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bh=xtMe2pzANcCv923DR4DGNj4wwpKFAlvPFXGuh7j582g=; b=CJa74u/rK3wE2u0Ipz9zWnG8bXBavMr3aD39umQSPPYw/IwjXY2qpEcYcWGbtPfPN7NzFv 1LbEYj8jn2TeBbDQ== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 5DB971392A; Fri, 8 Aug 2025 09:56:04 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id gNGMFrTJlWixXQAAD6G6ig (envelope-from ); Fri, 08 Aug 2025 09:56:04 +0000 Message-ID: <06849b87-ccca-4dbc-8b35-a537bb6a9859@suse.cz> Date: Fri, 8 Aug 2025 11:56:04 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH HOTFIX 6.17] mm/mremap: avoid expensive folio lookup on mremap folio pte batch Content-Language: en-US To: Lorenzo Stoakes , Andrew Morton Cc: "Liam R . Howlett" , Jann Horn , Pedro Falcato , Barry Song , Dev Jain , linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Hildenbrand References: <20250807185819.199865-1-lorenzo.stoakes@oracle.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: <20250807185819.199865-1-lorenzo.stoakes@oracle.com> Content-Type: text/plain; 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DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.cz:+] X-Spam-Score: -4.51 On 8/7/25 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 Acked-by: Vlastimil Babka Sadly, the improvement will be far from 3888.9% :( > --- > 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; > -- > 2.50.1