On Wed, May 29, 2019 at 2:20 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > [Sorry for a late reply] > > On Thu 23-05-19 11:58:45, Pingfan Liu wrote: > > On Wed, May 22, 2019 at 7:16 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > > > On Wed 22-05-19 15:12:16, Pingfan Liu wrote: > [...] > > > > But in fact, we already have for_each_node_state(nid, N_MEMORY) to > > > > cover this purpose. > > > > > > I do not really think we want to spread N_MEMORY outside of the core MM. > > > It is quite confusing IMHO. > > > . > > But it has already like this. Just git grep N_MEMORY. > > I might be wrong but I suspect a closer review would reveal that the use > will be inconsistent or dubious so following the existing users is not > the best approach. > > > > > Furthermore, changing the definition of online may > > > > break something in the scheduler, e.g. in task_numa_migrate(), where > > > > it calls for_each_online_node. > > > > > > Could you be more specific please? Why should numa balancing consider > > > nodes without any memory? > > > > > As my understanding, the destination cpu can be on a memory less node. > > BTW, there are several functions in the scheduler facing the same > > scenario, task_numa_migrate() is an example. > > Even if the destination node is memoryless then any migration would fail > because there is no memory. Anyway I still do not see how using online > node would break anything. > Suppose we have nodes A, B,C, where C is memory less but has little distance to B, comparing with the one from A to B. Then if a task is running on A, but prefer to run on B due to memory footprint. task_numa_migrate() allows us to migrate the task to node C. Changing for_each_online_node will break this. Regards, Pingfan > > > > By keeping the node owning cpu as online, Michal's patch can avoid > > > > such corner case and keep things easy. Furthermore, if needed, the > > > > other patch can use for_each_node_state(nid, N_MEMORY) to replace > > > > for_each_online_node is some space. > > > > > > Ideally no code outside of the core MM should care about what kind of > > > memory does the node really own. The external code should only care > > > whether the node is online and thus usable or offline and of no > > > interest. > > > > Yes, but maybe it will pay great effort on it. > > Even if that is the case it would be preferable because the current > situation is just not sustainable wrt maintenance cost. It is just too > simple to break the existing logic as this particular report outlines. > -- > Michal Hocko > SUSE Labs
