Merge tag 'scrub-fstrim-minlen-freesp-histogram-6.10_2024-07-29' of https://git.kerne...

xfs_scrub: use free space histograms to reduce fstrim runtime [v30.9 13/28]

This patchset dramatically reduces the runtime of the FITRIM calls made
during phase 8 of xfs_scrub.  It turns out that phase 8 can really get
bogged down if the free space contains a large number of very small
extents.  In these cases, the runtime can increase by an order of
magnitude to free less than 1%% of the free space.  This is not worth the
time, since we're spending a lot of time to do very little work.  The
FITRIM ioctl allows us to specify a minimum extent length, so we can use
statistical methods to compute a minlen parameter.

It turns out xfs_db/spaceman already have the code needed to create
histograms of free space extent lengths.  We add the ability to compute
a CDF of the extent lengths, which make it easy to pick a minimum length
corresponding to 99%% of the free space.  In most cases, this results in
dramatic reductions in phase 8 runtime.  Hence, move the histogram code
to libfrog, and wire up xfs_scrub, since phase 7 already walks the
fsmap.

We also add a new -o suboption to xfs_scrub so that people who /do/ want
to examine every free extent can do so.

This has been running on the djcloud for months with no problems.  Enjoy!

Signed-off-by: Darrick J. Wong <djwong@kernel.org>