On Wed 22-01-20 19:15:47, David Hildenbrand wrote: > On 22.01.20 17:46, Michal Hocko wrote: > > On Wed 22-01-20 12:58:16, David Hildenbrand wrote: [...] > >> Especially interesting for IBM z Systems, whereby memory > >> onlining/offlining will trigger the actual population of memory in the > >> hypervisor. So if an admin wants to offline some memory (to give it back > >> to the hypervisor), it would use lsmem to identify such blocks first, > >> instead of trying random blocks until one offlining request succeeds. > > > > I am sorry for being dense here but I still do not understand why s390 > > It's good that we talk about it :) It's hard to reconstruct actual use > cases from tools and some documentation only ... > > Side note (just FYI): One difference on s390x compared to other > architectures (AFAIKS) is that once memory is offline, you might not be > allowed (by the hypervisor) to online it again - because it was > effectively unplugged. Such memory is not removed via remove_memory(), > it's simply kept offline. I have a very vague understanding of s390 specialities but this is not really relevant to the discussion AFAICS because this happens _after_ offlining. > > and the way how it does the hotremove matters here. Afterall there are > > no arch specific operations done until the memory is offlined. Also > > randomly checking memory blocks and then hoping that the offline will > > succeed is not way much different from just trying the offline the > > block. Both have to crawl through the pfn range and bail out on the > > unmovable memory. > > I think in general we have to approaches to memory unplugging. > > 1. Know explicitly what you want to unplug (e.g., a DIMM spanning > multiple memory blocks). > > 2. Find random memory blocks you can offline/unplug. > > > For 1, I think we both agree that we don't need this. Just try to > offline and you know if it worked. > > Now of course, for 2 you can try random blocks until you succeeded. From > a sysadmin point of view that's very inefficient. From a powerpc-utils > point of view, that's inefficient. How exactly is check + offline more optimal then offline which makes check as its first step? I will get to your later points after this is clarified. -- Michal Hocko SUSE Labs
