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

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 11/11/21 8:20 PM, David Hildenbrand wrote:
> 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

No, it includes multi-threading:

	while (1) {
		char *c;
		char *start = mmap_area[cpu];
		char *end = mmap_area[cpu] + FAULT_LENGTH;
		pthread_barrier_wait(&barrier);
		//printf("fault into %p-%p\n",start, end);

		for (c = start; c < end; c += PAGE_SIZE)
			*c = 0;

		pthread_barrier_wait(&barrier);
		for (i = 0; cpu==0 && i < num; i++)
			madvise(mmap_area[i], FAULT_LENGTH, MADV_DONTNEED);
		pthread_barrier_wait(&barrier);
	}

Thread on cpu0 will use madvise(MADV_DONTNEED) to release the physical
memory of threads on other cpu.

> 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).
> 
>