On Mon, Apr 08, 2019 at 08:18:35AM -0700, Alexander Duyck wrote: > On Mon, Apr 8, 2019 at 5:24 AM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote: > > > > > > On 4/5/19 8:09 PM, 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. > > > > > > 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. > > > > > > 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. > > > > > > 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. > > > > > > 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. > > any ideas about how are you planning to control this? I think supplying the 64M value from host is probably reasonable. > > You mean in terms of switching the hinting on/off? The setup should be > pretty simple. Basically we would still need a hook like the one you > added after the allocation to determine where the free page ultimately > landed and to do a check against the high water mark I mentioned. > Basically if there is something like 2X the number of pages needed to > fulfill the 64MB requirement we could then kick off a thread running > on the zone to begin populating the hints and notifying the > virtio-balloon interface. When we can no longer fill the ring we would > simply stop the thread until we get back to the 2X state for nr_freed > versus the last nr_freed value we had hinted upon. It wouldn't be > dissimilar to how we currently handle the Tx path in many NICs where > we shut off hinting. > > For examples of doing something like this you could look at the Rx > softIRQ handling in the NIC drivers. Basically the idea there is you > trigger the event once, and then the thread is running until all work > has been completed. The thread itself is limiting itself to only > processing some number of fixed buffers for each request, and when it > can no longer get a full set it stops and waits to be rescheduled by > an interrupt. > > > > 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. > > > > > > 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. > > > > > > 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. > > Is it possible that once the pages are offline, there is a large > > allocation request in the guest needing those offline pages as well? > > It is possible. However the behavior here would be no different from a > NIC driver. NIC drivers will sit on a swath of memory for Rx purposes > waiting for the DMA to occur. Here we are sitting on 64MB which for a > large allocation should not be that significant. > > As far as avoiding it, I don't think there is any way we can avoid > such an event completely. There are scenerios where the hitning will > get hung up while sitting on memory for an extended period of time. > That is why I am thinking our best mitigation for now would be to keep > the amount of hinting we are doing confined to something on the > smaller side such as 64M or less which I have already mentioned. By > doing that if we do hit one of the problematic scenarios we should > have minimal impact. > > > > > > > Anyway this email ended up being pretty massive by the time I was > > > done. Feel free to reply to parts of it and we can break it out into > > > separate threads of discussion as necessary. I will start working on > > > coding some parts of this next week. > > > > > > Thanks. > > > > > > - Alex > > -- > > Regards > > Nitesh > >
