Re: [PATCH v2] printk: avoid prb_first_valid_seq() where possible

[Petr Mladek <pmladek@suse.com>]

On Thu 2021-02-11 18:37:52, John Ogness wrote:
> If message sizes average larger than expected (more than 32
> characters), the data_ring will wrap before the desc_ring. Once the
> data_ring wraps, it will start invalidating descriptors. These
> invalid descriptors hang around until they are eventually recycled
> when the desc_ring wraps. Readers do not care about invalid
> descriptors, but they still need to iterate past them. If the
> average message size is much larger than 32 characters, then there
> will be many invalid descriptors preceding the valid descriptors.
> 
> The function prb_first_valid_seq() always begins at the oldest
> descriptor and searches for the first valid descriptor. This can
> be rather expensive for the above scenario. And, in fact, because
> of its heavy usage in /dev/kmsg, there have been reports of long
> delays and even RCU stalls.
> 
> For code that does not need to search from the oldest record,
> replace prb_first_valid_seq() usage with prb_read_valid_*()
> functions, which provide a start sequence number to search from.
> 
> Fixes: 896fbe20b4e2333fb55 ("printk: use the lockless ringbuffer")
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Reported-by: J. Avila <elavila@google.com>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Could you please push this fix into the stable releases
based on 5.10 and 5.11, please?

The patch fixes a visible performance regression. It has
landed in the mainline as the commit
13791c80b0cdf54d92fc542 ("printk: avoid prb_first_valid_seq() where
possible").

It should apply cleanly.

Best Regards,
Petr