padata: document multithreaded jobs

Add Documentation for multithreaded jobs.
Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Josh Triplett <josh@joshtriplett.org>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Robert Elliott <elliott@hpe.com>
Cc: Shile Zhang <shile.zhang@linux.alibaba.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Steven Sistare <steven.sistare@oracle.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Link: http://lkml.kernel.org/r/20200527173608.2885243-9-daniel.m.jordan@oracle.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/core-api/padata.rst

@@ -4,23 +4,26 @@
 The padata parallel execution mechanism
 =======================================
 
-:Date: December 2019
+:Date: May 2020
 
 Padata is a mechanism by which the kernel can farm jobs out to be done in
-parallel on multiple CPUs while retaining their ordering.  It was developed for
-use with the IPsec code, which needs to be able to perform encryption and
-decryption on large numbers of packets without reordering those packets.  The
-crypto developers made a point of writing padata in a sufficiently general
-fashion that it could be put to other uses as well.
+parallel on multiple CPUs while optionally retaining their ordering.
 
-Usage
-=====
+It was originally developed for IPsec, which needs to perform encryption and
+decryption on large numbers of packets without reordering those packets.  This
+is currently the sole consumer of padata's serialized job support.
+
+Padata also supports multithreaded jobs, splitting up the job evenly while load
+balancing and coordinating between threads.
+
+Running Serialized Jobs
+=======================
 
 Initializing
 ------------
 
-The first step in using padata is to set up a padata_instance structure for
-overall control of how jobs are to be run::
+The first step in using padata to run serialized jobs is to set up a
+padata_instance structure for overall control of how jobs are to be run::
 
     #include <linux/padata.h>
 
@@ -162,6 +165,24 @@ functions that correspond to the allocation in reverse::
 It is the user's responsibility to ensure all outstanding jobs are complete
 before any of the above are called.
 
+Running Multithreaded Jobs
+==========================
+
+A multithreaded job has a main thread and zero or more helper threads, with the
+main thread participating in the job and then waiting until all helpers have
+finished.  padata splits the job into units called chunks, where a chunk is a
+piece of the job that one thread completes in one call to the thread function.
+
+A user has to do three things to run a multithreaded job.  First, describe the
+job by defining a padata_mt_job structure, which is explained in the Interface
+section.  This includes a pointer to the thread function, which padata will
+call each time it assigns a job chunk to a thread.  Then, define the thread
+function, which accepts three arguments, ``start``, ``end``, and ``arg``, where
+the first two delimit the range that the thread operates on and the last is a
+pointer to the job's shared state, if any.  Prepare the shared state, which is
+typically allocated on the main thread's stack.  Last, call
+padata_do_multithreaded(), which will return once the job is finished.
+
 Interface
 =========