[RFC PATCH v2 1/7] ktask: add documentation

[Daniel Jordan]

Motivates and explains the ktask API for kernel clients.

Signed-off-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
Reviewed-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
Cc: Aaron Lu <aaron.lu@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxx>
---
 Documentation/core-api/index.rst |   1 +
 Documentation/core-api/ktask.rst | 104 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 105 insertions(+)
 create mode 100644 Documentation/core-api/ktask.rst

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 0606be3a3111..9096f4b77b42 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -15,6 +15,7 @@ Core utilities
    assoc_array
    atomic_ops
    cpu_hotplug
+   ktask
    local_ops
    workqueue
    genericirq
diff --git a/Documentation/core-api/ktask.rst b/Documentation/core-api/ktask.rst
new file mode 100644
index 000000000000..cb4b0d87c8c6
--- /dev/null
+++ b/Documentation/core-api/ktask.rst
@@ -0,0 +1,104 @@
+============================================
+ktask: parallelize cpu-intensive kernel work
+============================================
+
+:Date: July, 2017
+:Author: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
+
+
+Introduction
+============
+
+ktask is a generic framework for parallelizing cpu-intensive work in the
+kernel.  The intended use is for big machines that can use their cpu power to
+speed up large tasks that can't otherwise be multithreaded in userland.  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, starting these threads, and load balancing the work between
+them.
+
+
+Motivation
+==========
+
+Why do we need ktask when the kernel has other APIs for managing concurrency?
+After all, kthread_workers and workqueues already provide ways to start
+threads, and the kernel can handle large tasks with a single thread by
+periodically yielding the cpu with cond_resched (e.g. hugetlbfs_fallocate,
+clear_gigantic_page) or performing the work in fixed size batches (e.g. struct
+pagevec, struct mmu_gather).
+
+Of the existing concurrency facilities, kthread_worker isn't suited for
+providing parallelism because each comes with only a single thread.  Workqueues
+are a better fit for this, and in fact ktask is built on an unbound workqueue,
+but workqueues aren't designed for splitting up a large task.  ktask instead
+uses unbound workqueue threads to run "chunks" of a task.
+
+On top of workqueues, ktask takes care of dividing up the task into chunks,
+determining how many threads to use to complete those chunks, starting the
+threads, and load balancing across them.  This makes use of otherwise idle
+cpus, but if the system is under load, the scheduler still decides when the
+ktask threads run: existing cond_resched calls are retained in big loops that
+have been parallelized.
+
+This added concurrency boosts the performance of the system in a number of
+ways: system startup and shutdown are faster, page fault latency of a gigantic
+page goes down (zero the page in parallel), initializing many pages goes
+quicker (e.g. populating a range of pages via prefaulting, mlocking, or
+fallocating), and pages are freed back to the system in less time (e.g. on a
+large munmap(2) or on exit(2) of a large process).
+
+
+Configuration
+=============
+
+To use ktask, configure the kernel with CONFIG_KTASK=y.
+
+If CONFIG_KTASK=n, calls to the ktask API are simply #define'd to run the
+thread function that the client provides so that the task is completed without
+concurrency in the current thread.
+
+
+Concept
+=======
+
+A little terminology up front:  A 'task' is the total work there is to do and a
+'chunk' is a unit of work given to a thread.
+
+To complete a task using the ktask framework, a client provides a thread
+function that is responsible for completing one chunk.  The thread function is
+defined in a standard way, with start and end arguments that delimit the chunk
+as well as an argument that the client uses to pass data specific to the task.
+
+In addition, the client supplies an object representing the start of the task
+and an iterator function that knows how to advance some number of units in the
+task to yield another object representing the new task position.  The framework
+uses the start object and iterator internally to divide the task into chunks.
+
+Finally, the client passes the total task size and a minimum chunk size to
+indicate the minimum amount of work that's appropriate to do in one chunk.  The
+sizes are given in task-specific units (e.g. pages, inodes, bytes).  The
+framework uses these sizes, along with the number of online cpus and an
+internal maximum number of threads, to decide how many threads to start and how
+many chunks to divide the task into.
+
+For example, consider the task of clearing a gigantic page.  This used to be
+done in a single thread with a for loop that calls a page clearing function for
+each constituent base page.  To parallelize with ktask, the client first moves
+the for loop to the thread function, adapting it to operate on the range passed
+to the function.  In this simple case, the thread function's start and end
+arguments are just addresses delimiting the portion of the gigantic page to
+clear.  Then, where the for loop used to be, the client calls into ktask with
+the start address of the gigantic page, the total size of the gigantic page,
+and the thread function.  Internally, ktask will divide the address range into
+an appropriate number of chunks and start an appropriate number of threads to
+complete these chunks.
+
+
+Interface
+=========
+
+.. Include ktask.h inline here.  This file is heavily commented and documents
+.. the ktask interface.
+.. kernel-doc:: include/linux/ktask.h
-- 
2.12.2