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[2003:cb:c706:4e00:31ad:5274:e21c:b59]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-36b367c01f6sm15094953f8f.23.2024.07.30.09.11.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 30 Jul 2024 09:11:51 -0700 (PDT) Message-ID: <95ed1631-ff62-4627-8dc6-332096e673b4@redhat.com> Date: Tue, 30 Jul 2024 18:11:49 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/6] mm: split underutilized THPs To: Usama Arif , akpm@linux-foundation.org, linux-mm@kvack.org Cc: hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, roman.gushchin@linux.dev, yuzhao@google.com, baohua@kernel.org, ryan.roberts@arm.com, rppt@kernel.org, willy@infradead.org, cerasuolodomenico@gmail.com, corbet@lwn.net, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@meta.com References: <20240730125346.1580150-1-usamaarif642@gmail.com> <3cd1b07d-7b02-4d37-918a-5759b23291fb@gmail.com> <73b97a03-3742-472f-9a36-26ba9009d715@gmail.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: <73b97a03-3742-472f-9a36-26ba9009d715@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 30.07.24 17:19, Usama Arif wrote: > > > On 30/07/2024 16:14, Usama Arif wrote: >> >> >> On 30/07/2024 15:35, David Hildenbrand wrote: >>> On 30.07.24 14:45, Usama Arif wrote: >>>> The current upstream default policy for THP is always. However, Meta >>>> uses madvise in production as the current THP=always policy vastly >>>> overprovisions THPs in sparsely accessed memory areas, resulting in >>>> excessive memory pressure and premature OOM killing. >>>> Using madvise + relying on khugepaged has certain drawbacks over >>>> THP=always. Using madvise hints mean THPs aren't "transparent" and >>>> require userspace changes. Waiting for khugepaged to scan memory and >>>> collapse pages into THP can be slow and unpredictable in terms of performance >>>> (i.e. you dont know when the collapse will happen), while production >>>> environments require predictable performance. If there is enough memory >>>> available, its better for both performance and predictability to have >>>> a THP from fault time, i.e. THP=always rather than wait for khugepaged >>>> to collapse it, and deal with sparsely populated THPs when the system is >>>> running out of memory. >>>> >>>> This patch-series is an attempt to mitigate the issue of running out of >>>> memory when THP is always enabled. During runtime whenever a THP is being >>>> faulted in or collapsed by khugepaged, the THP is added to a list. >>>> Whenever memory reclaim happens, the kernel runs the deferred_split >>>> shrinker which goes through the list and checks if the THP was underutilized, >>>> i.e. how many of the base 4K pages of the entire THP were zero-filled. >>>> If this number goes above a certain threshold, the shrinker will attempt >>>> to split that THP. Then at remap time, the pages that were zero-filled are >>>> not remapped, hence saving memory. This method avoids the downside of >>>> wasting memory in areas where THP is sparsely filled when THP is always >>>> enabled, while still providing the upside THPs like reduced TLB misses without >>>> having to use madvise. >>>> >>>> Meta production workloads that were CPU bound (>99% CPU utilzation) were >>>> tested with THP shrinker. The results after 2 hours are as follows: >>>> >>>>                              | THP=madvise |  THP=always   | THP=always >>>>                              |             |               | + shrinker series >>>>                              |             |               | + max_ptes_none=409 >>>> ----------------------------------------------------------------------------- >>>> Performance improvement     |      -      |    +1.8%      |     +1.7% >>>> (over THP=madvise)          |             |               | >>>> ----------------------------------------------------------------------------- >>>> Memory usage                |    54.6G    | 58.8G (+7.7%) |   55.9G (+2.4%) >>>> ----------------------------------------------------------------------------- >>>> max_ptes_none=409 means that any THP that has more than 409 out of 512 >>>> (80%) zero filled filled pages will be split. >>>> >>>> To test out the patches, the below commands without the shrinker will >>>> invoke OOM killer immediately and kill stress, but will not fail with >>>> the shrinker: >>>> >>>> echo 450 > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none >>>> mkdir /sys/fs/cgroup/test >>>> echo $$ > /sys/fs/cgroup/test/cgroup.procs >>>> echo 20M > /sys/fs/cgroup/test/memory.max >>>> echo 0 > /sys/fs/cgroup/test/memory.swap.max >>>> # allocate twice memory.max for each stress worker and touch 40/512 of >>>> # each THP, i.e. vm-stride 50K. >>>> # With the shrinker, max_ptes_none of 470 and below won't invoke OOM >>>> # killer. >>>> # Without the shrinker, OOM killer is invoked immediately irrespective >>>> # of max_ptes_none value and kill stress. >>>> stress --vm 1 --vm-bytes 40M --vm-stride 50K >>>> >>>> Patches 1-2 add back helper functions that were previously removed >>>> to operate on page lists (needed by patch 3). >>>> Patch 3 is an optimization to free zapped tail pages rather than >>>> waiting for page reclaim or migration. >>>> Patch 4 is a prerequisite for THP shrinker to not remap zero-filled >>>> subpages when splitting THP. >>>> Patches 6 adds support for THP shrinker. >>>> >>>> (This patch-series restarts the work on having a THP shrinker in kernel >>>> originally done in >>>> https://lore.kernel.org/all/cover.1667454613.git.alexlzhu@fb.com/. >>>> The THP shrinker in this series is significantly different than the >>>> original one, hence its labelled v1 (although the prerequisite to not >>>> remap clean subpages is the same).) >>> >>> As shared previously, there is one issue with uffd (even when currently not active for a VMA!), where we must not zap present page table entries. >>> >>> Something that is always possible (assuming no GUP pins of course, which) is replacing the zero-filled subpages by shared zeropages. >>> >>> Is that being done in this patch set already, or are we creating pte_none() entries? >>> >> >> I think thats done in Patch 4/6. In function try_to_unmap_unused, we have below which I think does what you are suggesting? i.e. point to shared zeropage and not clear pte for uffd armed vma. >> >> if (userfaultfd_armed(pvmw->vma)) { >> newpte = pte_mkspecial(pfn_pte(page_to_pfn(ZERO_PAGE(pvmw->address)), >> pvmw->vma->vm_page_prot)); >> ptep_clear_flush(pvmw->vma, pvmw->address, pvmw->pte); >> set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte); >> } > > > Ah are you suggesting userfaultfd_armed(pvmw->vma) will evaluate to false even if its uffd? I think something like below would work in that case. I remember one ugly case in QEMU with postcopy live-migration where we must not zap zero-filled pages. I am not 100% regarding THP (if it could be enabled at that point), but imagine the following 1) mmap(), enable THP 2) Migrate a bunch of pages from the source during precopy (writing to the memory). Might end up creating THPs (during fault/khugepaged) 3) Register UFFD on the VMA 4) Disable new THPs from forming via MADV_NOHUGEPAGE on the VMA 5) Discard any pages that have been re-dirtied or not migrated yet 6) Migrate-on-demand any holes using uffd If we discard zero-filled pages between 2) and 3) we might get wrong uffd notifications in 6 for pages that have already been migrated). I'll have to check if that actually happens in that sequence in QEMU: if QEMU would disable THP right before 2) we would be safe. But I recall that it is not the case :/ -- Cheers, David / dhildenb