Commit a961ec4e authored by Isaac J. Manjarres's avatar Isaac J. Manjarres Committed by Petr Mladek
Browse files

printk: Improve memory usage logging during boot 



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 buffer data + meta data: 524288 + 1835008 = 2359296 bytes
Memory: ... (... 733252K reserved ...)

log_buf_len=1M:

printk: log buffer data + meta data: 1048576 + 3670016 = 4718592 bytes
Memory: ... (... 735556K reserved ...)

Signed-off-by: default avatarIsaac J. Manjarres <isaacmanjarres@google.com>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Tested-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240930184826.3595221-1-isaacmanjarres@google.com


[pmladek@suse.com: Updated the examples in the commit message, simplified comment for default buffer.]
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
parent 17b65575

kernel/printk/printk.c

+22 −4
Original line number Diff line number Diff line
@@ -1156,6 +1156,17 @@ static unsigned int __init add_to_rb(struct printk_ringbuffer *rb, 


static char setup_text_buf[PRINTKRB_RECORD_MAX] __initdata;



static void print_log_buf_usage_stats(void)

{

	unsigned int descs_count = log_buf_len >> PRB_AVGBITS;

	size_t meta_data_size;



	meta_data_size = descs_count * (sizeof(struct prb_desc) + sizeof(struct printk_info));



	pr_info("log buffer data + meta data: %u + %zu = %zu bytes\n",

		log_buf_len, meta_data_size, log_buf_len + meta_data_size);

}



void __init setup_log_buf(int early)

{

	struct printk_info *new_infos;
@@ -1185,20 +1196,25 @@ void __init setup_log_buf(int early) 
	if (!early && !new_log_buf_len)

		log_buf_add_cpu();



	if (!new_log_buf_len)

	if (!new_log_buf_len) {

		/* Show the memory stats only once. */

		if (!early)

			goto out;



		return;

	}



	new_descs_count = new_log_buf_len >> PRB_AVGBITS;

	if (new_descs_count == 0) {

		pr_err("new_log_buf_len: %lu too small\n", new_log_buf_len);

		return;

		goto out;

	}



	new_log_buf = memblock_alloc(new_log_buf_len, LOG_ALIGN);

	if (unlikely(!new_log_buf)) {

		pr_err("log_buf_len: %lu text bytes not available\n",

		       new_log_buf_len);

		return;

		goto out;

	}



	new_descs_size = new_descs_count * sizeof(struct prb_desc);
@@ -1261,7 +1277,7 @@ void __init setup_log_buf(int early) 
		       prb_next_seq(&printk_rb_static) - seq);

	}



	pr_info("log_buf_len: %u bytes\n", log_buf_len);

	print_log_buf_usage_stats();

	pr_info("early log buf free: %u(%u%%)\n",

		free, (free * 100) / __LOG_BUF_LEN);

	return;
@@ -1270,6 +1286,8 @@ void __init setup_log_buf(int early) 
	memblock_free(new_descs, new_descs_size);

err_free_log_buf:

	memblock_free(new_log_buf, new_log_buf_len);

out:

	print_log_buf_usage_stats();

}



static bool __read_mostly ignore_loglevel;