On 11.11.21 13:00, Qi Zheng wrote: > > > On 11/11/21 7:19 PM, David Hildenbrand wrote: >> On 11.11.21 12:08, Qi Zheng wrote: >>> >>> >>> On 11/11/21 5:22 PM, David Hildenbrand wrote: >>>> On 11.11.21 04:58, Qi Zheng wrote: >>>>> >>>>> >>>>> On 11/11/21 1:37 AM, David Hildenbrand wrote: >>>>>>>> It would still be a fairly coarse-grained locking, I am not sure if that >>>>>>>> is a step into the right direction. If you want to modify *some* page >>>>>>>> table in your process you have exclude each and every page table walker. >>>>>>>> Or did I mis-interpret what you were saying? >>>>>>> >>>>>>> That is one possible design, it favours fast walking and penalizes >>>>>>> mutation. We could also stick a lock in the PMD (instead of a >>>>>>> refcount) and still logically be using a lock instead of a refcount >>>>>>> scheme. Remember modify here is "want to change a table pointer into a >>>>>>> leaf pointer" so it isn't an every day activity.. >>>>>> >>>>>> It will be if we somewhat frequent when reclaim an empty PTE page table >>>>>> as soon as it turns empty. This not only happens when zapping, but also >>>>>> during writeback/swapping. So while writing back / swapping you might be >>>>>> left with empty page tables to reclaim. >>>>>> >>>>>> Of course, this is the current approach. Another approach that doesn't >>>>>> require additional refcounts is scanning page tables for empty ones and >>>>>> reclaiming them. This scanning can either be triggered manually from >>>>>> user space or automatically from the kernel. >>>>> >>>>> Whether it is introducing a special rwsem or scanning an empty page >>>>> table, there are two problems as follows: >>>>> >>>>> #1. When to trigger the scanning or releasing? >>>> >>>> For example when reclaiming memory, when scanning page tables in >>>> khugepaged, or triggered by user space (note that this is the approach I >>>> originally looked into). But it certainly requires more locking thought >>>> to avoid stopping essentially any page table walker. >>>> >>>>> #2. Every time to release a 4K page table page, 512 page table >>>>> entries need to be scanned. >>>> >>>> It would happen only when actually trigger reclaim of page tables >>>> (again, someone has to trigger it), so it's barely an issue. >>>> >>>> For example, khugepaged already scans the page tables either way. >>>> >>>>> >>>>> For #1, if the scanning is triggered manually from user space, the >>>>> kernel is relatively passive, and the user does not fully know the best >>>>> timing to scan. If the scanning is triggered automatically from the >>>>> kernel, that is great. But the timing is not easy to confirm, is it >>>>> scanned and reclaimed every time zap or try_to_unmap? >>>>> >>>>> For #2, refcount has advantages. >>>>> >>>>>> >>>>>>> >>>>>>> There is some advantage with this thinking because it harmonizes well >>>>>>> with the other stuff that wants to convert tables into leafs, but has >>>>>>> to deal with complicated locking. >>>>>>> >>>>>>> On the other hand, refcounts are a degenerate kind of rwsem and only >>>>>>> help with freeing pages. It also puts more atomics in normal fast >>>>>>> paths since we are refcounting each PTE, not read locking the PMD. >>>>>>> >>>>>>> Perhaps the ideal thing would be to stick a rwsem in the PMD. read >>>>>>> means a table cannot be come a leaf. I don't know if there is space >>>>>>> for another atomic in the PMD level, and we'd have to use a hitching >>>>>>> post/hashed waitq scheme too since there surely isn't room for a waitq >>>>>>> too.. >>>>>>> >>>>>>> I wouldn't be so quick to say one is better than the other, but at >>>>>>> least let's have thought about a locking solution before merging >>>>>>> refcounts :) >>>>>> >>>>>> Yes, absolutely. I can see the beauty in the current approach, because >>>>>> it just reclaims "automatically" once possible -- page table empty and >>>>>> nobody is walking it. The downside is that it doesn't always make sense >>>>>> to reclaim an empty page table immediately once it turns empty. >>>>>> >>>>>> Also, it adds complexity for something that is only a problem in some >>>>>> corner cases -- sparse memory mappings, especially relevant for some >>>>>> memory allocators after freeing a lot of memory or running VMs with >>>>>> memory ballooning after inflating the balloon. Some of these use cases >>>>>> might be good with just triggering page table reclaim manually from user >>>>>> space. >>>>>> >>>>> >>>>> Yes, this is indeed a problem. Perhaps some flags can be introduced so >>>>> that the release of page table pages can be delayed in some cases. >>>>> Similar to the lazyfree mechanism in MADV_FREE? >>>> >>>> The issue AFAIU is that once your refcount hits 0 (no more references, >>>> no more entries), the longer you wait with reclaim, the longer others >>>> have to wait for populating a fresh page table because the "page table >>>> to be reclaimed" is still stuck around. You'd have to keep the refcount >>>> increased for a while, and only drop it after a while. But when? And >>>> how? IMHO it's not trivial, but maybe there is an easy way to achieve it. >>>> >>> >>> For running VMs with memory ballooning after inflating the balloon, is >>> this a hot behavior? Even if it is, it is already facing the release and >>> reallocation of physical pages. The overhead after introducing >>> pte_refcount is that we need to release and re-allocate page table page. >>> But 2MB physical pages only corresponds to 4KiB of PTE page table page. >>> So maybe the overhead is not big. >> >> The cases that come to my mind are >> >> a) Swapping on shared memory with concurrent access >> b) Reclaim on file-backed memory with concurrent access >> c) Free page reporting as implemented by virtio-balloon >> >> In all of these cases, you can have someone immediately re-access the >> page table and re-populate it. > > In the performance test shown on the cover, we repeatedly performed > touch and madvise(MADV_DONTNEED) actions, which simulated the case > you said above. > > We did find a small amount of performance regression, but I think it is > acceptable, and no new perf hotspots have been added. That test always accesses 2MiB and does it from a single thread. Things might (IMHO will) look different when only accessing individual pages and doing the access from one/multiple separate threads (that's what a),b) and c) essentially do, they don't do it in the pattern you measured. what you measured matches rather a typical memory allocator). -- Thanks, David / dhildenb
