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[2003:cb:c707:7600:5c18:5a7d:c5b7:e7a9]) by smtp.gmail.com with ESMTPSA id v17-20020adfe291000000b0033d56aa4f45sm11925656wri.112.2024.02.27.11.17.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 27 Feb 2024 11:17:19 -0800 (PST) Message-ID: <2934125a-f2e2-417c-a9f9-3cb1e074a44f@redhat.com> Date: Tue, 27 Feb 2024 20:17:18 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags To: Ryan Roberts , Andrew Morton , Matthew Wilcox , Huang Ying , Gao Xiang , Yu Zhao , Yang Shi , Michal Hocko , Kefeng Wang Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20231025144546.577640-1-ryan.roberts@arm.com> <20231025144546.577640-2-ryan.roberts@arm.com> <6541e29b-f25a-48b8-a553-fd8febe85e5a@redhat.com> From: David Hildenbrand 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: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 3CE7F80018 X-Stat-Signature: qo51ixckj3g1mtj5ch5a6z8udtattonc X-Rspam-User: X-HE-Tag: 1709061446-326660 X-HE-Meta: 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 BhCE+/Ly TjA9NkrwXGdBwlQ9HAxDmvahQxtxjsthe4Zn/X9WOxPwlaat6BzVPE+zm/0O4K6ktaqk8Suw43fDezFv7znionaHV8f0EKSbIYWDUs9Rm/hiyM24ayL/Er/At8A1/qA4M8YoExfDv5tsKnt4stSTMV0kPxYa1CU91FCN40ZtSeR0pe7jV4BF+X8/EEYNwUwggyuzZalcqYNNWljjFkhPuoaczCw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 27.02.24 18:10, Ryan Roberts wrote: > Hi David, > > On 26/02/2024 17:41, Ryan Roberts wrote: >> On 22/02/2024 10:20, David Hildenbrand wrote: >>> On 22.02.24 11:19, David Hildenbrand wrote: >>>> On 25.10.23 16:45, Ryan Roberts wrote: >>>>> As preparation for supporting small-sized THP in the swap-out path, >>>>> without first needing to split to order-0, Remove the CLUSTER_FLAG_HUGE, >>>>> which, when present, always implies PMD-sized THP, which is the same as >>>>> the cluster size. >>>>> >>>>> The only use of the flag was to determine whether a swap entry refers to >>>>> a single page or a PMD-sized THP in swap_page_trans_huge_swapped(). >>>>> Instead of relying on the flag, we now pass in nr_pages, which >>>>> originates from the folio's number of pages. This allows the logic to >>>>> work for folios of any order. >>>>> >>>>> The one snag is that one of the swap_page_trans_huge_swapped() call >>>>> sites does not have the folio. But it was only being called there to >>>>> avoid bothering to call __try_to_reclaim_swap() in some cases. >>>>> __try_to_reclaim_swap() gets the folio and (via some other functions) >>>>> calls swap_page_trans_huge_swapped(). So I've removed the problematic >>>>> call site and believe the new logic should be equivalent. >>>> >>>> That is the  __try_to_reclaim_swap() -> folio_free_swap() -> >>>> folio_swapped() -> swap_page_trans_huge_swapped() call chain I assume. >>>> >>>> The "difference" is that you will now (1) get another temporary >>>> reference on the folio and (2) (try)lock the folio every time you >>>> discard a single PTE of a (possibly) large THP. >>>> >>> >>> Thinking about it, your change will not only affect THP, but any call to >>> free_swap_and_cache(). >>> >>> Likely that's not what we want. :/ >>> >> >> Is folio_trylock() really that expensive given the code path is already locking >> multiple spinlocks, and I don't think we would expect the folio lock to be very >> contended? >> >> I guess filemap_get_folio() could be a bit more expensive, but again, is this >> really a deal-breaker? >> >> >> I'm just trying to refamiliarize myself with this series, but I think I ended up >> allocating a cluster per cpu per order. So one potential solution would be to >> turn the flag into a size and store it in the cluster info. (In fact I think I >> was doing that in an early version of this series - will have to look at why I >> got rid of that). Then we could avoid needing to figure out nr_pages from the folio. > > I ran some microbenchmarks to see if these extra operations cause a performance > issue - it all looks OK to me. Sorry, I'm drowning in reviews right now. I was hoping to get some of my own stuff figured out today ... maybe tomorrow. > > I modified your "pte-mapped-folio-benchmarks" to add a "munmap-swapped-forked" > mode, which prepares the 1G memory mapping by first paging it out with > MADV_PAGEOUT, then it forks a child (and keeps that child alive) so that the > swap slots have 2 references, then it measures the duration of munmap() in the > parent on the entire range. The idea is that free_swap_and_cache() is called for > each PTE during munmap(). Prior to my change, swap_page_trans_huge_swapped() > will return true, due to the child's references, and __try_to_reclaim_swap() is > not called. After my change, we no longer have this short cut. > > In both cases the results are within 1% (confirmed across multiple runs of 20 > seconds each): > > mm-stable: Average: 0.004997 > + change: Average: 0.005037 > > (these numbers are for Ampere Altra. I also tested on M2 VM - no regression > their either). > > Do you still have a concern about this change? The main concern I had was not about overhead due to atomic operations in the non-concurrent case that you are measuring. We might now unnecessarily be incrementing the folio refcount and taking the folio lock. That will affects large folios in the swapcache now IIUC. Small folios should be unaffected. The side effects of that can be: * Code checking for additional folio reference could now detect some and back out. (the "mapcount + swapcache*folio_nr_pages != folio_refcount" stuff) * Code that might really benefit from trylocking the folio might fail to do so. For example, splitting a large folio might now fail more often simply because some process zaps a swap entry and the additional reference+page lock were optimized out previously. How relevant is it? Relevant enough that someone decided to put that optimization in? I don't know :) Arguably, zapping a present PTE also leaves the refcount elevated for a while until the mapcount was freed. But here, it could be avoided. Digging a bit, it was introduced in: commit e07098294adfd03d582af7626752255e3d170393 Author: Huang Ying Date: Wed Sep 6 16:22:16 2017 -0700 mm, THP, swap: support to reclaim swap space for THP swapped out The normal swap slot reclaiming can be done when the swap count reaches SWAP_HAS_CACHE. But for the swap slot which is backing a THP, all swap slots backing one THP must be reclaimed together, because the swap slot may be used again when the THP is swapped out again later. So the swap slots backing one THP can be reclaimed together when the swap count for all swap slots for the THP reached SWAP_HAS_CACHE. In the patch, the functions to check whether the swap count for all swap slots backing one THP reached SWAP_HAS_CACHE are implemented and used when checking whether a swap slot can be reclaimed. To make it easier to determine whether a swap slot is backing a THP, a new swap cluster flag named CLUSTER_FLAG_HUGE is added to mark a swap cluster which is backing a THP (Transparent Huge Page). Because THP swap in as a whole isn't supported now. After deleting the THP from the swap cache (for example, swapping out finished), the CLUSTER_FLAG_HUGE flag will be cleared. So that, the normal pages inside THP can be swapped in individually. With your change, if we have a swapped out THP with 512 entries and exit(), we would now 512 times in a row grab a folio reference and trylock the folio. In the past, we would have done that at most once. That doesn't feel quite right TBH ... so I'm wondering if there are any low-hanging fruits to avoid that. -- Cheers, David / dhildenb