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#10981Closes#11680
Instead of lengthy blurbs, switch to single-line, machine-readable
standardized (https://spdx.dev) license identifiers. The Linux kernel
switched long ago, so there is strong precedent.
Three cases are handled: AGPL-only, Apache-only, and dual licensed.
For the latter case, I chose (AGPL-3.0-or-later and Apache-2.0),
reasoning that our changes are extensive enough to apply our license.
The changes we applied mechanically with a script, except to
licenses/README.md.
Closes#9937
