Re: [PATCH v3 00/15] Free user PTE page table pages

[David Hildenbrand <david@redhat.com>]

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