linux-kernel - Re: [PATCH] watchdog: Avoid 100% CPU usage during reading watchdog when a task get signal

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Date:   Thu, 18 May 2023 10:49:04 -0500
From:   Corey Minyard <minyard@....org>
To:     Yu Chen <chen.yu@...ystack.cn>
Cc:     openipmi-developer@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] watchdog: Avoid 100% CPU usage during reading watchdog
 when a task get signal

On Mon, May 15, 2023 at 05:19:41AM -0700, Yu Chen wrote:
> A simple reproducer demonstrating the problem: (use ipmi_watchdog.ko)
> 
> In one terminal:
> 
> $ cat /dev/watchdog
> ...
> 
> In another terminal:
> 
> $ ps -aux | grep cat
> 14755 pts/1    R+    43:00 cat /dev/watchdog
> 51943 pts/2    S+     0:00 grep --color=auto cat
> 
> $ kill -9 14755
> $
> $ cat /proc/14755/status | grep SigPnd
> SigPnd: 0000000000000100
> $
> $ top
> 
> Tasks: 1049 total,   2 running, 1047 sleeping,   0 stopped,   0 zombie
> %Cpu(s): 0.0 us, 1.0 sy, 0.0 ni, 98.9 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
> MiB Mem : 522594.8 total, 517241.4 free,  2922.1 used,   2431.2 buff/cache
> MiB Swap:      0.0 total,      0.0 free,     0.0 used. 516589.2 avail Mem
> 
> PID USER    PR  NI    VIRT    RES    SHR S  %CPU  %MEM  TIME+ COMMAND
> 14755 root  20   0  215552   1024    576 R 100.0  0.0  0:15.12 cat
> 53417 root  20   0  224960   7040   3648 R   0.7  0.0  0:00.10 top
> 11 root     20   0       0      0      0 I   0.3  0.0  0:02.85 rcu_sched
> 1772 root   20   0  512256 387776 380800 S   0.3  0.1  0:32.05 python
> 
> We can see that when the cat process gets the signal, the CPU usage
> is 100%, Since signal_pending is true, the pick_next_task function
> in schedule always returns itself, it retries schedule indefinitely.
> ipmi_read() will busyloop.
> 
> Signed-off-by: Yu Chen <chen.yu@...ystack.cn>
> ---
>  drivers/char/ipmi/ipmi_watchdog.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/char/ipmi/ipmi_watchdog.c b/drivers/char/ipmi/ipmi_watchdog.c
> index 0d4a8dcac..173ed4266 100644
> --- a/drivers/char/ipmi/ipmi_watchdog.c
> +++ b/drivers/char/ipmi/ipmi_watchdog.c
> @@ -803,6 +803,11 @@ static ssize_t ipmi_read(struct file *file,
>  		init_waitqueue_entry(&wait, current);
>  		add_wait_queue(&read_q, &wait);
>  		while (!data_to_read) {
> +			if (signal_pending(current)) {
> +				remove_wait_queue(&read_q, &wait);
> +				rv = -ERESTARTSYS;
> +				goto out;
> +			}

This skips the call to remove_from_wait_queue(), which is bad.  I
already have a fix for this from someone else.

-corey

>  			set_current_state(TASK_INTERRUPTIBLE);
>  			spin_unlock_irq(&ipmi_read_lock);
>  			schedule();
> @@ -810,10 +815,6 @@ static ssize_t ipmi_read(struct file *file,
>  		}
>  		remove_wait_queue(&read_q, &wait);
>  
> -		if (signal_pending(current)) {
> -			rv = -ERESTARTSYS;
> -			goto out;
> -		}
>  	}
>  	data_to_read = 0;
>  
> -- 
> 2.27.0
>