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Howlett" , Christoph Lameter , David Rientjes Cc: Roman Gushchin , Harry Yoo , Uladzislau Rezki , Sidhartha Kumar , linux-mm@kvack.org, linux-kernel@vger.kernel.org, rcu@vger.kernel.org, maple-tree@lists.infradead.org, linux-modules@vger.kernel.org, Luis Chamberlain , Petr Pavlu , Sami Tolvanen , Aaron Tomlin , Lucas De Marchi , "linux-tegra@vger.kernel.org" References: <20250910-slub-percpu-caches-v8-0-ca3099d8352c@suse.cz> <20250910-slub-percpu-caches-v8-4-ca3099d8352c@suse.cz> <0406562e-2066-4cf8-9902-b2b0616dd742@kernel.org> Content-Language: en-US From: Jon Hunter In-Reply-To: <0406562e-2066-4cf8-9902-b2b0616dd742@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: LO2P265CA0448.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:e::28) To SJ2PR12MB8784.namprd12.prod.outlook.com (2603:10b6:a03:4d0::11) Precedence: bulk X-Mailing-List: linux-modules@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SJ2PR12MB8784:EE_|CY8PR12MB8215:EE_ X-MS-Office365-Filtering-Correlation-Id: 31710289-d6da-444f-d745-08de2da98d57 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016|10070799003; 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Once full, >> the sheaf is detached and submitted to call_rcu() with a handler that >> will try to put it in the barn, or flush to slab pages using bulk free, >> when the barn is full. Then a new empty sheaf must be obtained to put >> more objects there. >> >> It's possible that no free sheaves are available to use for a new >> rcu_free sheaf, and the allocation in kfree_rcu() context can only use >> GFP_NOWAIT and thus may fail. In that case, fall back to the existing >> kfree_rcu() implementation. >> >> Expected advantages: >> - batching the kfree_rcu() operations, that could eventually replace the >> existing batching >> - sheaves can be reused for allocations via barn instead of being >> flushed to slabs, which is more efficient >> - this includes cases where only some cpus are allowed to process rcu >> callbacks (Android) >> >> Possible disadvantage: >> - objects might be waiting for more than their grace period (it is >> determined by the last object freed into the sheaf), increasing memory >> usage - but the existing batching does that too. >> >> Only implement this for CONFIG_KVFREE_RCU_BATCHED as the tiny >> implementation favors smaller memory footprint over performance. >> >> Also for now skip the usage of rcu sheaf for CONFIG_PREEMPT_RT as the >> contexts where kfree_rcu() is called might not be compatible with taking >> a barn spinlock or a GFP_NOWAIT allocation of a new sheaf taking a >> spinlock - the current kfree_rcu() implementation avoids doing that. >> >> Teach kvfree_rcu_barrier() to flush all rcu_free sheaves from all caches >> that have them. This is not a cheap operation, but the barrier usage is >> rare - currently kmem_cache_destroy() or on module unload. >> >> Add CONFIG_SLUB_STATS counters free_rcu_sheaf and free_rcu_sheaf_fail to >> count how many kfree_rcu() used the rcu_free sheaf successfully and how >> many had to fall back to the existing implementation. >> >> Signed-off-by: Vlastimil Babka > > Hi Vlastimil, > > This patch increases kmod selftest (stress module loader) runtime by about > ~50-60%, from ~200s to ~300s total execution time. My tested kernel has > CONFIG_KVFREE_RCU_BATCHED enabled. Any idea or suggestions on what might be > causing this, or how to address it? > I have been looking into a regression for Linux v6.18-rc where time taken to run some internal graphics tests on our Tegra234 device has increased from around 35% causing the tests to timeout. Bisect is pointing to this commit and I also see we have CONFIG_KVFREE_RCU_BATCHED=y. I have not tried disabling CONFIG_KVFREE_RCU_BATCHED=y but I can. I am not sure if there are any downsides to disabling this? Thanks Jon -- nvpublic