Re: [PATCH v2 1/2] printk_ringbuffer: don't needlessly wrap data blocks around

Daniil Tatianin
2025-09-26T14:36:03Z

Hi all!

Now that the fix for large messages is committed, I think this patch
should be good to take,
and the second one we can just drop since it's no longer relevant.

Thanks,
Daniil

On 9/5/25 5:41 PM, Daniil Tatianin wrote:
> Previously, data blocks that perfectly fit the data ring buffer would
> get wrapped around to the beginning for no reason since the calculated
> offset of the next data block would belong to the next wrap. Since this
> offset is not actually part of the data block, but rather the offset of
> where the next data block is going to start, there is no reason to
> include it when deciding whether the current block fits the buffer.
>
> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
> ---
> kernel/printk/printk_ringbuffer.c | 27 +++++++++++++++++++--------
> 1 file changed, 19 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
> index d9fb053cff67..99989a9ce4b4 100644
> --- a/kernel/printk/printk_ringbuffer.c
> +++ b/kernel/printk/printk_ringbuffer.c
> @@ -1002,6 +1002,17 @@ static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
> return true;
> }
>
> +static bool is_blk_wrapped(struct prb_data_ring *data_ring,
> + unsigned long begin_lpos, unsigned long next_lpos)
> +{
> + /*
> + * Subtract one from next_lpos since it's not actually part of this data
> + * block. This allows perfectly fitting records to not wrap.
> + */
> + return DATA_WRAPS(data_ring, begin_lpos) !=
> + DATA_WRAPS(data_ring, next_lpos - 1);
> +}
> +
> /* Determine the end of a data block. */
> static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
> unsigned long lpos, unsigned int size)
> @@ -1013,7 +1024,7 @@ static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
> next_lpos = lpos + size;
>
> /* First check if the data block does not wrap. */
> - if (DATA_WRAPS(data_ring, begin_lpos) == DATA_WRAPS(data_ring, next_lpos))
> + if (!is_blk_wrapped(data_ring, begin_lpos, next_lpos))
> return next_lpos;
>
> /* Wrapping data blocks store their data at the beginning. */
> @@ -1081,7 +1092,7 @@ static char *data_alloc(struct printk_ringbuffer *rb, unsigned int size,
> blk = to_block(data_ring, begin_lpos);
> blk->id = id; /* LMM(data_alloc:B) */
>
> - if (DATA_WRAPS(data_ring, begin_lpos) != DATA_WRAPS(data_ring, next_lpos)) {
> + if (is_blk_wrapped(data_ring, begin_lpos, next_lpos)) {
> /* Wrapping data blocks store their data at the beginning. */
> blk = to_block(data_ring, 0);
>
> @@ -1125,7 +1136,7 @@ static char *data_realloc(struct printk_ringbuffer *rb, unsigned int size,
> return NULL;
>
> /* Keep track if @blk_lpos was a wrapping data block. */
> - wrapped = (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, blk_lpos->next));
> + wrapped = is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next);
>
> size = to_blk_size(size);
>
> @@ -1151,7 +1162,7 @@ static char *data_realloc(struct printk_ringbuffer *rb, unsigned int size,
>
> blk = to_block(data_ring, blk_lpos->begin);
>
> - if (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, next_lpos)) {
> + if (is_blk_wrapped(data_ring, blk_lpos->begin, next_lpos)) {
> struct prb_data_block *old_blk = blk;
>
> /* Wrapping data blocks store their data at the beginning. */
> @@ -1187,7 +1198,7 @@ static unsigned int space_used(struct prb_data_ring *data_ring,
> if (BLK_DATALESS(blk_lpos))
> return 0;
>
> - if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next)) {
> + if (!is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next)) {
> /* Data block does not wrap. */
> return (DATA_INDEX(data_ring, blk_lpos->next) -
> DATA_INDEX(data_ring, blk_lpos->begin));
> @@ -1234,14 +1245,14 @@ static const char *get_data(struct prb_data_ring *data_ring,
> }
>
> /* Regular data block: @begin less than @next and in same wrap. */
> - if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next) &&
> + if (!is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next) &&
> blk_lpos->begin < blk_lpos->next) {
> db = to_block(data_ring, blk_lpos->begin);
> *data_size = blk_lpos->next - blk_lpos->begin;
>
> /* Wrapping data block: @begin is one wrap behind @next. */
> - } else if (DATA_WRAPS(data_ring, blk_lpos->begin + DATA_SIZE(data_ring)) ==
> - DATA_WRAPS(data_ring, blk_lpos->next)) {
> + } else if (!is_blk_wrapped(data_ring,
> + blk_lpos->begin + DATA_SIZE(data_ring), blk_lpos->next)) {
> db = to_block(data_ring, 0);
> *data_size = DATA_INDEX(data_ring, blk_lpos->next);
>