linux-kernel - truncate dict: was: Re: [PATCH v3 3/3] printk: use the lockless ringbuffer

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Message-ID: <20200625081650.GE6156@alley>
Date:   Thu, 25 Jun 2020 10:16:50 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...nel.org>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: truncate dict: was: Re: [PATCH v3 3/3] printk: use the lockless
 ringbuffer

On Thu 2020-06-18 16:55:19, John Ogness wrote:
> Replace the existing ringbuffer usage and implementation with
> lockless ringbuffer usage. Even though the new ringbuffer does not
> require locking, all existing locking is left in place. Therefore,
> this change is purely replacing the underlining ringbuffer.

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -594,22 +473,26 @@ static u32 msg_used_size(u16 text_len, u16 dict_len, u32 *pad_len)
>  #define MAX_LOG_TAKE_PART 4
>  static const char trunc_msg[] = "<truncated>";
>  
> -static u32 truncate_msg(u16 *text_len, u16 *trunc_msg_len,
> -			u16 *dict_len, u32 *pad_len)
> +static void truncate_msg(u16 *text_len, u16 *trunc_msg_len, u16 *dict_len)
>  {
>  	/*
>  	 * The message should not take the whole buffer. Otherwise, it might
>  	 * get removed too soon.
>  	 */
>  	u32 max_text_len = log_buf_len / MAX_LOG_TAKE_PART;
> +
>  	if (*text_len > max_text_len)
>  		*text_len = max_text_len;
> -	/* enable the warning message */
> +
> +	/* enable the warning message (if there is room) */
>  	*trunc_msg_len = strlen(trunc_msg);
> +	if (*text_len >= *trunc_msg_len)
> +		*text_len -= *trunc_msg_len;
> +	else
> +		*trunc_msg_len = 0;
> +
>  	/* disable the "dict" completely */
>  	*dict_len = 0;

The dictionary does not longer need to be removed at this point.
It is stored in a separate buffer. It is ignored by prb_reserve()
when there is not enough space for it.

> -	/* compute the size again, count also the warning message */
> -	return msg_used_size(*text_len + *trunc_msg_len, 0, pad_len);
>  }
>  
>  /* insert record into the buffer, discard old ones, update heads */

Best Regards,
Petr

PS: I am still in the middle of review of this patch. I decided to
send the tree comments that I already have separately.