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Date: Wed, 14 Nov 2012 23:26:57 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: Ingo Molnar <mingo@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH 7/7] printk: Wake up klogd using irq_work
On Wed, 2012-11-14 at 21:37 +0100, Frederic Weisbecker wrote:
> klogd is woken up asynchronously from the tick in order
> to do it safely.
>
> However if printk is called when the tick is stopped, the reader
> won't be woken up until the next interrupt, which might not fire
> for a while. As a result, the user may miss some message.
>
> To fix this, lets implement the printk tick using a lazy irq work.
> This subsystem takes care of the timer tick state and can
> fix up accordingly.
>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Paul Gortmaker <paul.gortmaker@...driver.com>
> ---
> include/linux/printk.h | 3 ---
> init/Kconfig | 1 +
> kernel/printk.c | 36 ++++++++++++++++++++----------------
> kernel/time/tick-sched.c | 2 +-
> kernel/timer.c | 1 -
> 5 files changed, 22 insertions(+), 21 deletions(-)
>
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 9afc01e..86c4b62 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -98,9 +98,6 @@ int no_printk(const char *fmt, ...)
> extern asmlinkage __printf(1, 2)
> void early_printk(const char *fmt, ...);
>
> -extern int printk_needs_cpu(int cpu);
> -extern void printk_tick(void);
> -
> #ifdef CONFIG_PRINTK
> asmlinkage __printf(5, 0)
> int vprintk_emit(int facility, int level,
> diff --git a/init/Kconfig b/init/Kconfig
> index cdc152c..c575566 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1196,6 +1196,7 @@ config HOTPLUG
> config PRINTK
> default y
> bool "Enable support for printk" if EXPERT
> + select IRQ_WORK
> help
> This option enables normal printk support. Removing it
> eliminates most of the message strings from the kernel image
> diff --git a/kernel/printk.c b/kernel/printk.c
> index 2d607f4..c9104fe 100644
> --- a/kernel/printk.c
> +++ b/kernel/printk.c
> @@ -42,6 +42,7 @@
> #include <linux/notifier.h>
> #include <linux/rculist.h>
> #include <linux/poll.h>
> +#include <linux/irq_work.h>
>
> #include <asm/uaccess.h>
>
> @@ -1955,30 +1956,32 @@ int is_console_locked(void)
> static DEFINE_PER_CPU(int, printk_pending);
> static DEFINE_PER_CPU(char [PRINTK_BUF_SIZE], printk_sched_buf);
>
> -void printk_tick(void)
> +static void wake_up_klogd_work_func(struct irq_work *irq_work)
> {
> - if (__this_cpu_read(printk_pending)) {
> - int pending = __this_cpu_xchg(printk_pending, 0);
> - if (pending & PRINTK_PENDING_SCHED) {
> - char *buf = __get_cpu_var(printk_sched_buf);
> - printk(KERN_WARNING "[sched_delayed] %s", buf);
> - }
> - if (pending & PRINTK_PENDING_WAKEUP)
> - wake_up_interruptible(&log_wait);
> + int pending = __this_cpu_xchg(printk_pending, 0);
> +
> + if (pending & PRINTK_PENDING_SCHED) {
> + char *buf = __get_cpu_var(printk_sched_buf);
> + printk(KERN_WARNING "[sched_delayed] %s", buf);
> }
> -}
>
> -int printk_needs_cpu(int cpu)
> -{
> - if (cpu_is_offline(cpu))
> - printk_tick();
I'm a little worried about this patch, because of the above code.
(see below)
> - return __this_cpu_read(printk_pending);
> + 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)
> {
> - if (waitqueue_active(&log_wait))
> + preempt_disable();
> + if (waitqueue_active(&log_wait)) {
> this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
> + irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
> + }
> + preempt_enable();
> }
>
> static void console_cont_flush(char *text, size_t size)
> @@ -2458,6 +2461,7 @@ int printk_sched(const char *fmt, ...)
> va_end(args);
>
> __this_cpu_or(printk_pending, PRINTK_PENDING_SCHED);
> + irq_work_queue(&__get_cpu_var(wake_up_klogd_work));
> local_irq_restore(flags);
>
> return r;
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index f249e8c..822d757 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -289,7 +289,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> time_delta = timekeeping_max_deferment();
> } while (read_seqretry(&xtime_lock, seq));
>
> - if (rcu_needs_cpu(cpu, &rcu_delta_jiffies) || printk_needs_cpu(cpu) ||
> + if (rcu_needs_cpu(cpu, &rcu_delta_jiffies) ||
If the CPU is going offline, the printk_tick() would be executed here.
But now that printk_tick() is done with the irq_work code, it wont be
executed till the next tick. Could this cause a missed printk because
of this, if the cpu is going offline?
Actually, how does irq_work in general handle cpu offline work?
-- Steve
> arch_needs_cpu(cpu) || irq_work_needs_cpu()) {
> next_jiffies = last_jiffies + 1;
> delta_jiffies = 1;
> diff --git a/kernel/timer.c b/kernel/timer.c
> index 367d008..ff3b516 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -1351,7 +1351,6 @@ void update_process_times(int user_tick)
> account_process_tick(p, user_tick);
> run_local_timers();
> rcu_check_callbacks(cpu, user_tick);
> - printk_tick();
> #ifdef CONFIG_IRQ_WORK
> if (in_irq())
> irq_work_run();
--
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