On 22.11.18 22:28, Wei Yang wrote: > On Thu, Nov 22, 2018 at 04:26:40PM +0100, 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. >> >> mem_hotplug_begin() is just an internal optimization. (we don't want >> everybody to take the device lock) >> > > Hi, David > > Thanks for your comment. My last sentence should have been "we don't want everybody to take the device hotplug lock" :) That caused confusion. > > Hmm... I didn't catch your point. > > Related to memory hot-plug, there are (at least) three locks, > > * device_hotplug_lock (global) > * device lock (device scope) > * mem_hotplug_lock (global) > > But with two different hold sequence in two cases: > > * device_online() > > device_hotplug_lock > device_lock > mem_hotplug_lock > > * add_memory_resource() > > device_hotplug_lock > mem_hotplug_lock > device_lock > ^ > | > I don't find where this is hold in add_memory_resource(). > Would you mind giving me a hint? > > If my understanding is correct, what is your point? > The point I was trying to make: Right now all onlining/offlining/adding/removing is protected by the device_hotplug_lock (and that's a good thing, things are fragile enough already). mem_hotplug_lock() is used in addition for get_online_mems(). "This patch is a preparation for removing the global lock during online_pages phase." - is more like "one global lock". > I guess your point is : just remove mem_hotplug_lock is not enough to > resolve the scalability issue? Depends on which scalability issue :) Getting rid of / removing the impact of mem_hotplug_lock is certainly a very good idea. And improves scalability of all callers of get_online_mems(). If that is the intention, very good :) If the intention is to make onlining/offlining more scalable (e.g. in parallel or such), then scalability is limited by device_hotplug_lock. > > Please correct me, if I am not. :-) > Guess I was just wondering which scalability issue we are trying to solve :) -- Thanks, David / dhildenb
