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Mon, 7 Oct 2024 07:00:06 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface MIME-Version: 1.0 Date: Mon, 07 Oct 2024 10:59:36 +0000 From: "Arnd Bergmann" To: "Sebastian Andrzej Siewior" , "Arnd Bergmann" Cc: "Dennis Zhou" , "Tejun Heo" , "Christoph Lameter" , "Thomas Gleixner" , "Will Deacon" , "kernel test robot" , "Andrew Morton" , "Suren Baghdasaryan" , "Kent Overstreet" , "Hou Tao" , "Jan Kara" , "Jianhui Zhou" <912460177@qq.com>, "Yury Norov" , linux-mm@kvack.org, linux-kernel@vger.kernel.org Message-Id: <98957f9e-aa9b-4d8c-8254-968905e85023@app.fastmail.com> In-Reply-To: <20241007104348.ImJPXDl9@linutronix.de> References: <20241004095702.637528-1-arnd@kernel.org> <20241007104348.ImJPXDl9@linutronix.de> Subject: Re: [PATCH] preempt_rt: increase PERCPU_DYNAMIC_SIZE_SHIFT for slab randomization Content-Type: text/plain Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 5BABF140021 X-Stat-Signature: hse9rzfrg481f3gag8wjrrj6zbdsk3rp X-Rspam-User: X-HE-Tag: 1728298808-784976 X-HE-Meta: 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 S51fgrPL /FgUtc7McPG/0asXgyoFpBGgZRjaCvMKW+qGfoQYbLOY9QREwg79HOFWZaC7Ld0ErXOwZZKlMpZ+CQw84PhhWHGgedJqeKGKHV/oIHLM2EBtTjkuHuoyIryoqFsqNBBEiJ4kDF5Preh7F6hDSAl3G04Ioy3scjXayfFCbJthZhnlvSjnu7TJ+ys1mwtGXha8O9VgWgeOTB5/mgAuqKC6vfa7uY7oGr/MFlpMYhKJNBHxqY2+6Ocg2VsXb2vygvVjukRr6CwAigua2W+4hsB+N2Stity5lE6T4MVdXFdUSDir5NF8= 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 Mon, Oct 7, 2024, at 10:43, Sebastian Andrzej Siewior wrote: > On 2024-10-04 09:56:56 [+0000], Arnd Bergmann wrote: > How bad is it, to have PERCPU_DYNAMIC_SIZE_SHIFT unconditionally set to > 13? If it is bad could we restrict it with LOCKDEP and PAGE_SIZE > 4KiB? > > So maybe something like this: > > diff --git a/include/linux/percpu.h b/include/linux/percpu.h > index b6321fc491598..52b5ea663b9f0 100644 > --- a/include/linux/percpu.h > +++ b/include/linux/percpu.h > @@ -41,7 +41,11 @@ > PCPU_MIN_ALLOC_SHIFT) > > #ifdef CONFIG_RANDOM_KMALLOC_CACHES > -#define PERCPU_DYNAMIC_SIZE_SHIFT 12 > +# if defined(CONFIG_LOCKDEP) && !defined(CONFIG_PAGE_SIZE_4KB) > +# define PERCPU_DYNAMIC_SIZE_SHIFT 13 > +# else > +# define PERCPU_DYNAMIC_SIZE_SHIFT 12 > +#endif /* LOCKDEP and PAGE_SIZE > 4KiB */ > #else > #define PERCPU_DYNAMIC_SIZE_SHIFT 10 > #endif I think that's fine. If you have lockdep and large page sizes, the percpu memory area is entirely lost in the noise of the overhead you already get. For your version: Acked-by: Arnd Bergmann Reported-by: Arnd Bergmann Can you pick that up for rt fixes (if you already have a tree) or send it to Andrew for the mm tree? Feel free to take my changelog text. Arnd