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Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Barry Song , Dev Jain , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20250807185819.199865-1-lorenzo.stoakes@oracle.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/g1oFAmgsLPQFCRvGjuMACgkQTd4Q 9wD/g1o0bxAAqYC7gTyGj5rZwvy1VesF6YoQncH0yI79lvXUYOX+Nngko4v4dTlOQvrd/vhb 02e9FtpA1CxgwdgIPFKIuXvdSyXAp0xXuIuRPQYbgNriQFkaBlHe9mSf8O09J3SCVa/5ezKM OLW/OONSV/Fr2VI1wxAYj3/Rb+U6rpzqIQ3Uh/5Rjmla6pTl7Z9/o1zKlVOX1SxVGSrlXhqt kwdbjdj/csSzoAbUF/duDuhyEl11/xStm/lBMzVuf3ZhV5SSgLAflLBo4l6mR5RolpPv5wad GpYS/hm7HsmEA0PBAPNb5DvZQ7vNaX23FlgylSXyv72UVsObHsu6pT4sfoxvJ5nJxvzGi69U s1uryvlAfS6E+D5ULrV35taTwSpcBAh0/RqRbV0mTc57vvAoXofBDcs3Z30IReFS34QSpjvl Hxbe7itHGuuhEVM1qmq2U72ezOQ7MzADbwCtn+yGeISQqeFn9QMAZVAkXsc9Wp0SW/WQKb76 FkSRalBZcc2vXM0VqhFVzTb6iNqYXqVKyuPKwhBunhTt6XnIfhpRgqveCPNIasSX05VQR6/a OBHZX3seTikp7A1z9iZIsdtJxB88dGkpeMj6qJ5RLzUsPUVPodEcz1B5aTEbYK6428H8MeLq NFPwmknOlDzQNC6RND8Ez7YEhzqvw7263MojcmmPcLelYbfOwU0EVcufkQEQAOfX3n0g0fZz 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+DWgUCaCwtJQUJG8aPFAAKCRBN3hD3AP+DWlDnD/4k2TW+HyOOOePVm23F5HOhNNd7nNv3 Vq2cLcW1DteHUdxMO0X+zqrKDHI5hgnE/E2QH9jyV8mB8l/ndElobciaJcbl1cM43vVzPIWn 01vW62oxUNtEvzLLxGLPTrnMxWdZgxr7ACCWKUnMGE2E8eca0cT2pnIJoQRz242xqe/nYxBB /BAK+dsxHIfcQzl88G83oaO7vb7s/cWMYRKOg+WIgp0MJ8DO2IU5JmUtyJB+V3YzzM4cMic3 bNn8nHjTWw/9+QQ5vg3TXHZ5XMu9mtfw2La3bHJ6AybL0DvEkdGxk6YHqJVEukciLMWDWqQQ RtbBhqcprgUxipNvdn9KwNpGciM+hNtM9kf9gt0fjv79l/FiSw6KbCPX9b636GzgNy0Ev2UV m00EtcpRXXMlEpbP4V947ufWVK2Mz7RFUfU4+ETDd1scMQDHzrXItryHLZWhopPI4Z+ps0rB CQHfSpl+wG4XbJJu1D8/Ww3FsO42TMFrNr2/cmqwuUZ0a0uxrpkNYrsGjkEu7a+9MheyTzcm vyU2knz5/stkTN2LKz5REqOe24oRnypjpAfaoxRYXs+F8wml519InWlwCra49IUSxD1hXPxO WBe5lqcozu9LpNDH/brVSzHCSb7vjNGvvSVESDuoiHK8gNlf0v+epy5WYd7CGAgODPvDShGN g3eXuA== Organization: Red Hat In-Reply-To: <20250807185819.199865-1-lorenzo.stoakes@oracle.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: MSo4gat89UjC2LWpdCte9NFbeR5-aqNx--t-0afAeoA_1754593855 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: k9sir5uctg3e78g5z1pr6bcf6dmcosur X-Rspam-User: X-Rspamd-Queue-Id: 0E5C620009 X-Rspamd-Server: rspam02 X-HE-Tag: 1754593857-660094 X-HE-Meta: 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 CGpWbJrU 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 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 07.08.25 20:58, Lorenzo Stoakes wrote: > It was discovered in the attached report that commit f822a9a81a31 ("mm: > optimize mremap() by PTE batching") introduced a significant performance > regression on a number of metrics on x86-64, most notably > stress-ng.bigheap.realloc_calls_per_sec - indicating a 37.3% regression in > number of mremap() calls per second. > > I was able to reproduce this locally on an intel x86-64 raptor lake system, > noting an average of 143,857 realloc calls/sec (with a stddev of 4,531 or > 3.1%) prior to this patch being applied, and 81,503 afterwards (stddev of > 2,131 or 2.6%) - a 43.3% regression. > > During testing I was able to determine that there was no meaningful > difference in efforts to optimise the folio_pte_batch() operation, nor > checking folio_test_large(). > > This is within expectation, as a regression this large is likely to > indicate we are accessing memory that is not yet in a cache line (and > perhaps may even cause a main memory fetch). > > The expectation by those discussing this from the start was that > vm_normal_folio() (invoked by mremap_folio_pte_batch()) would likely be the > culprit due to having to retrieve memory from the vmemmap (which mremap() > page table moves does not otherwise do, meaning this is inevitably cold > memory). > > I was able to definitively determine that this theory is indeed correct and > the cause of the issue. > > The solution is to restore part of an approach previously discarded on > review, that is to invoke pte_batch_hint() which explicitly determines, > through reference to the PTE alone (thus no vmemmap lookup), what the PTE > batch size may be. > > On platforms other than arm64 this is currently hardcoded to return 1, so > this naturally resolves the issue for x86-64, and for arm64 introduces > little to no overhead as the pte cache line will be hot. > > With this patch applied, we move from 81,503 realloc calls/sec to > 138,701 (stddev of 496.1 or 0.4%), which is a -3.6% regression, however > accounting for the variance in the original result, this is broadly > restoring performance to its prior state. > > Reported-by: kernel test robot > Closes: https://lore.kernel.org/oe-lkp/202508071609.4e743d7c-lkp@intel.com > Fixes: f822a9a81a31 ("mm: optimize mremap() by PTE batching") > Signed-off-by: Lorenzo Stoakes > --- > mm/mremap.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/mremap.c b/mm/mremap.c > index 677a4d744df9..9afa8cd524f5 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -179,6 +179,10 @@ static int mremap_folio_pte_batch(struct vm_area_struct *vma, unsigned long addr > if (max_nr == 1) > return 1; > > + /* Avoid expensive folio lookup if we stand no chance of benefit. */ > + if (pte_batch_hint(ptep, pte) == 1) > + return 1; > + > folio = vm_normal_folio(vma, addr, pte); > if (!folio || !folio_test_large(folio)) > return 1; Acked-by: David Hildenbrand Wondering whether we could then just use the patch hint instead of going via the folio. IOW, return pte_batch_hint(ptep, pte); Not sure if that was discussed at some point before we went into the direction of using folios. But there really doesn't seem to be anything gained for other architectures here (as raised by Jann). -- Cheers, David / dhildenb