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Re: [RFC][PATCHv5 06/13] printk: register PM notifier

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On Tuesday, August 15, 2017 4:56:18 AM CEST Sergey Senozhatsky wrote:
> It's not always possible/safe to wake_up() printk kernel
> thread. For example, late suspend/early resume may printk()
> while timekeeping is not initialized yet, so calling into the
> scheduler may result in recursive warnings.
> 
> Another thing to notice is the fact that PM at some point
> freezes user space and kernel threads: freeze_processes()
> and freeze_kernel_threads(), correspondingly. Thus we need
> printk() to operate in emergency mode there and attempt to
> immediately flush pending kernel message to the console.
> 
> This patch registers PM notifier, so PM can switch printk
> to emergency mode from PM_FOO_PREPARE notifiers and return

Isn't that too early?  That's before user space is frozen even.

> back to printk threaded mode from PM_POST_FOO notifiers.

And isn't that too late?

> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Suggested-by: Andreas Mohr <andi@lisas.de>
> ---
>  kernel/printk/printk.c | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 05165f008bc8..d3f149fad85c 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -49,6 +49,7 @@
>  #include <linux/sched/debug.h>
>  #include <linux/sched/task_stack.h>
>  #include <linux/kthread.h>
> +#include <linux/suspend.h>
>  
>  #include <linux/uaccess.h>
>  #include <asm/sections.h>
> @@ -2913,6 +2914,33 @@ static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
>  	.flags = IRQ_WORK_LAZY,
>  };
>  
> +static int printk_pm_notify(struct notifier_block *notify_block,
> +			    unsigned long mode, void *unused)
> +{
> +	switch (mode) {
> +	case PM_HIBERNATION_PREPARE:
> +	case PM_SUSPEND_PREPARE:
> +	case PM_RESTORE_PREPARE:
> +		printk_emergency_begin_sync();

I'm not sure what would be wrong with calling this directly
from dpm_suspend_noirq().

> +		break;
> +
> +	case PM_POST_SUSPEND:
> +	case PM_POST_HIBERNATION:
> +	case PM_POST_RESTORE:
> +		printk_emergency_end_sync();

And this could be called from dpm_resume_noirq().

In which case you wouldn't really need the stuff below.

Thanks,
Rafael

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Thread

[RFC][PATCHv5 00/13] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:00 +0200
  [RFC][PATCHv5 12/13] printk: do not cond_resched() when we can offload Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 10/13] printk: force printk_kthread to offload printing Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 04/13] printk: add enforce_emergency parameter Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 07/13] printk: register syscore notifier Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
    Re: [RFC][PATCHv5 07/13] printk: register syscore notifier "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2017-08-15 14:10 +0200
      Re: [RFC][PATCHv5 07/13] printk: register syscore notifier Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-08-16 09:00 +0200
        Re: [RFC][PATCHv5 07/13] printk: register syscore notifier "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2017-08-16 15:10 +0200
  [RFC][PATCHv5 11/13] printk: always offload printing from user-space processes Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 05/13] printk: enable printk offloading Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 01/13] printk: move printk_pending out of per-cpu Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 02/13] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 08/13] printk: set watchdog_thresh as maximum value for atomic_print_limit Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 03/13] printk: add sync printk_emergency API Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 09/13] printk: add auto-emergency enforcement mechanism Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 13/13] printk: move offloading logic to per-cpu Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
  [RFC][PATCHv5 06/13] printk: register PM notifier Sergey Senozhatsky <sergey.senozhatsky@gmail.com> - 2017-08-15 05:10 +0200
    Re: [RFC][PATCHv5 06/13] printk: register PM notifier "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2017-08-15 14:00 +0200
      Re: [RFC][PATCHv5 06/13] printk: register PM notifier Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-08-16 09:40 +0200
        Re: [RFC][PATCHv5 06/13] printk: register PM notifier "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2017-08-16 15:10 +0200
          Re: [RFC][PATCHv5 06/13] printk: register PM notifier Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-08-17 08:00 +0200
            Re: [RFC][PATCHv5 06/13] printk: register PM notifier "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2017-08-17 18:00 +0200
              Re: [RFC][PATCHv5 06/13] printk: register PM notifier Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-08-18 01:20 +0200
  Re: [RFC][PATCHv5 00/13] printk: introduce printing kernel thread Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2017-08-23 10:40 +0200

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