From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754046AbcDFAR4 (ORCPT ); Tue, 5 Apr 2016 20:17:56 -0400 Received: from mail-pf0-f194.google.com ([209.85.192.194]:36467 "EHLO mail-pf0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752819AbcDFARy (ORCPT ); Tue, 5 Apr 2016 20:17:54 -0400 Date: Wed, 6 Apr 2016 09:19:19 +0900 From: Sergey Senozhatsky To: Andrew Morton Cc: Sergey Senozhatsky , Sergey Senozhatsky , Jan Kara , Petr Mladek , Tejun Heo , Tetsuo Handa , linux-kernel@vger.kernel.org, Byungchul Park , Jan Kara Subject: Re: [PATCH v10 1/2] printk: Make printk() completely async Message-ID: <20160406001919.GA601@swordfish> References: <1459789048-1337-1-git-send-email-sergey.senozhatsky@gmail.com> <1459789048-1337-2-git-send-email-sergey.senozhatsky@gmail.com> <20160404155149.a3e3307def2d1315e2099c63@linux-foundation.org> <20160405051717.GB1954@swordfish> <20160405073932.GA513@swordfish> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160405073932.GA513@swordfish> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, so I currently have something like this. untouched things: - module param is RO; offload printing to kthread (I'll wait for opinions/replies). ======= >>From 1e7de1f9a590d8f23609f943362768e4c14580cc Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Tue, 5 Apr 2016 01:57:27 +0900 Subject: [PATCH] printk: Make printk() completely async Currently, printk() sometimes waits for message to be printed to console and sometimes it does not (when console_sem is held by some other process). In case printk() grabs console_sem and starts printing to console, it prints messages from kernel printk buffer until the buffer is empty. When serial console is attached, printing is slow and thus other CPUs in the system have plenty of time to append new messages to the buffer while one CPU is printing. Thus the CPU can spend unbounded amount of time doing printing in console_unlock(). This is especially serious problem if the printk() calling console_unlock() was called with interrupts disabled. In practice users have observed a CPU can spend tens of seconds printing in console_unlock() (usually during boot when hundreds of SCSI devices are discovered) resulting in RCU stalls (CPU doing printing doesn't reach quiescent state for a long time), softlockup reports (IPIs for the printing CPU don't get served and thus other CPUs are spinning waiting for the printing CPU to process IPIs), and eventually a machine death (as messages from stalls and lockups append to printk buffer faster than we are able to print). So these machines are unable to boot with serial console attached. Another observed issue is that due to slow printk, hardware discovery is slow and udev times out before kernel manages to discover all the attached HW. Also during artificial stress testing SATA disk disappears from the system because its interrupts aren't served for too long. This patch makes printk() completely asynchronous (similar to what printk_deferred() did until now). It appends message to the kernel printk buffer and wake_up()s a special dedicated kthread to do the printing to console. This has the advantage that printing always happens from a schedulable contex and thus we don't lockup any particular CPU or even interrupts. Also it has the advantage that printk() is fast and thus kernel booting is not slowed down by slow serial console. Disadvantage of this method is that in case of crash there is higher chance that important messages won't appear in console output (we may need working scheduling to print message to console). We somewhat mitigate this risk by switching printk to the original method of immediate printing to console if oops is in progress. Also for debugging purposes we provide printk.synchronous kernel parameter which resorts to the original printk behavior. printk() is expected to work under different conditions and in different scenarios, including corner cases of OOM when all of the workers are busy (e.g. allocating memory), thus printk() uses its own dedicated printing kthread, rather than relying on workqueue (even with WQ_MEM_RECLAIM bit set we potentially can receive delays in printing until workqueue declares a ->mayday, as noted by Tetsuo Handa). Signed-off-by: Jan Kara Signed-off-by: Sergey Senozhatsky --- Documentation/kernel-parameters.txt | 10 ++++ kernel/printk/printk.c | 94 ++++++++++++++++++++++++++++++++++--- 2 files changed, 97 insertions(+), 7 deletions(-) diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index 46c0b19..733a13d 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -3117,6 +3117,16 @@ bytes respectively. Such letter suffixes can also be entirely omitted. printk.time= Show timing data prefixed to each printk message line Format: (1/Y/y=enable, 0/N/n=disable) + printk.synchronous= + By default kernel messages are printed to console + asynchronously (except during early boot or when oops + is happening). That avoids kernel stalling behind slow + serial console and thus avoids softlockups, interrupt + timeouts, or userspace timing out during heavy printing. + However for debugging problems, printing messages to + console immediately may be desirable. This option + enables such behavior. + processor.max_cstate= [HW,ACPI] Limit processor to maximum C-state max_cstate=9 overrides any DMI blacklist limit. diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index bfbf284..217f27a 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -46,6 +46,7 @@ #include #include #include +#include #include #include @@ -284,6 +285,16 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN); static char *log_buf = __log_buf; static u32 log_buf_len = __LOG_BUF_LEN; +/* Control whether printing to console must be synchronous. */ +static bool __read_mostly printk_sync = !IS_ENABLED(CONFIG_SMP); +module_param_named(synchronous, printk_sync, bool, S_IRUGO); +MODULE_PARM_DESC(synchronous, "make printing to console synchronous"); + +/* Printing kthread for async printk */ +static struct task_struct *printk_kthread; +/* When `true' printing thread has messages to print */ +static bool printk_kthread_need_flush_console; + /* Return log buffer address */ char *log_buf_addr_get(void) { @@ -1608,6 +1619,8 @@ asmlinkage int vprintk_emit(int facility, int level, const char *dict, size_t dictlen, const char *fmt, va_list args) { + /* cpu currently holding logbuf_lock in this function */ + static unsigned int logbuf_cpu = UINT_MAX; static bool recursion_bug; static char textbuf[LOG_LINE_MAX]; char *text = textbuf; @@ -1617,8 +1630,6 @@ asmlinkage int vprintk_emit(int facility, int level, int this_cpu; int printed_len = 0; bool in_sched = false; - /* cpu currently holding logbuf_lock in this function */ - static unsigned int logbuf_cpu = UINT_MAX; if (level == LOGLEVEL_SCHED) { level = LOGLEVEL_DEFAULT; @@ -1757,12 +1768,35 @@ asmlinkage int vprintk_emit(int facility, int level, if (!in_sched) { lockdep_off(); /* - * Try to acquire and then immediately release the console - * semaphore. The release will print out buffers and wake up - * /dev/kmsg and syslog() users. + * Attempt to print the messages to console asynchronously so + * that the kernel doesn't get stalled due to slow serial + * console. That can lead to softlockups, lost interrupts, or + * userspace timing out under heavy printing load. + * + * However we resort to synchronous printing of messages during + * early boot, when synchronous printing was explicitly + * requested by a kernel parameter, or when console_verbose() + * was called to print everything during panic / oops. + * Unlike bust_spinlocks() and oops_in_progress, + * console_verbose() sets console_loglevel to MOTORMOUTH and + * never clears it, while oops_in_progress can go back to 0, + * switching printk back to async mode; we want printk to + * operate in sync mode once panic() occurred. */ - if (console_trylock()) - console_unlock(); + if (console_loglevel != CONSOLE_LOGLEVEL_MOTORMOUTH && + printk_kthread) { + /* Offload printing to a schedulable context. */ + printk_kthread_need_flush_console = true; + wake_up_process(printk_kthread); + } else { + /* + * Try to acquire and then immediately release the + * console semaphore. The release will print out + * buffers and wake up /dev/kmsg and syslog() users. + */ + if (console_trylock()) + console_unlock(); + } lockdep_on(); } @@ -2722,6 +2756,52 @@ static int __init printk_late_init(void) late_initcall(printk_late_init); #if defined CONFIG_PRINTK +static int printk_kthread_func(void *data) +{ + while (1) { + set_current_state(TASK_INTERRUPTIBLE); + if (!printk_kthread_need_flush_console) + schedule(); + + __set_current_state(TASK_RUNNING); + /* + * Avoid an infinite loop when console_unlock() cannot + * access consoles, e.g. because console_suspended is + * true. schedule(), someone else will print the messages + * from resume_console(). + */ + printk_kthread_need_flush_console = false; + + console_lock(); + console_unlock(); + } + + return 0; +} + +static int __init init_printk_kthread(void) +{ + struct task_struct *thread; + + if (printk_sync) + return 0; + + thread = kthread_run(printk_kthread_func, NULL, "printk"); + if (IS_ERR(thread)) { + pr_err("printk: unable to create printing thread\n"); + printk_sync = true; + } else { + struct sched_param param = { + .sched_priority = MAX_RT_PRIO - 1, + }; + + sched_setscheduler(thread, SCHED_FIFO, ¶m); + printk_kthread = thread; + } + return 0; +} +core_initcall(init_printk_kthread); + /* * Delayed printk version, for scheduler-internal messages: */ -- 2.8.0.rc0.1.gd285ab0