On Mon, Apr 08, 2019 at 02:09:59PM -0400, Nitesh Narayan Lal wrote: > On 4/8/19 12:36 PM, David Hildenbrand wrote: > > On 06.04.19 02:09, Alexander Duyck wrote: > >> So I am starting this thread as a spot to collect my thoughts on the > >> current guest free page hinting design as well as point out a few > >> possible things we could do to improve upon it. > >> > >> 1. The current design isn't likely going to scale well to multiple > >> VCPUs. The issue specifically is that the zone lock must be held to > >> pull pages off of the free list and to place them back there once they > >> have been hinted upon. As a result it would likely make sense to try > >> to limit ourselves to only having one thread performing the actual > >> hinting so that we can avoid running into issues with lock contention > >> between threads. > > Makes sense. > > > >> 2. There are currently concerns about the hinting triggering false OOM > >> situations if too much memory is isolated while it is being hinted. My > >> thought on this is to simply avoid the issue by only hint on a limited > >> amount of memory at a time. Something like 64MB should be a workable > >> limit without introducing much in the way of regressions. However as a > >> result of this we can easily be overrun while waiting on the host to > >> process the hinting request. As such we will probably need a way to > >> walk the free list and free pages after they have been freed instead > >> of trying to do it as they are freed. > > We will need such a way in case we care about dropped hinting requests, yes. > > > >> 3. Even with the current buffering which is still on the larger side > >> it is possible to overrun the hinting limits if something causes the > >> host to stall and a large swath of memory is released. As such we are > >> still going to need some sort of scanning mechanism or will have to > >> live with not providing accurate hints. > > Yes, usually if there is a lot of guest activity, you could however > > assume that free pages might get reused either way soon. Of course, > > special cases are "freeing XGB and being idle afterwards". > > > >> 4. In my opinion, the code overall is likely more complex then it > >> needs to be. We currently have 2 allocations that have to occur every > >> time we provide a hint all the way to the host, ideally we should not > >> need to allocate more memory to provide hints. We should be able to > >> hold the memory use for a memory hint device constant and simply map > >> the page address and size to the descriptors of the virtio-ring. > > I don't think the two allocations are that complex. The only thing I > > consider complex is isolation a lot of pages from different zones etc. > > Two allocations, nobody really cares about that. Of course, the fact > > that we have to allocate memory from the VCPUs where we currently freed > > a page is not optimal. I consider that rather a problem/complex. > > > > Especially you have a point regarding scalability and multiple VCPUs. > > > >> With that said I have a few ideas that may help to address the 4 > >> issues called out above. The basic idea is simple. We use a high water > >> mark based on zone->free_area[order].nr_free to determine when to wake > >> up a thread to start hinting memory out of a given free area. From > >> there we allocate non-"Offline" pages from the free area and assign > >> them to the hinting queue up to 64MB at a time. Once the hinting is > >> completed we mark them "Offline" and add them to the tail of the > >> free_area. Doing this we should cycle the non-"Offline" pages slowly > >> out of the free_area. In addition the search cost should be minimal > >> since all of the "Offline" pages should be aggregated to the tail of > >> the free_area so all pages allocated off of the free_area will be the > >> non-"Offline" pages until we shift over to them all being "Offline". > >> This should be effective for MAX_ORDER - 1 and MAX_ORDER - 2 pages > >> since the only real consumer of add_to_free_area_tail is > >> __free_one_page which uses it to place a page with an order less than > >> MAX_ORDER - 2 on the tail of a free_area assuming that it should be > >> freeing the buddy of that page shortly. The only other issue with > >> adding to tail would be the memory shuffling which was recently added, > >> but I don't see that as being something that will be enabled in most > >> cases so we could probably just make the features mutually exclusive, > >> at least for now. > >> > >> So if I am not mistaken this would essentially require a couple > >> changes to the mm infrastructure in order for this to work. > >> > >> First we would need to split nr_free into two counters, something like > >> nr_freed and nr_bound. You could use nr_freed - nr_bound to get the > >> value currently used for nr_free. When we pulled the pages for hinting > >> we would reduce the nr_freed value and then add back to it when the > >> pages are returned. When pages are allocated they would increment the > >> nr_bound value. The idea behind this is that we can record nr_free > >> when we collect the pages and save it to some local value. This value > >> could then tell us how many new pages have been added that have not > >> been hinted upon. > > I can imagine that quite some people will have problems with such > > "virtualization specific changes" splattered around core memory > > management. Would there be a way to manage this data at a different > > place, out of core-mm and somehow work on it via callbacks? > > > >> In addition we will need some way to identify which pages have been > >> hinted on and which have not. The way I believe easiest to do this > >> would be to overload the PageType value so that we could essentially > >> have two values for "Buddy" pages. We would have our standard "Buddy" > >> pages, and "Buddy" pages that also have the "Offline" value set in the > >> PageType field. Tracking the Online vs Offline pages this way would > >> actually allow us to do this with almost no overhead as the mapcount > >> value is already being reset to clear the "Buddy" flag so adding a > >> "Offline" flag to this clearing should come at no additional cost. > > Just nothing here that this will require modifications to kdump > > (makedumpfile to be precise and the vmcore information exposed from the > > kernel), as kdump only checks for the the actual mapcount value to > > detect buddy and offline pages (to exclude them from dumps), they are > > not treated as flags. > > > > For now, any mapcount values are really only separate values, meaning > > not the separate bits are of interest, like flags would be. Reusing > > other flags would make our life a lot easier. E.g. PG_young or so. But > > clearing of these is then the problematic part. > > > > Of course we could use in the kernel two values, Buddy and BuddyOffline. > > But then we have to check for two different values whenever we want to > > identify a buddy page in the kernel. > > > >> Lastly we would need to create a specialized function for allocating > >> the non-"Offline" pages, and to tweak __free_one_page to tail enqueue > >> "Offline" pages. I'm thinking the alloc function it would look > >> something like __rmqueue_smallest but without the "expand" and needing > >> to modify the !page check to also include a check to verify the page > >> is not "Offline". As far as the changes to __free_one_page it would be > >> a 2 line change to test for the PageType being offline, and if it is > >> to call add_to_free_area_tail instead of add_to_free_area. > > As already mentioned, there might be scenarios where the additional > > hinting thread might consume too much CPU cycles, especially if there is > > little guest activity any you mostly spend time scanning a handful of > > free pages and reporting them. I wonder if we can somehow limit the > > amount of wakeups/scans for a given period to mitigate this issue. > > > > One main issue I see with your approach is that we need quite a lot of > > core memory management changes. This is a problem. I wonder if we can > > factor out most parts into callbacks. > > > > E.g. in order to detect where to queue a certain page (front/tail), call > > a callback if one is registered, mark/check pages in a core-mm unknown > > way as offline etc. > > > > I still wonder if there could be an easier way to combine recording of > > hints and one hinting thread, essentially avoiding scanning and some of > > the required core-mm changes. > In order to resolve the scalability issues associated with my > patch-series without compromising with free memory hints, I may explore > the idea described below: > - Use xbitmap (if possible - earlier suggested by Rik and Wei) > corresponding to each zone on a granularity of MAX_ORDER - 2, to track > the freed PFN's. MAX_ORDER - 2 is what? 2Mbyte? > - Define and use counters corresponding to each zone to monitor the > amount of memory freed. > - As soon as the 64MB free memory threshold is hit wake up the kernel > thread which will scan this xbitmap and try to isolate the pages and > clear the corresponding bits. (We still have to acquire zone lock to > protect the respective xbitmap) So that's 32 pages then? I'd say just keep them in an array, list, tree or hash, bitmap is for when you have nots of pages. > - Report the isolated pages back to the host in a synchronous manner. > I still have to work on several details of this idea including xbitmap, > but first would like to hear any suggestions/thoughts. > > > -- > Regards > Nitesh > -- MST
