On 22/03/18 17:50, Waiman Long wrote: > On 03/22/2018 04:41 AM, Juri Lelli wrote: > > On 21/03/18 12:21, Waiman Long wrote: [...] > >> + cpuset.sched_load_balance > >> + A read-write single value file which exists on non-root cgroups. > >> + The default is "1" (on), and the other possible value is "0" > >> + (off). > >> + > >> + 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. > >> + > >> + This flag is hierarchical and is inherited by child cpusets. It > >> + can be turned off only when the CPUs in this cpuset aren't > >> + listed in the cpuset.cpus of other sibling cgroups, and all > >> + the child cpusets, if present, have this flag turned off. > >> + > >> + Once it is off, it cannot be turned back on as long as the > >> + parent cgroup still has this flag in the off state. > >> + > > I'm afraid that this will not work for SCHED_DEADLINE (at least for how > > it is implemented today). As you can see in Documentation [1] the only > > way a user has to perform partitioned/clustered scheduling is to create > > subset of exclusive cpusets and then assign deadline tasks to them. The > > other thing to take into account here is that a root_domain is created > > for each exclusive set and we use such root_domain to keep information > > about admitted bandwidth and speed up load balancing decisions (there is > > a max heap tracking deadlines of active tasks on each root_domain). > > Now, AFAIR distinct root_domain(s) are created when parent group has > > sched_load_balance disabled and cpus_exclusive set (in cgroup v1 that > > is). So, what we normally do is create, say, cpus_exclusive groups for > > the different clusters and then disable sched_load_balance at root level > > (so that each cluster gets its own root_domain). Also, > > sched_load_balance is enabled in children groups (as load balancing > > inside clusters is what we actually needed :). > > That looks like an undocumented side effect to me. I would rather see an > explicit control file that enable root_domain and break it free from > cpu_exclusive && !sched_load_balance, e.g. sched_root_domain(?). Mmm, it actually makes some sort of sense to me that as long as parent groups can't load balance (because !sched_load_balance) and this group can't have CPUs overlapping with some other group (because cpu_exclusive) a data structure (root_domain) is created to handle load balancing for this isolated subsystem. I agree that it should be better documented, though. > > IIUC your proposal this will not be permitted with cgroup v2 because > > sched_load_balance won't be present at root level and children groups > > won't be able to set sched_load_balance back to 1 if that was set to 0 > > in some parent. Is that true? > > Yes, that is the current plan. OK, thanks for confirming. Can you tell again however why do you think we need to remove sched_load_balance from root level? Won't we end up having tasks put on isolated sets? Also, I guess children groups with more than one CPU will need to be able to load balance across their CPUs, no matter what their parent group does? Thanks, - Juri -- 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
