Michał Chojnowski 4563cbe595 logalloc: prevent false positives in reclaim_timer ...

reclaim_timer uses a coarse clock, but does not account for
the measurement error introduced by that -- it can falsely
report reclaims as stalls, even if they are shorter by a full
coarse clock tick from the requested threshold
(blocked-reactor-notify-ms).

Notably, if the stall threshold happens to be smaller or equal to coarse
clock resolution, Scylla's log gets spammed with false stall reports.
The resolution of coarse clocks in Linux is 1/CONFIG_HZ. This is
typically equal to 1 ms or 4 ms, and stall thresholds of this order
can occur in practice.

Eliminate false positives by requiring the measured reclaim duration to
be at least 1 clock tick longer than the configured threshold for it to
be considered a stall.

Fixes #10981

Closes #11680

2022-10-02 13:41:40 +03:00

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