Re: [RFC][PATCH v4 1/2] printk: Make printk() completely async

Byungchul Park
2016-03-16T05:40:22Z

On Tue, Mar 15, 2016 at 11:07:38PM +0900, Sergey Senozhatsky wrote:
>
> something like this (*minimally tested so far*).
>
> -- move wake_up() and friends under the logbuf section; so we can detect
^^^^^^^^^^^^^^
section protected by logbuf_lock?

> printk() recursion from wake_up()

Excuse me, but I fear that it can cause an unnecessary deadlock.
Furthermore it cannot be handled if it is caused by logbuf_lock. IMHO,
nothing including synced printk can help us in this case. Is there
something I missed? I'am sorry if I did, but could you let me know if so?

I mean that it would be better to keep the wake_up and friend out of the
critical section by logbuf_lock.

Thanks,
Byungchul

>
> -- in recursion_bug branch switch to sync printk. we don't know why did we
> recurse, may be because of wake_up()->spin_lock(). doing
> kthread_stop()->wake_up_process() can be unsafe, I guess, just set
> `printk_sync' to true.
>
>
> Signed-off-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
> Documentation/kernel-parameters.txt | 10 ++
> kernel/printk/printk.c | 202 +++++++++++++++++++++++++-----------
> 2 files changed, 154 insertions(+), 58 deletions(-)
>
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 1e58ae9..454999e 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -3114,6 +3114,16 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> printk.time= Show timing data prefixed to each printk message line
> Format: <bool> (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 d5fd844..38baed1 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -46,6 +46,7 @@
> #include <linux/utsname.h>
> #include <linux/ctype.h>
> #include <linux/uio.h>
> +#include <linux/kthread.h>
>
> #include <asm/uaccess.h>
> #include <asm-generic/sections.h>
> @@ -284,6 +285,105 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
> static char *log_buf = __log_buf;
> static u32 log_buf_len = __LOG_BUF_LEN;
>
> +/*
> + * When true, printing to console will happen synchronously unless someone else
> + * is already printing messages.
> + *
> + * The default value on UP systems is 'true'.
> + */
> +static bool __read_mostly printk_sync = !IS_ENABLED(CONFIG_SMP);
> +module_param_named(synchronous, printk_sync, bool, S_IRUGO | S_IWUSR);
> +MODULE_PARM_DESC(synchronous, "make printing to console synchronous");
> +
> +/* Printing kthread for async vprintk_emit() */
> +static struct task_struct *printk_thread;
> +/* Wait for printing wakeups from async vprintk_emit() */
> +static DECLARE_WAIT_QUEUE_HEAD(printing_wait);
> +
> +static int printing_func(void *data)
> +{
> + while (1) {
> + DECLARE_WAITQUEUE(wait, current);
> + set_current_state(TASK_INTERRUPTIBLE);
> + add_wait_queue(&printing_wait, &wait);
> +
> + schedule();
> + remove_wait_queue(&printing_wait, &wait);
> +
> + console_lock();
> + console_unlock();
> + }
> +
> + return 0;
> +}
> +
> +static int __init init_printk_thread(void)
> +{
> + if (printk_sync)
> + return 0;
> +
> + printk_thread = kthread_run(printing_func, NULL, "printk");
> + BUG_ON(IS_ERR(printk_thread));
> + return 0;
> +}
> +late_initcall(init_printk_thread);
> +
> +/*
> + * Delayed printk version, for scheduler-internal messages:
> + */
> +#define PRINTK_PENDING_WAKEUP (1<<0)
> +#define PRINTK_PENDING_OUTPUT (1<<1)
> +
> +static DEFINE_PER_CPU(int, printk_pending);
> +
> +static void wake_up_klogd_work_func(struct irq_work *irq_work)
> +{
> + int pending = __this_cpu_xchg(printk_pending, 0);
> +
> + if (pending & PRINTK_PENDING_OUTPUT) {
> + if (!printk_sync && printk_thread) {
> + wake_up(&printing_wait);
> + } else {
> + /*
> + * If trylock fails, someone else is doing
> + * the printing
> + */
> + if (console_trylock())
> + console_unlock();
> + }
> + }
> +
> + if (pending & PRINTK_PENDING_WAKEUP)
> + wake_up_interruptible(&log_wait);
> +}
> +
> +static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
> + .func = wake_up_klogd_work_func,
> + .flags = IRQ_WORK_LAZY,
> +};
> +
> +void wake_up_klogd(void)
> +{
> + preempt_disable();
> + if (waitqueue_active(&log_wait)) {
> + this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
> + irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> + }
> + preempt_enable();
> +}
> +
> +int printk_deferred(const char *fmt, ...)
> +{
> + va_list args;
> + int r;
> +
> + va_start(args, fmt);
> + r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
> + va_end(args);
> +
> + return r;
> +}
> +
> /* Return log buffer address */
> char *log_buf_addr_get(void)
> {
> @@ -1609,6 +1709,8 @@ 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;
> @@ -1618,12 +1720,17 @@ 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;
> + bool in_panic = console_loglevel == CONSOLE_LOGLEVEL_MOTORMOUTH;
> + bool sync_print = printk_sync;
>
> if (level == LOGLEVEL_SCHED) {
> level = LOGLEVEL_DEFAULT;
> + /*
> + * Deferred sched messages must not be printed
> + * synchronously regardless the @printk_sync or @in_panic.
> + */
> in_sched = true;
> + sync_print = false;
> }
>
> boot_delay_msec(level);
> @@ -1660,6 +1767,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> static const char recursion_msg[] =
> "BUG: recent printk recursion!";
>
> + printk_sync = true;
> recursion_bug = false;
> /* emit KERN_CRIT message */
> printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,
> @@ -1749,13 +1857,45 @@ asmlinkage int vprintk_emit(int facility, int level,
> dict, dictlen, text, text_len);
> }
>
> + /*
> + * By default we print message to console asynchronously so that 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
> + * kernel parameter, or when console_verbose() was called to print
> + * everything during panic / oops.
> + */
> + if (!sync_print) {
> + if (in_sched) {
> + /*
> + * @in_sched messages may come too early, when we don't
> + * yet have @printk_thread. We can't print deferred
> + * messages directly, because this may deadlock, route
> + * them via IRQ context.
> + */
> + __this_cpu_or(printk_pending,
> + PRINTK_PENDING_OUTPUT);
> + irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> + } else if (printk_thread && !in_panic) {
> + /*
> + * This will wakeup the printing kthread and offload
> + * printing to a schedulable context.
> + */
> + wake_up(&printing_wait);
> + } else {
> + sync_print = true;
> + }
> + }
> +
> logbuf_cpu = UINT_MAX;
> raw_spin_unlock(&logbuf_lock);
> lockdep_on();
> local_irq_restore(flags);
>
> - /* If called from the scheduler, we can not call up(). */
> - if (!in_sched) {
> + if (sync_print) {
> lockdep_off();
> /*
> * Try to acquire and then immediately release the console
> @@ -2724,60 +2864,6 @@ late_initcall(printk_late_init);
>
> #if defined CONFIG_PRINTK
> /*
> - * Delayed printk version, for scheduler-internal messages:
> - */
> -#define PRINTK_PENDING_WAKEUP 0x01
> -#define PRINTK_PENDING_OUTPUT 0x02
> -
> -static DEFINE_PER_CPU(int, printk_pending);
> -
> -static void wake_up_klogd_work_func(struct irq_work *irq_work)
> -{
> - int pending = __this_cpu_xchg(printk_pending, 0);
> -
> - if (pending & PRINTK_PENDING_OUTPUT) {
> - /* If trylock fails, someone else is doing the printing */
> - if (console_trylock())
> - console_unlock();
> - }
> -
> - if (pending & PRINTK_PENDING_WAKEUP)
> - wake_up_interruptible(&log_wait);
> -}
> -
> -static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
> - .func = wake_up_klogd_work_func,
> - .flags = IRQ_WORK_LAZY,
> -};
> -
> -void wake_up_klogd(void)
> -{
> - preempt_disable();
> - if (waitqueue_active(&log_wait)) {
> - this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
> - irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> - }
> - preempt_enable();
> -}
> -
> -int printk_deferred(const char *fmt, ...)
> -{
> - va_list args;
> - int r;
> -
> - preempt_disable();
> - va_start(args, fmt);
> - r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
> - va_end(args);
> -
> - __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> - irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> - preempt_enable();
> -
> - return r;
> -}
> -
> -/*
> * printk rate limiting, lifted from the networking subsystem.
> *
> * This enforces a rate limit: not more than 10 kernel messages
> --
> 2.8.0.rc0