[PATCH 28/31] sched_ext: Add Documentation/scheduler/sched-ext.rst

[Tejun Heo <tj@kernel.org>]

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
---
 Documentation/scheduler/index.rst     |   1 +
 Documentation/scheduler/sched-ext.rst | 230 ++++++++++++++++++++++++++
 include/linux/sched/ext.h             |   2 +
 kernel/sched/ext.c                    |   2 +
 kernel/sched/ext.h                    |   2 +
 5 files changed, 237 insertions(+)
 create mode 100644 Documentation/scheduler/sched-ext.rst

diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
index b430d856056a..8a27a9967284 100644
--- a/Documentation/scheduler/index.rst
+++ b/Documentation/scheduler/index.rst
@@ -18,6 +18,7 @@ Linux Scheduler
     sched-nice-design
     sched-rt-group
     sched-stats
+    sched-ext
     sched-debug
 
     text_files
diff --git a/Documentation/scheduler/sched-ext.rst b/Documentation/scheduler/sched-ext.rst
new file mode 100644
index 000000000000..81f78e05a6c2
--- /dev/null
+++ b/Documentation/scheduler/sched-ext.rst
@@ -0,0 +1,230 @@
+==========================
+Extensible Scheduler Class
+==========================
+
+sched_ext is a scheduler class whose behavior can be defined by a set of BPF
+programs - the BPF scheduler.
+
+* sched_ext exports a full scheduling interface so that any scheduling
+  algorithm can be implemented on top.
+
+* The BPF scheduler can group CPUs however it sees fit and schedule them
+  together, as tasks aren't tied to specific CPUs at the time of wakeup.
+
+* The BPF scheduler can be turned on and off dynamically anytime.
+
+* The system integrity is maintained no matter what the BPF scheduler does.
+  The default scheduling behavior is restored anytime an error is detected,
+  a runnable task stalls, or on sysrq-S.
+
+Switching to and from sched_ext
+===============================
+
+``CONFIG_SCHED_CLASS_EXT`` is the config option to enable sched_ext and
+``tools/sched_ext`` contains the example schedulers.
+
+sched_ext is used only when the BPF scheduler is loaded and running.
+
+If a task explicitly sets its scheduling policy to ``SCHED_EXT``, it will be
+treated as ``SCHED_NORMAL`` and scheduled by CFS until the BPF scheduler is
+loaded. On load, such tasks will be switched to and scheduled by sched_ext.
+
+The BPF scheduler can choose to schedule all normal and lower class tasks by
+calling ``scx_bpf_switch_all()`` from its ``init()`` operation. In this
+case, all ``SCHED_NORMAL``, ``SCHED_BATCH``, ``SCHED_IDLE`` and
+``SCHED_EXT`` tasks are scheduled by sched_ext. In the example schedulers,
+this mode can be selected with the ``-a`` option.
+
+Terminating the sched_ext scheduler program, triggering sysrq-S, or
+detection of any internal error including stalled runnable tasks aborts the
+BPF scheduler and reverts all tasks back to CFS.
+
+.. code-block:: none
+
+    # make -j16 -C tools/sched_ext
+    # tools/sched_ext/scx_example_dummy -a
+    local=0 global=3
+    local=5 global=24
+    local=9 global=44
+    local=13 global=56
+    local=17 global=72
+    ^CEXIT: BPF scheduler unregistered
+
+If ``CONFIG_SCHED_DEBUG`` is set, the current status of the BPF scheduler
+and whether a given task is on sched_ext can be determined as follows:
+
+.. code-block:: none
+
+    # cat /sys/kernel/debug/sched/ext
+    ops                           : dummy
+    enabled                       : 1
+    switching_all                 : 1
+    switched_all                  : 1
+    enable_state                  : enabled
+
+    # grep ext /proc/self/sched
+    ext.enabled                                  :                    1
+
+The Basics
+==========
+
+Userspace can implement an arbitrary BPF scheduler by loading a set of BPF
+programs that implement ``struct sched_ext_ops``. The only mandatory field
+is ``.name`` which must be a valid BPF object name. All operations are
+optional. The following modified excerpt is from
+``tools/sched/scx_example_dummy.bpf.c`` showing a minimal global FIFO
+scheduler.
+
+.. code-block:: c
+
+    s32 BPF_STRUCT_OPS(dummy_init)
+    {
+            if (switch_all)
+                    scx_bpf_switch_all();
+            return 0;
+    }
+
+    void BPF_STRUCT_OPS(dummy_enqueue, struct task_struct *p, u64 enq_flags)
+    {
+            if (enq_flags & SCX_ENQ_LOCAL)
+                    scx_bpf_dispatch(p, SCX_DSQ_LOCAL, enq_flags);
+            else
+                    scx_bpf_dispatch(p, SCX_DSQ_GLOBAL, enq_flags);
+    }
+
+    void BPF_STRUCT_OPS(dummy_exit, struct scx_exit_info *ei)
+    {
+            exit_type = ei->type;
+    }
+
+    SEC(".struct_ops")
+    struct sched_ext_ops dummy_ops = {
+            .enqueue                = (void *)dummy_enqueue,
+            .init                   = (void *)dummy_init,
+            .exit                   = (void *)dummy_exit,
+            .name                   = "dummy",
+    };
+
+Dispatch Queues
+---------------
+
+To match the impedance between the scheduler core and the BPF scheduler,
+sched_ext uses simple FIFOs called dsq's (dispatch queues). By default,
+there is one global FIFO (``SCX_DSQ_GLOBAL``), and one local dsq per CPU
+(``SCX_DSQ_LOCAL``). The BPF scheduler can manage an arbitrary number of
+dsq's using ``scx_bpf_create_dsq()`` and ``scx_bpf_destroy_dsq()``.
+
+A task is always *dispatch*ed to a dsq for execution. The task starts
+execution when a CPU *consume*s the task from the dsq.
+
+Internally, a CPU only executes tasks which are running on its local dsq,
+and the ``.consume()`` operation is in fact a transfer of a task from a
+remote dsq to the CPU's local dsq. A CPU therefore only consumes from other
+dsq's when its local dsq is empty, and dispatching a task to a local dsq
+will cause it to be executed before the CPU attempts to consume tasks which
+were previously dispatched to other dsq's.
+
+Scheduling Cycle
+----------------
+
+The following briefly shows how a waking task is scheduled and executed.
+
+1. When a task is waking up, ``.select_cpu()`` is the first operation
+   invoked. This serves two purposes. First, CPU selection optimization
+   hint. Second, waking up the selected CPU if idle.
+
+   The CPU selected by ``.select_cpu()`` is an optimization hint and not
+   binding. The actual decision is made at the last step of scheduling.
+   However, there is a small performance gain if the CPU ``.select_cpu()``
+   returns matches the CPU the task eventually runs on.
+
+   A side-effect of selecting a CPU is waking it up from idle. While a BPF
+   scheduler can wake up any cpu using the ``scx_bpf_kick_cpu()`` helper,
+   using ``.select_cpu()`` judiciously can be simpler and more efficient.
+
+   Note that the scheduler core will ignore an invalid CPU selection, for
+   example, if it's outside the allowed cpumask of the task.
+
+2. Once the target CPU is selected, ``.enqueue()`` is invoked. It can make
+   one of the following decisions:
+
+   * Immediately dispatch the task to either the global or local dsq by
+     calling ``scx_bpf_dispatch()`` with ``SCX_DSQ_GLOBAL`` or
+     ``SCX_DSQ_LOCAL``, respectively.
+
+   * Immediately dispatch the task to a user-created dsq by calling
+     ``scx_bpf_dispatch()`` with a dsq ID which is smaller than 2^63.
+
+   * Queue the task on the BPF side.
+
+3. When a CPU is ready to schedule, it first looks at its local dsq. If
+   empty, it invokes ``.consume()`` which should make one or more
+   ``scx_bpf_consume()`` calls to consume tasks from dsq's. If a
+   ``scx_bpf_consume()`` call succeeds, the CPU has the next task to run and
+   ``.consume()`` can return.
+
+   If ``.consume()`` is not implemented, the built-in ``SCX_DSQ_GLOBAL`` dsq
+   is consumed by default.
+
+4. If there's still no task to run, ``.dispatch()`` is invoked which should
+   make one or more ``scx_bpf_dispatch()`` calls to dispatch tasks from the
+   BPF scheduler to one of the dsq's. If more than one task has been
+   dispatched, go back to the previous consumption step.
+
+5. If there's still no task to run, ``.consume_final()`` is invoked.
+   ``.consume_final()`` is equivalent to ``.consume()``, but is invoked
+   right before the CPU goes idle. This provide schedulers with a hook that
+   can be used to implement, e.g., more aggressive work stealing from remote
+   dsq's.
+
+Note that the BPF scheduler can always choose to dispatch tasks immediately
+in ``.enqueue()`` as illustrated in the above dummy example. In such case,
+there's no need to implement ``.dispatch()`` as a task is never queued on
+the BPF side.
+
+Where to Look
+=============
+
+* ``include/linux/sched/ext.h`` defines the core data structures, ops table
+  and constants.
+
+* ``kernel/sched/ext.c`` contains sched_ext core implementation and helpers.
+  The functions prefixed with ``scx_bpf_`` can be called from the BPF
+  scheduler.
+
+* ``tools/sched_ext/`` hosts example BPF scheduler implementations.
+
+  * ``scx_example_dummy[.bpf].c``: Minimal global FIFO scheduler example
+    using a custom dsq.
+
+  * ``scx_example_qmap[.bpf].c``: A multi-level FIFO scheduler supporting
+    five levels of priority implemented with ``BPF_MAP_TYPE_QUEUE``.
+
+ABI Instability
+===============
+
+The APIs provided by sched_ext to BPF schedulers programs have no stability
+guarantees. This includes the ops table callbacks and constants defined in
+``include/linux/sched/ext.h``, as well as the ``scx_bpf_`` kfuncs defined in
+``kernel/sched/ext.c``.
+
+While we will attempt to provide a relatively stable API surface when
+possible, they are subject to change without warning between kernel
+versions.
+
+Caveats
+=======
+
+* The current implementation isn't safe in that the BPF scheduler can crash
+  the kernel.
+
+  * Unsafe cpumask helpers should be replaced by proper generic BPF helpers.
+
+  * Currently, all kfunc helpers can be called by any operation as BPF
+    doesn't yet support filtering kfunc calls per struct_ops operation. Some
+    helpers are context sensitive as should be restricted accordingly.
+
+  * Timers used by the BPF scheduler should be shut down when aborting.
+
+* There are a couple BPF hacks which are still needed even for sched_ext
+  proper. They should be removed in the near future.
diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index d9f941e23011..49eda3adeecf 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -1,5 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /*
+ * BPF extensible scheduler class: Documentation/scheduler/sched-ext.rst
+ *
  * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
  * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
  * Copyright (c) 2022 David Vernet <dvernet@meta.com>
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index aab9ae13b88f..a28144220501 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1,5 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /*
+ * BPF extensible scheduler class: Documentation/scheduler/sched-ext.rst
+ *
  * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
  * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
  * Copyright (c) 2022 David Vernet <dvernet@meta.com>
diff --git a/kernel/sched/ext.h b/kernel/sched/ext.h
index e1eaaba3d4c7..b97dbb840ac9 100644
--- a/kernel/sched/ext.h
+++ b/kernel/sched/ext.h
@@ -1,5 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /*
+ * BPF extensible scheduler class: Documentation/scheduler/sched-ext.rst
+ *
  * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
  * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
  * Copyright (c) 2022 David Vernet <dvernet@meta.com>
-- 
2.38.1