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

[David Hildenbrand <david@redhat.com>]

On 11.11.21 13:32, Qi Zheng wrote:
> 
> 
> 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:
> 

Oh sorry, I totally skipped [2].

> 	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.
> 

I'll have a more detailed look at the benchmark. On a quick glimpse,
looks like the threads are also accessing a full 2MiB range, one page at
a time, and one thread is zapping the whole 2MiB range. A single CPU
only accesses memory within one 2MiB range IIRC.

Having multiple threads just access individual pages within a single 2
MiB region, and having one thread zap that memory (e.g., simulate
swapout) could be another benchmark.

We have to make sure to run with THP disabled (e.g., using
madvise(MADV_NOHUGEPAGE) on the complete mapping in the benchmark
eventually), because otherwise you might just be populating+zapping THPs
if they would otherwise be allowed in the environment.

-- 
Thanks,

David / dhildenb