On Fri, Nov 23, 2018 at 09:46:52AM +0100, David Hildenbrand wrote: >On 23.11.18 09:42, Michal Hocko wrote: >> On Thu 22-11-18 16:26:40, David Hildenbrand wrote: >>> On 22.11.18 11:12, Wei Yang wrote: >>>> During online_pages phase, pgdat->nr_zones will be updated in case this >>>> zone is empty. >>>> >>>> Currently the online_pages phase is protected by the global lock >>>> mem_hotplug_begin(), which ensures there is no contention during the >>>> update of nr_zones. But this global lock introduces scalability issues. >>>> >>>> This patch is a preparation for removing the global lock during >>>> online_pages phase. Also this patch changes the documentation of >>>> node_size_lock to include the protectioin of nr_zones. >>> >>> I looked into locking recently, and there is more to it. >>> >>> Please read: >>> >>> commit dee6da22efac451d361f5224a60be2796d847b51 >>> Author: David Hildenbrand <david@xxxxxxxxxx> >>> Date: Tue Oct 30 15:10:44 2018 -0700 >>> >>> memory-hotplug.rst: add some details about locking internals >>> >>> Let's document the magic a bit, especially why device_hotplug_lock is >>> required when adding/removing memory and how it all play together with >>> requests to online/offline memory from user space. >>> >>> Short summary: Onlining/offlining of memory requires the device_hotplug_lock >>> as of now. >> >> Well, I would tend to disagree here. You might be describing the current >> state of art but the device_hotplug_lock doesn't make much sense for the >> memory hotplug in principle. There is absolutely nothing in the core MM > >There are collisions with CPU hotplug that require this lock (when nodes >come and go as far as I remember). And there is the problematic lock >inversion that can happen when adding/remving memory. This all has to be >sorted out, we'll have to see if we really need it for >onlining/offlining, though, however ... > Seems I get a little understanding on this part. There are two hotplug: * CPU hotplug * Memory hotplug. There are two phase for Memory hotplug: * physical add/remove * logical online/offline All of them are protected by device_hotplug_lock now, so we need to be careful to release this in any case. Is my understanding correct? >> that would require this lock. The current state just uses a BKL in some >> sense and we really want to get rid of that longterm. This patch is a tiny >> step in that direction and I suspect many more will need to come on the >> way. We really want to end up with a clear scope of each lock being >> taken. A project for a brave soul... > >... for now I don't consider "optimize for parallel >onlining/offlining/adding/removing of memory and cpus" really necessary. >What is necessary indeed is to not slowdown the whole system just >because some memory is coming/going. Therefore I agree, this patch is a >step into the right direction. > Agree. The target is to accelerate the hot-plug without slow down the normal process. >-- > >Thanks, > >David / dhildenb -- Wei Yang Help you, Help me
