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29cb955934
The bio submission time may be a few jiffies more than the expected
waiting time, due to 'extra_bytes' can't be divided in
tg_within_bps_limit(), and also due to timer wakeup delay.
In this case, adjust slice_start to jiffies will discard the extra wait time,
causing lower rate than expected.
Current in-tree code already covers deviation by rounddown(), but turns
out it is not enough, because jiffies - slice_start can be a multiple of
throtl_slice.
For example, assume bps_limit is 1000bytes, 1 jiffes is 10ms, and
slice is 20ms(2 jiffies), expected rate is 1000 / 1000 * 20 = 20 bytes
per slice.
If user issues two 21 bytes IO, then wait time will be 30ms for the
first IO:
bytes_allowed = 20, extra_bytes = 1;
jiffy_wait = 1 + 2 = 3 jiffies
and consider
extra 1 jiffies by timer, throtl_trim_slice() will be called at:
jiffies = 40ms
slice_start = 0ms, slice_end= 40ms
bytes_disp = 21
In this case, before the patch, real rate in the first two slices is
10.5 bytes per slice, and slice will be updated to:
jiffies = 40ms
slice_start = 40ms, slice_end = 60ms,
bytes_disp = 0;
Hence the second IO will have to wait another 30ms;
With the patch, the real rate in the first slice is 20 bytes per slice,
which is the same as expected, and slice will be updated:
jiffies=40ms,
slice_start = 20ms, slice_end = 60ms,
bytes_disp = 1;
And now, there is still 19 bytes allowed in the second slice, and the
second IO will only have to wait 10ms;
This problem will cause blktests throtl/001 failure in case of
CONFIG_HZ_100=y, fix it by preserving one extra finished slice in
throtl_trim_slice().
Fixes:
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.