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[79.242.62.232]) by smtp.gmail.com with ESMTPSA id c6sm11100631wmq.46.2021.11.11.04.51.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Nov 2021 04:51:36 -0800 (PST) Message-ID: <2ffa76f5-ca39-2044-61fa-5398faf16a6c@redhat.com> Date: Thu, 11 Nov 2021 13:51:35 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Content-Language: en-US To: Qi Zheng , Jason Gunthorpe Cc: akpm@linux-foundation.org, tglx@linutronix.de, kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, songmuchun@bytedance.com, zhouchengming@bytedance.com References: <20211110105428.32458-1-zhengqi.arch@bytedance.com> <20211110125601.GQ1740502@nvidia.com> <8d0bc258-58ba-52c5-2e0d-a588489f2572@redhat.com> <20211110143859.GS1740502@nvidia.com> <6ac9cc0d-7dea-0e19-51b3-625ec6561ac7@redhat.com> <20211110163925.GX1740502@nvidia.com> <7c97d86f-57f4-f764-3e92-1660690a0f24@redhat.com> <60515562-5f93-11cd-6c6a-c7cc92ff3bf8@bytedance.com> <9ee06b52-4844-7996-fa34-34fc7d4fdc10@bytedance.com> <27d73395-70b4-fe4a-4c8d-415b43ff9c1f@redhat.com> <2e19ad1b-15f3-7508-c5d5-6c31765f26d3@bytedance.com> <1489f02f-d024-b9ec-2ab6-e6efc8a022f1@redhat.com> <791ddf94-5ad1-b431-85a1-db9a07579057@bytedance.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v3 00/15] Free user PTE page table pages In-Reply-To: <791ddf94-5ad1-b431-85a1-db9a07579057@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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