Issue in dmesg time with lockless ring buffer

[J. Avila]

Hello,

When doing some internal testing on a 5.10.4 kernel, we found that the
time taken for dmesg seemed to increase from the order of milliseconds to
the order of seconds when the dmesg size approached the ~1.2MB limit. After
doing some digging, we found that by reverting all of the patches in printk/
up to and including 896fbe20b4e2333fb55cc9b9b783ebcc49eee7c7 ("use the
lockless ringbuffer"), we were able to once more see normal dmesg times.

This kernel had no meaningful diffs in the printk/ dir when compared to
Linus' tree. This behavior was consistently reproducible using the
following steps:

1) In one shell, run "time dmesg > /dev/null"
2) In another, constantly write to /dev/kmsg

Within ~5 minutes, we saw that dmesg times increased to 1 second, only
increasing further from there. Is this a known issue?

Thank you,

Avila