Messages in this thread | | | Date | Thu, 11 Nov 2021 20:00:06 +0800 | Subject | Re: [PATCH v3 00/15] Free user PTE page table pages | From | Qi Zheng <> |
| |
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.
> > For something mostly static (balloon inflation, memory allocator), it's > not that big of a deal I guess. >
| |