On 05/31/2018 08:26 AM, Peter Zijlstra wrote: > On Tue, May 29, 2018 at 09:41:30AM -0400, Waiman Long wrote: >> The sched.load_balance flag is needed to enable CPU isolation similar to >> what can be done with the "isolcpus" kernel boot parameter. Its value >> can only be changed in a scheduling domain with no child cpusets. On >> a non-scheduling domain cpuset, the value of sched.load_balance is >> inherited from its parent. This is to make sure that all the cpusets >> within the same scheduling domain or partition has the same load >> balancing state. >> >> This flag is set by the parent and is not delegatable. >> + cpuset.sched.domain_root >> + A read-write single value file which exists on non-root >> + cpuset-enabled cgroups. It is a binary value flag that accepts >> + either "0" (off) or "1" (on). This flag is set by the parent >> + and is not delegatable. >> + >> + If set, it indicates that the current cgroup is the root of a >> + new scheduling domain or partition that comprises itself and >> + all its descendants except those that are scheduling domain >> + roots themselves and their descendants. The root cgroup is >> + always a scheduling domain root. >> + >> + There are constraints on where this flag can be set. It can >> + only be set in a cgroup if all the following conditions are true. >> + >> + 1) The "cpuset.cpus" is not empty and the list of CPUs are >> + exclusive, i.e. they are not shared by any of its siblings. >> + 2) The parent cgroup is also a scheduling domain root. >> + 3) There is no child cgroups with cpuset enabled. This is >> + for eliminating corner cases that have to be handled if such >> + a condition is allowed. >> + >> + Setting this flag will take the CPUs away from the effective >> + CPUs of the parent cgroup. Once it is set, this flag cannot >> + be cleared if there are any child cgroups with cpuset enabled. >> + Further changes made to "cpuset.cpus" is allowed as long as >> + the first condition above is still true. >> + >> + A parent scheduling domain root cgroup cannot distribute all >> + its CPUs to its child scheduling domain root cgroups unless >> + its load balancing flag is turned off. >> + >> + cpuset.sched.load_balance >> + A read-write single value file which exists on non-root >> + cpuset-enabled cgroups. It is a binary value flag that accepts >> + either "0" (off) or "1" (on). This flag is set by the parent >> + and is not delegatable. It is on by default in the root cgroup. >> + >> + When it is on, tasks within this cpuset will be load-balanced >> + by the kernel scheduler. Tasks will be moved from CPUs with >> + high load to other CPUs within the same cpuset with less load >> + periodically. >> + >> + When it is off, there will be no load balancing among CPUs on >> + this cgroup. Tasks will stay in the CPUs they are running on >> + and will not be moved to other CPUs. >> + >> + The load balancing state of a cgroup can only be changed on a >> + scheduling domain root cgroup with no cpuset-enabled children. >> + All cgroups within a scheduling domain or partition must have >> + the same load balancing state. As descendant cgroups of a >> + scheduling domain root are created, they inherit the same load >> + balancing state of their root. > I still find all that a bit weird. > > So load_balance=0 basically changes a partition into a > 'fully-partitioned partition' with the seemingly random side-effect that > now sub-partitions are allowed to consume all CPUs. Are you suggesting that we should allow sub-partition to consume all the CPUs no matter the load balance state? I can live with that if you think it is more logical. > The rationale, only given in the Changelog above, seems to be to allow > 'easy' emulation of isolcpus. > > I'm still not convinced this is a useful knob to have. You can do > fully-partitioned by simply creating a lot of 1 cpu parititions. That is certainly true. However, I think there are some additional overhead in the scheduler side in maintaining those 1-cpu partitions. Right? > So this one knob does two separate things, both of which seem, to me, > redundant. > > Can we please get better rationale for this? I am fine getting rid of the load_balance flag if this is the consensus. However, we do need to come up with a good migration story for those users that need the isolcpus capability. I think Mike was the one asking for supporting isolcpus. So Mike, what is your take on that. Cheers, Longman -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
