From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B8CBFC32789 for ; Sat, 3 Nov 2018 01:56:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6E7FD2082D for ; Sat, 3 Nov 2018 01:56:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6E7FD2082D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=i-love.sakura.ne.jp Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728473AbeKCLFd (ORCPT ); Sat, 3 Nov 2018 07:05:33 -0400 Received: from www262.sakura.ne.jp ([202.181.97.72]:63484 "EHLO www262.sakura.ne.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728233AbeKCLFc (ORCPT ); Sat, 3 Nov 2018 07:05:32 -0400 Received: from fsav404.sakura.ne.jp (fsav404.sakura.ne.jp [133.242.250.103]) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTP id wA31tuI5080820; Sat, 3 Nov 2018 10:55:56 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Received: from www262.sakura.ne.jp (202.181.97.72) by fsav404.sakura.ne.jp (F-Secure/fsigk_smtp/530/fsav404.sakura.ne.jp); Sat, 03 Nov 2018 10:55:56 +0900 (JST) X-Virus-Status: clean(F-Secure/fsigk_smtp/530/fsav404.sakura.ne.jp) Received: from [192.168.1.8] (softbank060157065137.bbtec.net [60.157.65.137]) (authenticated bits=0) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTPSA id wA31tuRv080814 (version=TLSv1.2 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO); Sat, 3 Nov 2018 10:55:56 +0900 (JST) (envelope-from penguin-kernel@i-love.sakura.ne.jp) Subject: Re: [PATCH v6 1/3] printk: Add line-buffered printk() API. To: Matthew Wilcox Cc: Petr Mladek , Sergey Senozhatsky , Sergey Senozhatsky , Dmitriy Vyukov , Steven Rostedt , Alexander Potapenko , Fengguang Wu , Josh Poimboeuf , LKML , Linus Torvalds , Andrew Morton , linux-mm@kvack.org, Ingo Molnar , Peter Zijlstra , Will Deacon References: <1541165517-3557-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> <20181102144028.GQ10491@bombadil.infradead.org> From: Tetsuo Handa Message-ID: <865018bd-6352-cb92-1b8a-9254768f0b5c@i-love.sakura.ne.jp> Date: Sat, 3 Nov 2018 10:55:57 +0900 User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: <20181102144028.GQ10491@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2018/11/02 23:40, Matthew Wilcox wrote: > On Fri, Nov 02, 2018 at 10:31:55PM +0900, Tetsuo Handa wrote: >> get_printk_buffer() tries to assign a "struct printk_buffer" from >> statically preallocated array. get_printk_buffer() returns NULL if >> all "struct printk_buffer" are in use, but the caller does not need to >> check for NULL. > > This seems like a great way of wasting 16kB of memory. Since you've > already made printk_buffered() work with a NULL initial argument, what's > the advantage over just doing kmalloc(1024, GFP_ATOMIC)? Like "[PATCH 2/3] mm: Use line-buffered printk() for show_free_areas()." demonstrates, kzalloc(sizeof(struct printk_buffer), GFP_ATOMIC) can fail. And using statically preallocated buffers helps avoiding (1) out of buffers when memory cannot be allocated (2) kernel stack overflow when kernel stack is already tight (e.g. a memory allocation attempt from an interrupt handler which was invoked from deep inside call chain of a process context) . Whether (A) tuning the number of statically preallocated buffers (B) allocating buffers on caller side (e.g. kzalloc() or in .bss section) are useful is a future decision, for too much concurrent printk() will lockup the system even if there are enough buffers. I think that starting with statically preallocated buffers is (at least for now) a good choice for minimizing risk of (1) (2) while offering practically acceptable result.