padata: document multithreaded jobs

Add Documentation for multithreaded jobs.

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]>

1 files changed, 31 insertions, 10 deletions

diff --git a/Documentation/core-api/padata.rst b/Documentation/core-api/padata.rst
index 9a24c111781d..0830e5b0e821 100644
--- a/Documentation/core-api/padata.rst
+++ b/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
 =========