ktask: multithread CPU-intensive kernel work

hulk inclusion
category: feature
bugzilla: 13228
CVE: NA
---------------------------

A single CPU can spend an excessive amount of time in the kernel
operating on large amounts of data.  Often these situations arise
during initialization- and destruction-related tasks, where the data
involved scales with system size. These long-running jobs can slow
startup and shutdown of applications and the system itself while extra
CPUs sit idle.

To ensure that applications and the kernel continue to perform well as
core counts and memory sizes increase, harness these idle CPUs to
complete such jobs more quickly.

ktask is a generic framework for parallelizing CPU-intensive work in the
kernel.  The API is generic enough to add concurrency to many different
kinds of tasks--for example, zeroing a range of pages or evicting a list
of inodes--and aims to save its clients the trouble of splitting up the
work, choosing the number of threads to use, maintaining an efficient
concurrency level, starting these threads, and load balancing the work
between them.

The Documentation patch earlier in this series, from which the above was
swiped, has more background.

Inspired by work from Pavel Tatashin, Steve Sistare, and Jonathan Adams.

Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Suggested-by: Pavel Tatashin <Pavel.Tatashin@microsoft.com>
Suggested-by: Steve Sistare <steven.sistare@oracle.com>
Suggested-by: Jonathan Adams <jwadams@google.com>
Signed-off-by: Hongbo Yao <yaohongbo@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Tested-by: Hongbo Yao <yaohongbo@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>