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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0AFCFC43334 for ; Sun, 19 Jun 2022 20:43:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237027AbiFSUnw (ORCPT ); Sun, 19 Jun 2022 16:43:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42276 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237414AbiFSUnm (ORCPT ); Sun, 19 Jun 2022 16:43:42 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CDDB75FA3 for ; Sun, 19 Jun 2022 13:43:39 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 6AD8961291 for ; Sun, 19 Jun 2022 20:43:39 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 59C50C34114; Sun, 19 Jun 2022 20:43:38 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="mndrBOb3" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1655671416; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=QqJKD6zg80rBX8SlDSTLLX0THmaiPu1I0aoLaI/MT+0=; b=mndrBOb3yGaFbzWLVLWMtuuskGO4+qlGc8Z7ojrxAWw/TeCqqisQ0mFrGWKs8FwQ1TS2y7 H87ZNHUmbl3DMyQMsZPRxIM4gSecyh6waLaP+mHD3dnsdMN6/4m80Dd9yaIGANX8HeyYa7 WpQfQIQvDShv8cC00FzL34ZX6YrvGC8= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 21cd46c7 (TLSv1.3:AEAD-AES256-GCM-SHA384:256:NO); Sun, 19 Jun 2022 20:43:36 +0000 (UTC) From: "Jason A. Donenfeld" To: John Ogness , Petr Mladek , Marco Elver , linux-kernel@vger.kernel.org Cc: "Jason A. Donenfeld" Subject: [PATCH v2] printk: allow direct console printing to be enabled always Date: Sun, 19 Jun 2022 22:43:26 +0200 Message-Id: <20220619204326.556923-1-Jason@zx2c4.com> In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org In 5.19, there are some changes in printk message ordering / interleaving which leads to confusion. The most obvious (and benign) example appears on system boot, in which the "Run /init as init process" message gets intermixed with the messages that init actually writes() to stdout. For example, here's a snippet from build.wireguard.com: [ 0.469732] Freeing unused kernel image (initmem) memory: 4576K [ 0.469738] Write protecting the kernel read-only data: 10240k [ 0.473823] Freeing unused kernel image (text/rodata gap) memory: 2044K [ 0.475228] Freeing unused kernel image (rodata/data gap) memory: 1136K [ 0.475236] Run /init as init process WireGuard Test Suite on Linux 5.19.0-rc2+ x86_64 [+] Mounting filesystems... [+] Module self-tests: * allowedips self-tests: pass * nonce counter self-tests: pass * ratelimiter self-tests: pass [+] Enabling logging... [+] Launching tests... [ 0.475237] with arguments: [ 0.475238] /init [ 0.475238] with environment: [ 0.475239] HOME=/ [ 0.475240] TERM=linux [+] ip netns add wg-test-46-0 [+] ip netns add wg-test-46-1 Before the "with arguments:" and such would print prior to the "wireguard test suite on linux 5.19" banner. Now it shows after. I see the same thing with "Freeing unused kernel image (text/rodata gap) memory" printing interwoven into the console of my initramfs on my laptop. And so forth. But the bigger issue for me is that it makes it very confusing to interpret CI results later on. Prior, I would nice a nice correlation of: [+] some userspace command [ 1.2345 ] some kernel log output [+] some userspace command [ 1.2346 ] some kernel log output [+] some userspace command [ 1.2347 ] some kernel log output Now, the kernel log outputs are all over the place and out of order with the sequence of commands. This makes debugging issues somewhat tricky, because post hoc ergo propter hoc winds up being a good intuition to follow when tracking down bugs, and now the post hoc part is muddled. This is caused by threaded printk. In order to restore this in debugging sessions and in CI, this commit adds the ability to always use direct printk, either set by default at compile time, or overridden with a runtime command line switch. Cc: John Ogness Cc: Petr Mladek Cc: Marco Elver Fixes: 09c5ba0aa2fc ("printk: add kthread console printers") Signed-off-by: Jason A. Donenfeld --- Documentation/admin-guide/kernel-parameters.txt | 7 +++++++ init/Kconfig | 12 ++++++++++++ kernel/printk/printk.c | 11 +++++++++++ 3 files changed, 30 insertions(+) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 8090130b544b..96e910a4e12b 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4389,6 +4389,13 @@ printk.time= Show timing data prefixed to each printk message line Format: (1/Y/y=enable, 0/N/n=disable) + printk.direct= Rather than using kthreads for printk output, always + attempt to write to the console immediately. This has + performance implications, but will result in a more + faithful ordering and interleaving with other + processes writing to the console. + Format: (1/Y/y=enable, 0/N/n=disable) + processor.max_cstate= [HW,ACPI] Limit processor to maximum C-state max_cstate=9 overrides any DMI blacklist limit. diff --git a/init/Kconfig b/init/Kconfig index c7900e8975f1..47466aa2b0e8 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -798,6 +798,18 @@ config PRINTK_INDEX There is no additional runtime cost to printk with this enabled. +config PRINTK_DIRECT + bool "Attempt to flush printk output immediately" + depends on PRINTK + help + Rather than using kthreads for printk output, always attempt to write + to the console immediately. This has performance implications, but + will result in a more faithful ordering and interleaving with other + processes writing to the console. + + Say N here unless you really need this. This may also be controlled + at boot time with printk.direct=0/1. + # # Architectures with an unreliable sched_clock() should select this: # diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index ea3dd55709e7..43f8a0074ed6 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -178,6 +178,14 @@ static int __init control_devkmsg(char *str) } __setup("printk.devkmsg=", control_devkmsg); +static bool printk_direct = IS_ENABLED(CONFIG_PRINTK_DIRECT); + +static int __init control_printk_direct(char *str) +{ + return kstrtobool(str, &printk_direct); +} +__setup("printk.direct=", control_printk_direct); + char devkmsg_log_str[DEVKMSG_STR_MAX_SIZE] = "ratelimit"; #if defined(CONFIG_PRINTK) && defined(CONFIG_SYSCTL) int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write, @@ -3602,6 +3610,9 @@ static int __init printk_activate_kthreads(void) { struct console *con; + if (printk_direct) + return 0; + console_lock(); printk_kthreads_available = true; for_each_console(con) -- 2.35.1