mean_and_variance: Drop always failing tests

mean_and_variance_test_2 and mean_and_variance_test_4 always fail.
The input parameters to those tests are identical to the input parameters
to tests 1 and 3, yet the expected result for tests 2 and 4 is different
for the mean and stddev tests. That will always fail.

Expected mean_and_variance_get_mean(mv) == mean[i], but
mean_and_variance_get_mean(mv) == 22 (0x16)
mean[i] == 10 (0xa)

Drop the bad tests.

Fixes: 65bc41090720 ("mean and variance: More tests")
Closes: https://lore.kernel.org/lkml/065b94eb-6a24-4248-b7d7-d3212efb4787@roeck-us.net/
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: mean_and_variance: put struct mean_and_variance_weighted on a diet

The only caller of this code (time_stats) always knows the weights and
whether or not any information has been collected. Pass this
information into the mean and variance code so that it doesn't have to
store that information. This reduces the structure size from 24 to 16
bytes, which shrinks each time_stats counter to 192 bytes from 208.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

mean and variance: More tests

Add some more tests that test conventional and weighted mean
simultaneously, and with a table of values that represents events that
we'll be using this to look for so we can verify-by-eyeball that the
output looks sane.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: Mean and variance

This module provides a fast 64bit implementation of basic statistics
functions, including mean, variance and standard deviation in both
weighted and unweighted variants, the unweighted variant has a 32bit
limitation per sample to prevent overflow when squaring.

Signed-off-by: Daniel Hill <daniel@gluo.nz>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>