linux-kernel - Re: [PATCH v2] printk: Improve memory usage logging during boot

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Message-ID: <ZvwZV5MDlQYGJv9N@pathway.suse.cz>
Date: Tue, 1 Oct 2024 17:46:31 +0200
From: Petr Mladek <pmladek@...e.com>
To: "Isaac J. Manjarres" <isaacmanjarres@...gle.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>, surenb@...gle.com,
	kernel-team@...roid.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] printk: Improve memory usage logging during boot

On Mon 2024-09-30 11:48:24, Isaac J. Manjarres wrote:
> When the initial printk ring buffer size is updated, setup_log_buf()
> allocates a new ring buffer, as well as a set of meta-data structures
> for the new ring buffer. The function also emits the new size of the
> ring buffer, but not the size of the meta-data structures.
> 
> This makes it difficult to assess how changing the log buffer size
> impacts memory usage during boot.
> 
> For instance, increasing the ring buffer size from 512 KB to 1 MB
> through the command line yields an increase of 2304 KB in reserved
> memory at boot, while the only obvious change is the 512 KB
> difference in the ring buffer sizes:
> 
> log_buf_len=512K:
> 
> printk: log_buf_len: 524288 bytes
> Memory: ... (... 733252K reserved ...)
> 
> log_buf_len=1M:
> 
> printk: log_buf_len: 1048576 bytes
> Memory: ... (... 735556K reserved ...)
> 
> This is because of how the size of the meta-data structures scale with
> the size of the ring buffer.
> 
> Even when there aren't changes to the printk ring buffer size (i.e. the
> initial size ==  1 << CONFIG_LOG_BUF_SHIFT), it is impossible to tell
> how much memory is consumed by the printk ring buffer during boot.
> 
> Therefore, unconditionally log the sizes of the printk ring buffer
> and its meta-data structures, so that it's easier to understand
> how changing the log buffer size (either through the command line or
> by changing CONFIG_LOG_BUF_SHIFT) affects boot time memory usage.
> 
> With the new logs, it is much easier to see exactly why the memory
> increased by 2304 KB:
> 
> log_buf_len=512K:
> 
> printk: log_buf_len: 524288 bytes
> printk: prb_descs size: 393216 bytes
> printk: printk_infos size: 1441792 bytes

This should get updated to the new format.
If I count correctly then it should be:

printk: log buffer data + meta data: 524288 + 1835008 = 2359296 bytes

> Memory: ... (... 733252K reserved ...)
> 
> log_buf_len=1M:
> 
> printk: log_buf_len: 1048576 bytes
> printk: prb_descs size: 786432 bytes
> printk: printk_infos size: 2883584 bytes

and here:

printk: log buffer data + meta data: 1048576 + 3670016 = 4718592 bytes

> Memory: ... (... 735556K reserved ...)
>
> Signed-off-by: Isaac J. Manjarres <isaacmanjarres@...gle.com>

Otherwise, it looks good. With the updated commit message:

Reviewed-by: Petr Mladek <pmladek@...e.com>
Tested-by: Petr Mladek <pmladek@...e.com>


Note need to send v3. I could update the commit message when committing
the patch.

I am going to wait few days for a potential another review
before pushing.

Best Regards,
Petr