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Tue, 9 Sep 2025 10:20:36 +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 p9sQOXT/v2g6HwAAD6G6ig (envelope-from ); Tue, 09 Sep 2025 10:20:36 +0000 Message-ID: <838888f2-7a0f-49f0-9e33-592f13b14ad3@suse.cz> Date: Tue, 9 Sep 2025 12:20:36 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 04/21] slab: add sheaf support for batching kfree_rcu() operations Content-Language: en-US To: Uladzislau Rezki Cc: Suren Baghdasaryan , "Liam R. Howlett" , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Sidhartha Kumar , linux-mm@kvack.org, linux-kernel@vger.kernel.org, rcu@vger.kernel.org, maple-tree@lists.infradead.org References: <20250903-slub-percpu-caches-v7-0-71c114cdefef@suse.cz> <20250903-slub-percpu-caches-v7-4-71c114cdefef@suse.cz> <6f8274da-a010-4bb3-b3d6-690481b5ace0@suse.cz> 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-Stat-Signature: iejc1o4x77urt8ndbc6chg46atfcomtj X-Rspamd-Queue-Id: B19911C0006 X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1757413240-584142 X-HE-Meta: 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 lmA8rLwW k/RNRQ/QlQoEHf76j+3PzR6BU9jgVI89FnRE4K8WSJA4FO5NywwYCkiJN95eSmfGqbkCKRsrAPZyt9HVOxU5ZAZCwTHk7tiZtSsVrZ43phEKyEFgHSJzgqRl15LnpQXnUWVn7n7KGC2gN98chN1XT4INe2z47kWcRqDDyYzlJxjS/gjiQDKkqlHMo4vKDH9/+sg4jngaSw+N9YHzAnuBOw2pvxkcWaiMgxroyPYwKPY/BvRKeOi+TueJr8CfGHlrbzRR6Havmv3l5ageKIzoVF18xuCfIql6Lmydne8Yo5ISsHZs= 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 9/9/25 11:08, Uladzislau Rezki wrote: > On Mon, Sep 08, 2025 at 02:45:11PM +0200, Vlastimil Babka wrote: >> >> Hm I could not find where that distinction is in the code, can you give a >> hint please. In __kfree_rcu_sheaf() I do only have a GFP_NOWAIT attempt. >> > For PREEMPT_RT a regular spin-lock is an rt-mutex which can sleep. We > made kvfree_rcu() to make it possible to invoke it from non-sleep contexts: Oh you mean it's not allocating even on !RT so there's no RT-specific code. > CONFIG_PREEMPT_RT > > preempt_disable() or something similar; > kvfree_rcu(); > GFP_NOWAIT - lock rt-mutex > > If GFP_NOWAIT semantic does not access any spin-locks then we are safe > or if it uses raw_spin_locks. It does access spinlocks so it's not safe. Thanks, I didn't realize that aspect of kfree_rcu(). We'll need to solve this before making sheaves enabled everywhere. I don't think the vma or maple tree code would kfree_rcu() vma or maple_node in such a restricted context. But to be safe I'll just disable the kfree rcu sheaves for PREEMPT_RT for now. >> > - call_rcu() can be slow, therefore we do not use it in the kvfree_rcu(). >> >> If call_rcu() is called once per 32 kfree_rcu() filling up the rcu sheaf, is >> it still too slow? >> > You do not know where in a queue this callback lands, in the beginning, > in the end, etc. It is part of generic list which is processed one by > one. It can contain thousands of callbacks. > > If performance is not needed then it is not an issue. But in > kvfree_rcu() we do not use it, because of we want to offload > fast. So IIUC one could call_rcu(obj, some_callback_accessing_obj) and then do a kfree_rcu(obj) and the latter can be processed first? > -- > Uladzislau Rezki