linux-kernel - Re: [PATCH] printk: stop spining waiter when console resume to flush prb

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Date:   Thu, 6 May 2021 09:13:23 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Luo Jiaxing <luojiaxing@...wei.com>
Cc:     <pmladek@...e.com>, <sergey.senozhatsky@...il.com>,
        <john.ogness@...utronix.de>, <linux-kernel@...r.kernel.org>,
        <linuxarm@...wei.com>, <bobo.shaobowang@...wei.com>
Subject: Re: [PATCH] printk: stop spining waiter when console resume to
 flush prb

On Thu, 6 May 2021 16:00:26 +0800
Luo Jiaxing <luojiaxing@...wei.com> wrote:

> Some threads still call printk() for printing when resume_console() is
> being executed. In practice, the printk() is executed for a period of time
> and then returned. The duration is determined by the number of prints
> cached in the prb during the suspend/resume process. At the same time,
> resume_console() returns quickly.
> 
> Base on owner/waiter machanism, the frist one who fail to lock console will
> become waiter, and start spining. When current owner finish print one
> informance, if a waiter is waitting, owner will give up and let waiter
> become a new owner. New owner need to flush the whole prb unitl prb empty
> or another new waiter come and take the job from him.
> 
> So the first waiter after resume_console() will take seconds to help to
> flush prb, but driver which call printk() may be bothered by this. New
> a flag to mark resume flushing prb. When the console resume, before the
> prb is empty, stop to set a new waiter temporarily.
> 
> Signed-off-by: Luo Jiaxing <luojiaxing@...wei.com>
> ---
>  kernel/printk/printk.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 575a34b..2c680a5 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -287,6 +287,9 @@ EXPORT_SYMBOL(console_set_on_cmdline);
>  /* Flag: console code may call schedule() */
>  static int console_may_schedule;
>  
> +/* Flags: console flushing prb when resume */
> +static atomic_t console_resume_flush_prb = ATOMIC_INIT(0);

Why are you using an atomic? It's accessed within locks.

static bool console_resuming;


> +
>  enum con_msg_format_flags {
>  	MSG_FORMAT_DEFAULT	= 0,
>  	MSG_FORMAT_SYSLOG	= (1 << 0),
> @@ -1781,7 +1784,8 @@ static int console_trylock_spinning(void)
>  	raw_spin_lock(&console_owner_lock);
>  	owner = READ_ONCE(console_owner);
>  	waiter = READ_ONCE(console_waiter);

	resuming = READ_ONCE(console_removing);

> -	if (!waiter && owner && owner != current) {

	if (!resuming && (!waiter ...

> +	if (!waiter && owner && owner != current &&
> +	    !atomic_read(&console_resume_flush_prb)) {
>  		WRITE_ONCE(console_waiter, true);
>  		spin = true;
>  	}
> @@ -2355,6 +2359,7 @@ void resume_console(void)
>  	if (!console_suspend_enabled)
>  		return;
>  	down_console_sem();
> +	atomic_set(&console_resume_flush_prb, 1);
>  	console_suspended = 0;

	resuming = true;

>  	console_unlock();

	/* Keep clearing resume from entering the console_unlock */
	smp_wmb();
	resuming = false;


>  }
> @@ -2592,6 +2597,8 @@ void console_unlock(void)
>  	raw_spin_unlock(&logbuf_lock);
>  
>  	up_console_sem();
> +	if (atomic_read(&console_resume_flush_prb))
> +		atomic_set(&console_resume_flush_prb, 0);

Get rid of the above.

-- Steve

>  
>  	/*
>  	 * Someone could have filled up the buffer again, so re-check if there's