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[2003:cb:c707:4f00:a44a:5ad6:765a:635]) by smtp.gmail.com with ESMTPSA id cl2-20020a5d5f02000000b0033e456f6e7csm2505046wrb.1.2024.03.12.06.19.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 12 Mar 2024 06:19:38 -0700 (PDT) Message-ID: <75630ba6-79b6-4105-b614-29cfb0331084@redhat.com> Date: Tue, 12 Mar 2024 14:19:37 +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 References: <20240308074921.45752-1-ioworker0@gmail.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-Queue-Id: 86D741C0009 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: phdhge1srrqrdnft58rk8hcwojwfk7i4 X-HE-Tag: 1710249584-802717 X-HE-Meta: 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 yM57frhf TDeuXyNrBEY1n56fSjXa+382Q8uDVlb6PVMjYt5riX78r/IFYDM4Y4EcYYcvkQ1BWELpJxNDFMmLZEsgYZ7Q5sMGjws23T1fg5U5gZTSxa3f0nJPFrNW+qrAh66b02FY981YjqOmx0Dj6gCS3d016bl2Pp2HJpYMB5Tx7uvDy/nQdRyKSLOhXLASdo999H+yky7Ppb1wEq8u60R/ZMJnSzdpbCak9a+H90/AoDWoZ57L8v3hAXX0wbCnD/BzmwNMU5hO0fVP4/MhK6JKDif6O3N7UXFBpycsP7/SynNxx068rdyetxl5lk8IZ/wVioFzxBUZlz+WdnUhcHK5WEk6aN2NpOLO9vviiU+J6VEaQaV1QCAqpbodf0xFA+N70vlN2v/LgmDAOH3XpCmrCTCbCytm4H+XMAHSS/QtBDub0u41af3WX1XyQeDmQscshIVgF6Io7gT1knlbN8mA2T5Aef++NBg== 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 12.03.24 14:09, Lance Yang wrote: > Hey David, > > Thanks for taking time to review! > > On Tue, Mar 12, 2024 at 12:19 AM David Hildenbrand wrote: >> >> On 08.03.24 08:49, Lance Yang wrote: >>> The patch reduces the process visible downtime during hugepage >>> collapse. This is achieved by pre-zeroing the hugepage before >>> acquiring mmap_lock(write mode) if nr_pte_none >= 256, without >>> affecting the efficiency of khugepaged. >>> >>> On an Intel Core i5 CPU, the process visible downtime during >>> hugepage collapse is as follows: >>> >>> | nr_ptes_none | w/o __GFP_ZERO | w/ __GFP_ZERO | Change | >>> --------------------------------------------------—---------- >>> | 511 | 233us | 95us | -59.21%| >>> | 384 | 376us | 219us | -41.20%| >>> | 256 | 421us | 323us | -23.28%| >>> | 128 | 523us | 507us | -3.06%| >>> >>> Of course, alloc_charge_hpage() will take longer to run with >>> the __GFP_ZERO flag. >>> >>> | Func | w/o __GFP_ZERO | w/ __GFP_ZERO | >>> |----------------------|----------------|---------------| >>> | alloc_charge_hpage | 198us | 295us | >>> >>> But it's not a big deal because it doesn't impact the total >>> time spent by khugepaged in collapsing a hugepage. In fact, >>> it would decrease. >> >> It does look sane to me and not overly complicated. >> >> But, it's an optimization really only when we have quite a bunch of >> pte_none(), possibly repeatedly so that it really makes a difference. >> >> Usually, when we repeatedly collapse that many pte_none() we're just >> wasting a lot of memory and should re-evaluate life choices :) > > Agreed! It seems that the default value of max_pte_none may be set too > high, which could result in the memory wastage issue we're discussing. IIRC, some companies disable it completely (set to 0) because of that. > >> >> 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? -- Cheers, David / dhildenb