padata: add basic support for multithreaded jobs

Sometimes the kernel doesn't take full advantage of system memory
bandwidth, leading to a single CPU spending excessive time in
initialization paths where the data scales with memory size.

Multithreading naturally addresses this problem.

Extend padata, a framework that handles many parallel yet singlethreaded
jobs, to also handle multithreaded jobs by adding support for splitting up
the work evenly, specifying a minimum amount of work that's appropriate
for one helper thread to do, load balancing between helpers, and
coordinating them.

This is inspired by work from Pavel Tatashin and Steve Sistare.

Signed-off-by: Daniel Jordan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Tested-by: Josh Triplett <[email protected]>
Cc: Alexander Duyck <[email protected]>
Cc: Alex Williamson <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Kirill Tkhai <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Robert Elliott <[email protected]>
Cc: Shile Zhang <[email protected]>
Cc: Steffen Klassert <[email protected]>
Cc: Steven Sistare <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Zi Yan <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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