On 22.01.20 17:46, Michal Hocko wrote: > On Wed 22-01-20 12:58:16, David Hildenbrand wrote: >> On 22.01.20 11:54, David Hildenbrand wrote: >>> On 22.01.20 11:42, Michal Hocko wrote: >>>> On Wed 22-01-20 11:39:08, David Hildenbrand wrote: >>>>>>>> Really, the interface is flawed and should have never been merged in the >>>>>>>> first place. We cannot simply remove it altogether I am afraid so let's >>>>>>>> at least remove the bogus code and pretend that the world is a better >>>>>>>> place where everything is removable except the reality sucks... >>>>>>> >>>>>>> As I expressed already, the interface works as designed/documented and >>>>>>> has been used like that for years. >>>>>> >>>>>> It seems we do differ in the usefulness though. Using a crappy interface >>>>>> for years doesn't make it less crappy. I do realize we cannot remove the >>>>>> interface but we can remove issues with the implementation and I dare to >>>>>> say that most existing users wouldn't really notice. >>>>> >>>>> Well, at least powerpc-utils (why this interface was introduced) will >>>>> notice a) performance wise and b) because more logging output will be >>>>> generated (obviously non-offlineable blocks will be tried to offline). >>>> >>>> I would really appreciate some specific example for a real usecase. I am >>>> not familiar with powerpc-utils worklflows myself. >>>> >>> >>> Not an expert myself: >>> >>> https://github.com/ibm-power-utilities/powerpc-utils >>> >>> -> src/drmgr/drslot_chrp_mem.c >>> >>> On request to remove some memory it will >>> >>> a) Read "->removable" of all memory blocks ("lmb") >>> b) Check if the request can be fulfilled using the removable blocks >>> c) Try to offline the memory blocks by trying to offline it. If that >>> succeeded, trigger removeal of it using some hypervisor hooks. >>> >>> Interestingly, with "AMS ballooning", it will already consider the >>> "removable" information useless (most probably, because of >>> non-migratable balloon pages that can be offlined - I assume the powerpc >>> code that I converted to proper balloon compaction just recently). a) >>> and b) is skipped. >>> >>> Returning "yes" on all blocks will make them handle it just like if "AMS >>> ballooning" is active. So any memory block will be tried. Should work >>> but will be slower if no ballooning is active. >>> >> >> On lsmem: >> >> https://www.ibm.com/support/knowledgecenter/linuxonibm/com.ibm.linux.z.lgdd/lgdd_r_lsmem_cmd.html >> >> " >> Removable >> yes if the memory range can be set offline, no if it cannot be set >> offline. A dash (-) means that the range is already offline. The kernel >> method that identifies removable memory ranges is heuristic and not >> exact. Occasionally, memory ranges are falsely reported as removable or >> falsely reported as not removable. >> " >> >> Usage of lsmem paird with chmem: >> >> https://access.redhat.com/solutions/3937181 >> >> >> 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. > 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. I learned just now, "chmem"[1] has a mode where you can specify a "size" and not only a range. So a sysadmin can still control onlining/offlining for this use case with a few commands. In other tools (e.g., powerpc-utils), well, you have to try to offline random memory blocks (just like chmem does). AFAIK, once we turn /sys/.../removable useless, I can see the following changes: 1. Trying to offline a certain amount of memory blocks gets slower/takes longer/is less efficient. Might be tolerable. The tools seem to keep working. 2. You can no longer make a rough estimate how much memory you could offline - before you actually try to offline it. I can only imagine that something like this makes sense in a virtual environment (e.g., IBM z) to balance memory between virtual machines, but I am not aware of a real user of something like that. So what I can do is a) Come up with a patch that rips that stuff out (well I have that already lying around) b) Describe the existing users + changes we will see c) CC relevant people I identify (lsmem/chmem/powerpc-utils/etc.) on the patch to see if we are missing other use cases/users/implications. Sounds like a plan? [1] https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/tree/sys-utils/chmem.c -- Thanks, David / dhildenb
