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Wed, 17 Sep 2025 12:13:11 +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 5nmbHdelymjhWQAAD6G6ig (envelope-from ); Wed, 17 Sep 2025 12:13:11 +0000 Message-ID: <98f10e2a-7a02-42e8-8042-984ed82d5ba7@suse.cz> Date: Wed, 17 Sep 2025 14:13:11 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 04/23] slab: add sheaf support for batching kfree_rcu() operations Content-Language: en-US To: paulmck@kernel.org Cc: Harry Yoo , Suren Baghdasaryan , "Liam R. Howlett" , Christoph Lameter , David Rientjes , Roman Gushchin , Uladzislau Rezki , Sidhartha Kumar , linux-mm@kvack.org, linux-kernel@vger.kernel.org, rcu@vger.kernel.org, maple-tree@lists.infradead.org References: <20250910-slub-percpu-caches-v8-0-ca3099d8352c@suse.cz> <20250910-slub-percpu-caches-v8-4-ca3099d8352c@suse.cz> <8e7055a9-f899-4aad-8ed7-6543077c05d1@paulmck-laptop> 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: <8e7055a9-f899-4aad-8ed7-6543077c05d1@paulmck-laptop> Content-Type: text/plain; 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McKenney wrote: > On Wed, Sep 17, 2025 at 11:55:10AM +0200, Vlastimil Babka wrote: >> On 9/17/25 10:30, Harry Yoo wrote: >> > On Wed, Sep 10, 2025 at 10:01:06AM +0200, Vlastimil Babka wrote: >> >> +/* needed for kvfree_rcu_barrier() */ >> >> +void flush_all_rcu_sheaves() >> >> +{ >> >> + struct slub_percpu_sheaves *pcs; >> >> + struct slub_flush_work *sfw; >> >> + struct kmem_cache *s; >> >> + bool flushed = false; >> >> + unsigned int cpu; >> >> + >> >> + cpus_read_lock(); >> >> + mutex_lock(&slab_mutex); >> >> + >> >> + list_for_each_entry(s, &slab_caches, list) { >> >> + if (!s->cpu_sheaves) >> >> + continue; >> >> + >> >> + mutex_lock(&flush_lock); >> >> + >> >> + for_each_online_cpu(cpu) { >> >> + sfw = &per_cpu(slub_flush, cpu); >> >> + pcs = per_cpu_ptr(s->cpu_sheaves, cpu); >> >> + >> >> + if (!pcs->rcu_free || !pcs->rcu_free->size) { >> > >> > Is the compiler allowed to compile this to read pcs->rcu_free twice? >> > Something like: >> > >> > flush_all_rcu_sheaves() __kfree_rcu_sheaf() >> > >> > pcs->rcu_free != NULL >> > pcs->rcu_free = NULL >> > pcs->rcu_free == NULL >> > /* NULL-pointer-deref */ >> > pcs->rcu_free->size >> >> Good point, I'll remove the size check and simply pcs->rcu_free non-null >> means we flush. >> >> >> + sfw->skip = true; >> >> + continue; >> >> + } >> >> >> >> + INIT_WORK(&sfw->work, flush_rcu_sheaf); >> >> + sfw->skip = false; >> >> + sfw->s = s; >> >> + queue_work_on(cpu, flushwq, &sfw->work); >> >> + flushed = true; >> >> + } >> >> + >> >> + for_each_online_cpu(cpu) { >> >> + sfw = &per_cpu(slub_flush, cpu); >> >> + if (sfw->skip) >> >> + continue; >> >> + flush_work(&sfw->work); >> >> + } >> >> + >> >> + mutex_unlock(&flush_lock); >> >> + } >> >> + >> >> + mutex_unlock(&slab_mutex); >> >> + cpus_read_unlock(); >> >> + >> >> + if (flushed) >> >> + rcu_barrier(); >> > >> > I think we need to call rcu_barrier() even if flushed == false? >> > >> > Maybe a kvfree_rcu()'d object was already waiting for the rcu callback to >> > be processed before flush_all_rcu_sheaves() is called, and >> > in flush_all_rcu_sheaves() we skipped all (cache, cpu) pairs, >> > so flushed == false but the rcu callback isn't processed yet >> > by the end of the function? >> > >> > That sounds like a very unlikely to happen in a realistic scenario, >> > but still possible... >> >> Yes also good point, will flush unconditionally. >> >> Maybe in __kfree_rcu_sheaf() I should also move the call_rcu(...) before >> local_unlock(). So we don't end up seeing a NULL pcs->rcu_free in >> flush_all_rcu_sheaves() because __kfree_rcu_sheaf() already set it to NULL, >> but didn't yet do the call_rcu() as it got preempted after local_unlock(). >> >> But then rcu_barrier() itself probably won't mean we make sure such cpus >> finished the local_locked section, if we didn't queue work on them. So maybe >> we need synchronize_rcu()? > > Do you need both rcu_barrier() and synchronize_rcu(), maybe along with We need the local_lock protected sections of __kfree_rcu_sheaf() to be finished (which might be doing call_rcu(rcu_sheaf) And then the pending call_rcu(rcu_sheaf) callbacks to be executed. I think that means both synchronize_rcu() and rcu_barrier(). Possibly a RCU critical section in __kfree_rcu_sheaf() unless local_lock implies that even on RT. > kvfree_rcu_barrier() as well? This (flush_all_rcu_sheaves()) is for the implementation of kvfree_rcu_barrier() in a world with rcu_free sheaves. So it's called from kvfree_rcu_barrier(). > It would not be hard to make such a thing, > using workqueues or some such. Not sure what the API should look like, So there should be no one else calling such an API. There might be new users of kvfree_rcu_barrier() doing this indirectly in the future. > especially should people want other RCU flavors to get into the act > as well. > > Thanx, Paul