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    SubjectIssue in dmesg time with lockless ring buffer
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    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

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