheses.
Almost all possible state transitions among "member", valid and invalid partition roots are allowed except from "member" to invalid partition root.
Before the "member" to partition root transition can happen, the following conditions must be met or the transition will not be allowed.
1) The "cpuset.cpus" is non-empty and exclusive, i.e. they are not shared by any of its siblings. 2) The parent cgroup is a valid partition root. 3) The "cpuset.cpus" is a subset of parent's "cpuset.cpus". 4) There is no child cgroups with cpuset enabled. This avoids cpu migrations of multiple cgroups simultaneously which can be problematic.
Once becoming a partition root, the following two rules restrict what changes can be made to "cpuset.cpus".
1) The value must be exclusive. 2) If child cpusets exist, the value must be a superset of what are defined in the child cpusets.
The second rule applies even for "member". Other changes to "cpuset.cpus" that do not violate the above rules are always allowed.
External events like hotplug or inappropriate changes to "cpuset.cpus" can cause a valid partition root to become invalid. Besides the constraints on changing "cpuset.cpus" listed above, the other conditions required to maintain the validity of a partition root are as follows:
1) The parent cgroup is a valid partition root. 2) If "cpuset.cpus.effective" is empty, the partition must have no task associated with it. Otherwise, the partition becomes invalid and "cpuset.cpus.effective" will fall back to that of the nearest non-empty ancestor.
A corollary of a valid partition root is that "cpuset.cpu.effective" is always a subset of "cpuset.cpus". Note that a task cannot be moved to a cgroup with empty "cpuset.cpus.effective".
Changing a partition root (valid or invalid) to "member" is always allowed. If there are child partition roots underneath it, however, they will be forced to be switched back to "member" too and lose their partitions. So care must be taken to double check for this condition before disabling a partition root.
A valid parent partition may distribute out all its CPUs to its child partitions as long as it is not the root cgroup and there is no task associated with it.
An invalid partition root can be reverted back to a valid one if none of the validity constraints of a valid partition root are violated.
Poll and inotify events are triggered whenever the state of "cpuset.cpus.partition" changes. That includes changes caused by write to "cpuset.cpus.partition", cpu hotplug and other changes that make the partition invalid. This will allow user space agents to monitor unexpected changes to "cpuset.cpus.partition" without the need to do continuous polling.