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Content-Language: en-US To: Linus Torvalds , Matthew Wilcox Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , Sebastian Andrzej Siewior , LKML , "linux-mm@kvack.org" , Waiman Long , Mel Gorman , Steven Rostedt , "David Hildenbrand (Arm)" References: 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspamd-Server: rspam11 X-Stat-Signature: g44iz8g4u1kfeha5kaypq5mizdwutks4 X-Rspam-User: X-Rspamd-Queue-Id: 8010E140003 X-HE-Tag: 1771237953-580949 X-HE-Meta: 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 oYH4wYIN smE0t4gs4h2qft6VazPDvXJjX4nBnafP6G/DSbaC4r9Dh5LUrjT37c6GGTnc8rIXQ2qdThJo/O819naSnrWOP0OMcuPrc9EPWRBly 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 2/14/26 17:32, Linus Torvalds wrote: > On Fri, 13 Feb 2026 at 22:29, Matthew Wilcox wrote: >> >> What if we use an atomic_t on UP to simulate there being a spinlock, >> but only for pcp? > > Yes. Please just wrap this - very unusual - use case with a special > wrapper that can then be entirely different for UP and SMP, and use > something like the suggested "pcp_lock" that becomes a spinlock on > smp, and just a tracking variable on UP. > > And I don't think it needs to even be marked as 'atomic_t' on UP - the > value is going to be idempotent even when modified from interrupts > (because it will just be modified back), so no need for any special > logic, I think. The generic 'atomic_t' ops on UP disable interrupts, > which is horrendous. I think we could be using local_trylock_t, which does exactly this without atomic_t. However David had an even better suggestion (in a chat) that the pcp cache is unnecessary on UP for scalability anyway, so we can just have the trylock wrapper do nothing and fail unconditionally with SMP=n, and then all the cleanups I wanted are immediately possible and we'll even save some memory that would be cached unnecessarily on those small systems (and some code will be eliminated as dead). So I'll be going with that. > Changing spinlocks globally on UP to be something they haven't been > before does not sound like a good idea, particularly since no actual > developer uses UP any more (and honestly, UP is dead outside of very > low-end platforms or legacy like 68k). Ack, thanks. > Linus