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[2003:cb:c719:7700:ef08:112b:278d:7d90]) by smtp.gmail.com with ESMTPSA id m8-20020a05600c4f4800b00413ea26f942sm8380355wmq.14.2024.03.15.05.18.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 15 Mar 2024 05:18:25 -0700 (PDT) Message-ID: <893fd7d3-6f11-45ee-8fe6-52f11dc42cc7@redhat.com> Date: Fri, 15 Mar 2024 13:18:24 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/1] mm/khugepaged: reduce process visible downtime by pre-zeroing hugepage To: Lance Yang Cc: akpm@linux-foundation.org, mhocko@suse.com, zokeefe@google.com, shy828301@gmail.com, xiehuan09@gmail.com, songmuchun@bytedance.com, minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, libang.li@antgroup.com References: <20240308074921.45752-1-ioworker0@gmail.com> <75630ba6-79b6-4105-b614-29cfb0331084@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: 7bit X-Rspamd-Queue-Id: 9AB9A1C0010 X-Rspam-User: X-Stat-Signature: 13z1hj6tm5eb64w5agk73jy4ujzffpg6 X-Rspamd-Server: rspam01 X-HE-Tag: 1710505112-205169 X-HE-Meta: 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 PkYqTvdP 94JhxhIgiekVuHKSJK81DcqW5NYaC33FdJ2nP3SIzNGuEXpjabJm5+9d3d5GzFXVDX9M78Mk0ebPmYsyzNtPZbCNzzFVbcio9FVRVK+IPJxOJXHFo6JVUaJURveAorQKX+P2/V/4ulNSc4bozA71l/nT/PwOZAYyLNkj8aEhbOS01LFqzKH0AodQKoSfLGzG6zX8CPZZ35Ws8nSU6GY6bwEfCP4M0BapwRh74 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 14.03.24 15:19, Lance Yang wrote: > Another thought suggested by Bang Li is that we record which pte is none > in hpage_collapse_scan_pmd. Then, before acquiring the mmap_lock (write mode), > we will pre-zero pages as needed. Here is my point of view: we cannot optimize the common case where we have mostly !pte_none() in a similar way. So why do we care about the less common case? Is the process visible downtime reduction for that less common case really noticable? Or is it rather something that looks good in a micro-benchmark, but won't really make any difference in actual applications (again, where the common case will still result the same downtime). I'm not against this, I'm rather wonder "do we really care". I'd like to hear other opinions. >>>>> >>>>> So my question is: do we really care about it that much that we care to >>>>> optimize? >>>> >>>> IMO, although it may not be our main concern, reducing the impact of >>>> khugepaged on the process remains crucial. I think that users also prefer >>>> minimal interference from khugepaged. >>> >>> The problem I am having with this is that for the *common* case where we >>> have a small number of pte_none(), we cannot really optimize because we >>> have to perform the copy. >>> >>> So this feels like we're rather optimizing a corner case, and I am not >>> so sure if that is really worth it. >>> >>> Other thoughts? >> >> Another thought is to introduce khugepaged/alloc_zeroed_hpage for THP >> sysfs settings. This would enable users to decide whether to avoid unnecessary >> copies when nr_ptes_none > 0. Hm, new toggles for that, not sure ... I much rather prefer something without any new toggles, especially for this case. -- Cheers, David / dhildenb