[Bug 219216] sched_bind() blocks if the entropy pool is starved

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=219216

--- Comment #10 from Conrad Meyer <cem at freebsd.org> ---
rand_harvestq has had CPU reduced significantly in 12.x timeframe — previously
it was attempting to obtain far more entropy and far more often than it
actually needed.  This was in part due to conservative estimates (r338293) and
in part due to bugs in random read (r338292, r339492).

r287023, r334450 may also be relevant.

If you have a concurrent reader, rand_harvestq will fight it for the global
Fortuna state mutex on head today (and spin while waiting).  You are somewhat
saved by READ_RANDOM_UIO dropping the Fortuna lock every 4kB (PAGE_SIZE).  I
have a revision proposed to reduce this conflict further:

https://people.freebsd.org/~cem/rand_harvestq.png
https://reviews.freebsd.org/D20313

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