sched_ext: Implement BPF extensible scheduler class

Implement a new scheduler class sched_ext (SCX), which allows scheduling
policies to be implemented as BPF programs to achieve the following:

1. Ease of experimentation and exploration: Enabling rapid iteration of new
   scheduling policies.

2. Customization: Building application-specific schedulers which implement
   policies that are not applicable to general-purpose schedulers.

3. Rapid scheduler deployments: Non-disruptive swap outs of scheduling
   policies in production environments.

sched_ext leverages BPF’s struct_ops feature to define a structure which
exports function callbacks and flags to BPF programs that wish to implement
scheduling policies. The struct_ops structure exported by sched_ext is
struct sched_ext_ops, and is conceptually similar to struct sched_class. The
role of sched_ext is to map the complex sched_class callbacks to the more
simple and ergonomic struct sched_ext_ops callbacks.

For more detailed discussion on the motivations and overview, please refer
to the cover letter.

Later patches will also add several example schedulers and documentation.

This patch implements the minimum core framework to enable implementation of
BPF schedulers. Subsequent patches will gradually add functionalities
including safety guarantee mechanisms, nohz and cgroup support.

include/linux/sched/ext.h defines struct sched_ext_ops. With the comment on
top, each operation should be self-explanatory. The followings are worth
noting:

- Both "sched_ext" and its shorthand "scx" are used. If the identifier
  already has "sched" in it, "ext" is used; otherwise, "scx".

- In sched_ext_ops, only .name is mandatory. Every operation is optional and
  if omitted a simple but functional default behavior is provided.

- A new policy constant SCHED_EXT is added and a task can select sched_ext
  by invoking sched_setscheduler(2) with the new policy constant. However,
  if the BPF scheduler is not loaded, SCHED_EXT is the same as SCHED_NORMAL
  and the task is scheduled by CFS. When the BPF scheduler is loaded, all
  tasks which have the SCHED_EXT policy are switched to sched_ext.

- To bridge the workflow imbalance between the scheduler core and
  sched_ext_ops callbacks, sched_ext uses simple FIFOs called dispatch
  queues (dsq's). By default, there is one global dsq (SCX_DSQ_GLOBAL), and
  one local per-CPU dsq (SCX_DSQ_LOCAL). SCX_DSQ_GLOBAL is provided for
  convenience and need not be used by a scheduler that doesn't require it.
  SCX_DSQ_LOCAL is the per-CPU FIFO that sched_ext pulls from when putting
  the next task on the CPU. The BPF scheduler can manage an arbitrary number
  of dsq's using scx_bpf_create_dsq() and scx_bpf_destroy_dsq().

- sched_ext guarantees system integrity no matter what the BPF scheduler
  does. To enable this, each task's ownership is tracked through
  p->scx.ops_state and all tasks are put on scx_tasks list. The disable path
  can always recover and revert all tasks back to CFS. See p->scx.ops_state
  and scx_tasks.

- A task is not tied to its rq while enqueued. This decouples CPU selection
  from queueing and allows sharing a scheduling queue across an arbitrary
  subset of CPUs. This adds some complexities as a task may need to be
  bounced between rq's right before it starts executing. See
  dispatch_to_local_dsq() and move_task_to_local_dsq().

- One complication that arises from the above weak association between task
  and rq is that synchronizing with dequeue() gets complicated as dequeue()
  may happen anytime while the task is enqueued and the dispatch path might
  need to release the rq lock to transfer the task. Solving this requires a
  bit of complexity. See the logic around p->scx.sticky_cpu and
  p->scx.ops_qseq.

- Both enable and disable paths are a bit complicated. The enable path
  switches all tasks without blocking to avoid issues which can arise from
  partially switched states (e.g. the switching task itself being starved).
  The disable path can't trust the BPF scheduler at all, so it also has to
  guarantee forward progress without blocking. See scx_ops_enable() and
  scx_ops_disable_workfn().

- When sched_ext is disabled, static_branches are used to shut down the
  entry points from hot paths.

v7: - scx_ops_bypass() was incorrectly and unnecessarily trying to grab
      scx_ops_enable_mutex which can lead to deadlocks in the disable path.
      Fixed.

    - Fixed TASK_DEAD handling bug in scx_ops_enable() path which could lead
      to use-after-free.

    - Consolidated per-cpu variable usages and other cleanups.

v6: - SCX_NR_ONLINE_OPS replaced with SCX_OPI_*_BEGIN/END so that multiple
      groups can be expressed. Later CPU hotplug operations are put into
      their own group.

    - SCX_OPS_DISABLING state is replaced with the new bypass mechanism
      which allows temporarily putting the system into simple FIFO
      scheduling mode bypassing the BPF scheduler. In addition to the shut
      down path, this will also be used to isolate the BPF scheduler across
      PM events. Enabling and disabling the bypass mode requires iterating
      all runnable tasks. rq->scx.runnable_list addition is moved from the
      later watchdog patch.

    - ops.prep_enable() is replaced with ops.init_task() and
      ops.enable/disable() are now called whenever the task enters and
      leaves sched_ext instead of when the task becomes schedulable on
      sched_ext and stops being so. A new operation - ops.exit_task() - is
      called when the task stops being schedulable on sched_ext.

    - scx_bpf_dispatch() can now be called from ops.select_cpu() too. This
      removes the need for communicating local dispatch decision made by
      ops.select_cpu() to ops.enqueue() via per-task storage.
      SCX_KF_SELECT_CPU is added to support the change.

    - SCX_TASK_ENQ_LOCAL which told the BPF scheudler that
      scx_select_cpu_dfl() wants the task to be dispatched to the local DSQ
      was removed. Instead, scx_bpf_select_cpu_dfl() now dispatches directly
      if it finds a suitable idle CPU. If such behavior is not desired,
      users can use scx_bpf_select_cpu_dfl() which returns the verdict in a
      bool out param.

    - scx_select_cpu_dfl() was mishandling WAKE_SYNC and could end up
      queueing many tasks on a local DSQ which makes tasks to execute in
      order while other CPUs stay idle which made some hackbench numbers
      really bad. Fixed.

    - The current state of sched_ext can now be monitored through files
      under /sys/sched_ext instead of /sys/kernel/debug/sched/ext. This is
      to enable monitoring on kernels which don't enable debugfs.

    - sched_ext wasn't telling BPF that ops.dispatch()'s @prev argument may
      be NULL and a BPF scheduler which derefs the pointer without checking
      could crash the kernel. Tell BPF. This is currently a bit ugly. A
      better way to annotate this is expected in the future.

    - scx_exit_info updated to carry pointers to message buffers instead of
      embedding them directly. This decouples buffer sizes from API so that
      they can be changed without breaking compatibility.

    - exit_code added to scx_exit_info. This is used to indicate different
      exit conditions on non-error exits and will be used to handle e.g. CPU
      hotplugs.

    - The patch "sched_ext: Allow BPF schedulers to switch all eligible
      tasks into sched_ext" is folded in and the interface is changed so
      that partial switching is indicated with a new ops flag
      %SCX_OPS_SWITCH_PARTIAL. This makes scx_bpf_switch_all() unnecessasry
      and in turn SCX_KF_INIT. ops.init() is now called with
      SCX_KF_SLEEPABLE.

    - Code reorganized so that only the parts necessary to integrate with
      the rest of the kernel are in the header files.

    - Changes to reflect the BPF and other kernel changes including the
      addition of bpf_sched_ext_ops.cfi_stubs.

v5: - To accommodate 32bit configs, p->scx.ops_state is now atomic_long_t
      instead of atomic64_t and scx_dsp_buf_ent.qseq which uses
      load_acquire/store_release is now unsigned long instead of u64.

    - Fix the bug where bpf_scx_btf_struct_access() was allowing write
      access to arbitrary fields.

    - Distinguish kfuncs which can be called from any sched_ext ops and from
      anywhere. e.g. scx_bpf_pick_idle_cpu() can now be called only from
      sched_ext ops.

    - Rename "type" to "kind" in scx_exit_info to make it easier to use on
      languages in which "type" is a reserved keyword.

    - Since cff9b2332a ("kernel/sched: Modify initial boot task idle
      setup"), PF_IDLE is not set on idle tasks which haven't been online
      yet which made scx_task_iter_next_filtered() include those idle tasks
      in iterations leading to oopses. Update scx_task_iter_next_filtered()
      to directly test p->sched_class against idle_sched_class instead of
      using is_idle_task() which tests PF_IDLE.

    - Other updates to match upstream changes such as adding const to
      set_cpumask() param and renaming check_preempt_curr() to
      wakeup_preempt().

v4: - SCHED_CHANGE_BLOCK replaced with the previous
      sched_deq_and_put_task()/sched_enq_and_set_tsak() pair. This is
      because upstream is adaopting a different generic cleanup mechanism.
      Once that lands, the code will be adapted accordingly.

    - task_on_scx() used to test whether a task should be switched into SCX,
      which is confusing. Renamed to task_should_scx(). task_on_scx() now
      tests whether a task is currently on SCX.

    - scx_has_idle_cpus is barely used anymore and replaced with direct
      check on the idle cpumask.

    - SCX_PICK_IDLE_CORE added and scx_pick_idle_cpu() improved to prefer
      fully idle cores.

    - ops.enable() now sees up-to-date p->scx.weight value.

    - ttwu_queue path is disabled for tasks on SCX to avoid confusing BPF
      schedulers expecting ->select_cpu() call.

    - Use cpu_smt_mask() instead of topology_sibling_cpumask() like the rest
      of the scheduler.

v3: - ops.set_weight() added to allow BPF schedulers to track weight changes
      without polling p->scx.weight.

    - move_task_to_local_dsq() was losing SCX-specific enq_flags when
      enqueueing the task on the target dsq because it goes through
      activate_task() which loses the upper 32bit of the flags. Carry the
      flags through rq->scx.extra_enq_flags.

    - scx_bpf_dispatch(), scx_bpf_pick_idle_cpu(), scx_bpf_task_running()
      and scx_bpf_task_cpu() now use the new KF_RCU instead of
      KF_TRUSTED_ARGS to make it easier for BPF schedulers to call them.

    - The kfunc helper access control mechanism implemented through
      sched_ext_entity.kf_mask is improved. Now SCX_CALL_OP*() is always
      used when invoking scx_ops operations.

v2: - balance_scx_on_up() is dropped. Instead, on UP, balance_scx() is
      called from put_prev_taks_scx() and pick_next_task_scx() as necessary.
      To determine whether balance_scx() should be called from
      put_prev_task_scx(), SCX_TASK_DEQD_FOR_SLEEP flag is added. See the
      comment in put_prev_task_scx() for details.

    - sched_deq_and_put_task() / sched_enq_and_set_task() sequences replaced
      with SCHED_CHANGE_BLOCK().

    - Unused all_dsqs list removed. This was a left-over from previous
      iterations.

    - p->scx.kf_mask is added to track and enforce which kfunc helpers are
      allowed. Also, init/exit sequences are updated to make some kfuncs
      always safe to call regardless of the current BPF scheduler state.
      Combined, this should make all the kfuncs safe.

    - BPF now supports sleepable struct_ops operations. Hacky workaround
      removed and operations and kfunc helpers are tagged appropriately.

    - BPF now supports bitmask / cpumask helpers. scx_bpf_get_idle_cpumask()
      and friends are added so that BPF schedulers can use the idle masks
      with the generic helpers. This replaces the hacky kfunc helpers added
      by a separate patch in V1.

    - CONFIG_SCHED_CLASS_EXT can no longer be enabled if SCHED_CORE is
      enabled. This restriction will be removed by a later patch which adds
      core-sched support.

    - Add MAINTAINERS entries and other misc changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Co-authored-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>
Cc: Andrea Righi <andrea.righi@canonical.com>

MAINTAINERS

@@ -19936,6 +19936,19 @@ F:	include/linux/wait.h
 F:	include/uapi/linux/sched.h
 F:	kernel/sched/
 
+SCHEDULER - SCHED_EXT
+R:	Tejun Heo <tj@kernel.org>
+R:	David Vernet <void@manifault.com>
+L:	linux-kernel@vger.kernel.org
+S:	Maintained
+W:	https://github.com/sched-ext/scx
+T:	git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git
+F:	include/linux/sched/ext.h
+F:	kernel/sched/ext.h
+F:	kernel/sched/ext.c
+F:	tools/sched_ext/
+F:	tools/testing/selftests/sched_ext
+
 SCSI LIBSAS SUBSYSTEM
 R:	John Garry <john.g.garry@oracle.com>
 R:	Jason Yan <yanaijie@huawei.com>

include/asm-generic/vmlinux.lds.h

@@ -133,6 +133,7 @@
 	*(__dl_sched_class)			\
 	*(__rt_sched_class)			\
 	*(__fair_sched_class)			\
+	*(__ext_sched_class)			\
 	*(__idle_sched_class)			\
 	__sched_class_lowest = .;
 

include/linux/sched.h

@@ -80,6 +80,8 @@ struct task_group;
 struct task_struct;
 struct user_event_mm;
 
+#include <linux/sched/ext.h>
+
 /*
  * Task state bitmask. NOTE! These bits are also
  * encoded in fs/proc/array.c: get_task_state().
@@ -802,6 +804,9 @@ struct task_struct {
 	struct sched_rt_entity		rt;
 	struct sched_dl_entity		dl;
 	struct sched_dl_entity		*dl_server;
+#ifdef CONFIG_SCHED_CLASS_EXT
+	struct sched_ext_entity		scx;
+#endif
 	const struct sched_class	*sched_class;
 
 #ifdef CONFIG_SCHED_CORE

include/linux/sched/ext.h

@@ -1,9 +1,148 @@
 /* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
+ * Copyright (c) 2022 David Vernet <dvernet@meta.com>
+ */
 #ifndef _LINUX_SCHED_EXT_H
 #define _LINUX_SCHED_EXT_H
 
 #ifdef CONFIG_SCHED_CLASS_EXT
-#error "NOT IMPLEMENTED YET"
+
+#include <linux/llist.h>
+#include <linux/rhashtable-types.h>
+
+enum scx_public_consts {
+	SCX_OPS_NAME_LEN	= 128,
+
+	SCX_SLICE_DFL		= 20 * 1000000,	/* 20ms */
+};
+
+/*
+ * DSQ (dispatch queue) IDs are 64bit of the format:
+ *
+ *   Bits: [63] [62 ..  0]
+ *         [ B] [   ID   ]
+ *
+ *    B: 1 for IDs for built-in DSQs, 0 for ops-created user DSQs
+ *   ID: 63 bit ID
+ *
+ * Built-in IDs:
+ *
+ *   Bits: [63] [62] [61..32] [31 ..  0]
+ *         [ 1] [ L] [   R  ] [    V   ]
+ *
+ *    1: 1 for built-in DSQs.
+ *    L: 1 for LOCAL_ON DSQ IDs, 0 for others
+ *    V: For LOCAL_ON DSQ IDs, a CPU number. For others, a pre-defined value.
+ */
+enum scx_dsq_id_flags {
+	SCX_DSQ_FLAG_BUILTIN	= 1LLU << 63,
+	SCX_DSQ_FLAG_LOCAL_ON	= 1LLU << 62,
+
+	SCX_DSQ_INVALID		= SCX_DSQ_FLAG_BUILTIN | 0,
+	SCX_DSQ_GLOBAL		= SCX_DSQ_FLAG_BUILTIN | 1,
+	SCX_DSQ_LOCAL		= SCX_DSQ_FLAG_BUILTIN | 2,
+	SCX_DSQ_LOCAL_ON	= SCX_DSQ_FLAG_BUILTIN | SCX_DSQ_FLAG_LOCAL_ON,
+	SCX_DSQ_LOCAL_CPU_MASK	= 0xffffffffLLU,
+};
+
+/*
+ * Dispatch queue (dsq) is a simple FIFO which is used to buffer between the
+ * scheduler core and the BPF scheduler. See the documentation for more details.
+ */
+struct scx_dispatch_q {
+	raw_spinlock_t		lock;
+	struct list_head	list;	/* tasks in dispatch order */
+	u32			nr;
+	u64			id;
+	struct rhash_head	hash_node;
+	struct llist_node	free_node;
+	struct rcu_head		rcu;
+};
+
+/* scx_entity.flags */
+enum scx_ent_flags {
+	SCX_TASK_QUEUED		= 1 << 0, /* on ext runqueue */
+	SCX_TASK_BAL_KEEP	= 1 << 1, /* balance decided to keep current */
+	SCX_TASK_RESET_RUNNABLE_AT = 1 << 2, /* runnable_at should be reset */
+	SCX_TASK_DEQD_FOR_SLEEP	= 1 << 3, /* last dequeue was for SLEEP */
+
+	SCX_TASK_STATE_SHIFT	= 8,	  /* bit 8 and 9 are used to carry scx_task_state */
+	SCX_TASK_STATE_BITS	= 2,
+	SCX_TASK_STATE_MASK	= ((1 << SCX_TASK_STATE_BITS) - 1) << SCX_TASK_STATE_SHIFT,
+
+	SCX_TASK_CURSOR		= 1 << 31, /* iteration cursor, not a task */
+};
+
+/* scx_entity.flags & SCX_TASK_STATE_MASK */
+enum scx_task_state {
+	SCX_TASK_NONE,		/* ops.init_task() not called yet */
+	SCX_TASK_INIT,		/* ops.init_task() succeeded, but task can be cancelled */
+	SCX_TASK_READY,		/* fully initialized, but not in sched_ext */
+	SCX_TASK_ENABLED,	/* fully initialized and in sched_ext */
+
+	SCX_TASK_NR_STATES,
+};
+
+/*
+ * Mask bits for scx_entity.kf_mask. Not all kfuncs can be called from
+ * everywhere and the following bits track which kfunc sets are currently
+ * allowed for %current. This simple per-task tracking works because SCX ops
+ * nest in a limited way. BPF will likely implement a way to allow and disallow
+ * kfuncs depending on the calling context which will replace this manual
+ * mechanism. See scx_kf_allow().
+ */
+enum scx_kf_mask {
+	SCX_KF_UNLOCKED		= 0,	  /* not sleepable, not rq locked */
+	/* all non-sleepables may be nested inside SLEEPABLE */
+	SCX_KF_SLEEPABLE	= 1 << 0, /* sleepable init operations */
+	/* ops.dequeue (in REST) may be nested inside DISPATCH */
+	SCX_KF_DISPATCH		= 1 << 2, /* ops.dispatch() */
+	SCX_KF_ENQUEUE		= 1 << 3, /* ops.enqueue() and ops.select_cpu() */
+	SCX_KF_SELECT_CPU	= 1 << 4, /* ops.select_cpu() */
+	SCX_KF_REST		= 1 << 5, /* other rq-locked operations */
+
+	__SCX_KF_RQ_LOCKED	= SCX_KF_DISPATCH |
+				  SCX_KF_ENQUEUE | SCX_KF_SELECT_CPU | SCX_KF_REST,
+};
+
+/*
+ * The following is embedded in task_struct and contains all fields necessary
+ * for a task to be scheduled by SCX.
+ */
+struct sched_ext_entity {
+	struct scx_dispatch_q	*dsq;
+	struct list_head	dsq_node;
+	u32			flags;		/* protected by rq lock */
+	u32			weight;
+	s32			sticky_cpu;
+	s32			holding_cpu;
+	u32			kf_mask;	/* see scx_kf_mask above */
+	atomic_long_t		ops_state;
+
+	struct list_head	runnable_node;	/* rq->scx.runnable_list */
+
+	u64			ddsp_dsq_id;
+	u64			ddsp_enq_flags;
+
+	/* BPF scheduler modifiable fields */
+
+	/*
+	 * Runtime budget in nsecs. This is usually set through
+	 * scx_bpf_dispatch() but can also be modified directly by the BPF
+	 * scheduler. Automatically decreased by SCX as the task executes. On
+	 * depletion, a scheduling event is triggered.
+	 */
+	u64			slice;
+
+	/* cold fields */
+	/* must be the last field, see init_scx_entity() */
+	struct list_head	tasks_node;
+};
+
+void sched_ext_free(struct task_struct *p);
+
 #else	/* !CONFIG_SCHED_CLASS_EXT */
 
 static inline void sched_ext_free(struct task_struct *p) {}

include/uapi/linux/sched.h

@@ -118,6 +118,7 @@ struct clone_args {
 /* SCHED_ISO: reserved but not implemented yet */
 #define SCHED_IDLE		5
 #define SCHED_DEADLINE		6
+#define SCHED_EXT		7
 
 /* Can be ORed in to make sure the process is reverted back to SCHED_NORMAL on fork */
 #define SCHED_RESET_ON_FORK     0x40000000

init/init_task.c

@@ -6,6 +6,7 @@
 #include <linux/sched/sysctl.h>
 #include <linux/sched/rt.h>
 #include <linux/sched/task.h>
+#include <linux/sched/ext.h>
 #include <linux/init.h>
 #include <linux/fs.h>
 #include <linux/mm.h>
@@ -98,6 +99,16 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = {
 #endif
 #ifdef CONFIG_CGROUP_SCHED
 	.sched_task_group = &root_task_group,
+#endif
+#ifdef CONFIG_SCHED_CLASS_EXT
+	.scx		= {
+		.dsq_node	= LIST_HEAD_INIT(init_task.scx.dsq_node),
+		.sticky_cpu	= -1,
+		.holding_cpu	= -1,
+		.runnable_node	= LIST_HEAD_INIT(init_task.scx.runnable_node),
+		.ddsp_dsq_id	= SCX_DSQ_INVALID,
+		.slice		= SCX_SLICE_DFL,
+	},
 #endif
 	.ptraced	= LIST_HEAD_INIT(init_task.ptraced),
 	.ptrace_entry	= LIST_HEAD_INIT(init_task.ptrace_entry),

kernel/Kconfig.preempt

@@ -133,4 +133,27 @@ config SCHED_CORE
 	  which is the likely usage by Linux distributions, there should
 	  be no measurable impact on performance.
 
+config SCHED_CLASS_EXT
+	bool "Extensible Scheduling Class"
+	depends on BPF_SYSCALL && BPF_JIT && !SCHED_CORE
+	help
+	  This option enables a new scheduler class sched_ext (SCX), which
+	  allows scheduling policies to be implemented as BPF programs to
+	  achieve the following:
 
+	  - Ease of experimentation and exploration: Enabling rapid
+	    iteration of new scheduling policies.
+	  - Customization: Building application-specific schedulers which
+	    implement policies that are not applicable to general-purpose
+	    schedulers.
+	  - Rapid scheduler deployments: Non-disruptive swap outs of
+	    scheduling policies in production environments.
+
+	  sched_ext leverages BPF struct_ops feature to define a structure
+	  which exports function callbacks and flags to BPF programs that
+	  wish to implement scheduling policies. The struct_ops structure
+	  exported by sched_ext is struct sched_ext_ops, and is conceptually
+	  similar to struct sched_class.
+
+	  For more information:
+	    https://github.com/sched-ext/scx

kernel/sched/build_policy.c

@@ -21,13 +21,18 @@
 
 #include <linux/cpuidle.h>
 #include <linux/jiffies.h>
+#include <linux/kobject.h>
 #include <linux/livepatch.h>
+#include <linux/pm.h>
 #include <linux/psi.h>
+#include <linux/rhashtable.h>
+#include <linux/seq_buf.h>
 #include <linux/seqlock_api.h>
 #include <linux/slab.h>
 #include <linux/suspend.h>
 #include <linux/tsacct_kern.h>
 #include <linux/vtime.h>
+#include <linux/percpu-rwsem.h>
 
 #include <uapi/linux/sched/types.h>
 
@@ -52,4 +57,8 @@
 #include "cputime.c"
 #include "deadline.c"
 
+#ifdef CONFIG_SCHED_CLASS_EXT
+# include "ext.c"
+#endif
+
 #include "syscalls.c"

kernel/sched/core.c

@@ -3790,6 +3790,15 @@ bool cpus_share_resources(int this_cpu, int that_cpu)
 
 static inline bool ttwu_queue_cond(struct task_struct *p, int cpu)
 {
+	/*
+	 * The BPF scheduler may depend on select_task_rq() being invoked during
+	 * wakeups. In addition, @p may end up executing on a different CPU
+	 * regardless of what happens in the wakeup path making the ttwu_queue
+	 * optimization less meaningful. Skip if on SCX.
+	 */
+	if (task_on_scx(p))
+		return false;
+
 	/*
 	 * Do not complicate things with the async wake_list while the CPU is
 	 * in hotplug state.
@@ -4357,6 +4366,10 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
 	p->rt.on_rq		= 0;
 	p->rt.on_list		= 0;
 
+#ifdef CONFIG_SCHED_CLASS_EXT
+	init_scx_entity(&p->scx);
+#endif
+
 #ifdef CONFIG_PREEMPT_NOTIFIERS
 	INIT_HLIST_HEAD(&p->preempt_notifiers);
 #endif
@@ -4604,6 +4617,10 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 		goto out_cancel;
 	} else if (rt_prio(p->prio)) {
 		p->sched_class = &rt_sched_class;
+#ifdef CONFIG_SCHED_CLASS_EXT
+	} else if (task_should_scx(p)) {
+		p->sched_class = &ext_sched_class;
+#endif
 	} else {
 		p->sched_class = &fair_sched_class;
 	}
@@ -5511,8 +5528,10 @@ void sched_tick(void)
 		wq_worker_tick(curr);
 
 #ifdef CONFIG_SMP
-	rq->idle_balance = idle_cpu(cpu);
-	sched_balance_trigger(rq);
+	if (!scx_switched_all()) {
+		rq->idle_balance = idle_cpu(cpu);
+		sched_balance_trigger(rq);
+	}
 #endif
 }
 
@@ -6902,6 +6921,10 @@ void __setscheduler_prio(struct task_struct *p, int prio)
 		p->sched_class = &dl_sched_class;
 	else if (rt_prio(prio))
 		p->sched_class = &rt_sched_class;
+#ifdef CONFIG_SCHED_CLASS_EXT
+	else if (task_should_scx(p))
+		p->sched_class = &ext_sched_class;
+#endif
 	else
 		p->sched_class = &fair_sched_class;
 
@@ -8203,6 +8226,10 @@ void __init sched_init(void)
 	BUG_ON(!sched_class_above(&dl_sched_class, &rt_sched_class));
 	BUG_ON(!sched_class_above(&rt_sched_class, &fair_sched_class));
 	BUG_ON(!sched_class_above(&fair_sched_class, &idle_sched_class));
+#ifdef CONFIG_SCHED_CLASS_EXT
+	BUG_ON(!sched_class_above(&fair_sched_class, &ext_sched_class));
+	BUG_ON(!sched_class_above(&ext_sched_class, &idle_sched_class));
+#endif
 
 	wait_bit_init();
 
@@ -10337,3 +10364,38 @@ void sched_mm_cid_fork(struct task_struct *t)
 	t->mm_cid_active = 1;
 }
 #endif
+
+#ifdef CONFIG_SCHED_CLASS_EXT
+void sched_deq_and_put_task(struct task_struct *p, int queue_flags,
+			    struct sched_enq_and_set_ctx *ctx)
+{
+	struct rq *rq = task_rq(p);
+
+	lockdep_assert_rq_held(rq);
+
+	*ctx = (struct sched_enq_and_set_ctx){
+		.p = p,
+		.queue_flags = queue_flags,
+		.queued = task_on_rq_queued(p),
+		.running = task_current(rq, p),
+	};
+
+	update_rq_clock(rq);
+	if (ctx->queued)
+		dequeue_task(rq, p, queue_flags | DEQUEUE_NOCLOCK);
+	if (ctx->running)
+		put_prev_task(rq, p);
+}
+
+void sched_enq_and_set_task(struct sched_enq_and_set_ctx *ctx)
+{
+	struct rq *rq = task_rq(ctx->p);
+
+	lockdep_assert_rq_held(rq);
+
+	if (ctx->queued)
+		enqueue_task(rq, ctx->p, ctx->queue_flags | ENQUEUE_NOCLOCK);
+	if (ctx->running)
+		set_next_task(rq, ctx->p);
+}
+#endif	/* CONFIG_SCHED_CLASS_EXT */

kernel/sched/debug.c

@@ -1090,6 +1090,9 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
 		P(dl.runtime);
 		P(dl.deadline);
 	}
+#ifdef CONFIG_SCHED_CLASS_EXT
+	__PS("ext.enabled", task_on_scx(p));
+#endif
 #undef PN_SCHEDSTAT
 #undef P_SCHEDSTAT
 

kernel/sched/ext.h

@@ -1,15 +1,76 @@
 /* SPDX-License-Identifier: GPL-2.0 */
-
+/*
+ * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
+ * Copyright (c) 2022 David Vernet <dvernet@meta.com>
+ */
 #ifdef CONFIG_SCHED_CLASS_EXT
-#error "NOT IMPLEMENTED YET"
+
+struct sched_enq_and_set_ctx {
+	struct task_struct	*p;
+	int			queue_flags;
+	bool			queued;
+	bool			running;
+};
+
+void sched_deq_and_put_task(struct task_struct *p, int queue_flags,
+			    struct sched_enq_and_set_ctx *ctx);
+void sched_enq_and_set_task(struct sched_enq_and_set_ctx *ctx);
+
+extern const struct sched_class ext_sched_class;
+
+DECLARE_STATIC_KEY_FALSE(__scx_ops_enabled);
+DECLARE_STATIC_KEY_FALSE(__scx_switched_all);
+#define scx_enabled()		static_branch_unlikely(&__scx_ops_enabled)
+#define scx_switched_all()	static_branch_unlikely(&__scx_switched_all)
+
+static inline bool task_on_scx(const struct task_struct *p)
+{
+	return scx_enabled() && p->sched_class == &ext_sched_class;
+}
+
+void init_scx_entity(struct sched_ext_entity *scx);
+void scx_pre_fork(struct task_struct *p);
+int scx_fork(struct task_struct *p);
+void scx_post_fork(struct task_struct *p);
+void scx_cancel_fork(struct task_struct *p);
+bool task_should_scx(struct task_struct *p);
+void init_sched_ext_class(void);
+
+static inline const struct sched_class *next_active_class(const struct sched_class *class)
+{
+	class++;
+	if (scx_switched_all() && class == &fair_sched_class)
+		class++;
+	if (!scx_enabled() && class == &ext_sched_class)
+		class++;
+	return class;
+}
+
+#define for_active_class_range(class, _from, _to)				\
+	for (class = (_from); class != (_to); class = next_active_class(class))
+
+#define for_each_active_class(class)						\
+	for_active_class_range(class, __sched_class_highest, __sched_class_lowest)
+
+/*
+ * SCX requires a balance() call before every pick_next_task() call including
+ * when waking up from idle.
+ */
+#define for_balance_class_range(class, prev_class, end_class)			\
+	for_active_class_range(class, (prev_class) > &ext_sched_class ?		\
+			       &ext_sched_class : (prev_class), (end_class))
+
 #else	/* CONFIG_SCHED_CLASS_EXT */
 
 #define scx_enabled()		false
+#define scx_switched_all()	false
 
 static inline void scx_pre_fork(struct task_struct *p) {}
 static inline int scx_fork(struct task_struct *p) { return 0; }
 static inline void scx_post_fork(struct task_struct *p) {}
 static inline void scx_cancel_fork(struct task_struct *p) {}
+static inline bool task_on_scx(const struct task_struct *p) { return false; }
 static inline void init_sched_ext_class(void) {}
 
 #define for_each_active_class		for_each_class
@@ -18,7 +79,13 @@ static inline void init_sched_ext_class(void) {}
 #endif	/* CONFIG_SCHED_CLASS_EXT */
 
 #if defined(CONFIG_SCHED_CLASS_EXT) && defined(CONFIG_SMP)
-#error "NOT IMPLEMENTED YET"
+void __scx_update_idle(struct rq *rq, bool idle);
+
+static inline void scx_update_idle(struct rq *rq, bool idle)
+{
+	if (scx_enabled())
+		__scx_update_idle(rq, idle);
+}
 #else
 static inline void scx_update_idle(struct rq *rq, bool idle) {}
 #endif

kernel/sched/sched.h

@@ -194,6 +194,10 @@ static inline int idle_policy(int policy)
 
 static inline int normal_policy(int policy)
 {
+#ifdef CONFIG_SCHED_CLASS_EXT
+	if (policy == SCHED_EXT)
+		return true;
+#endif
 	return policy == SCHED_NORMAL;
 }
 
@@ -719,6 +723,16 @@ struct cfs_rq {
 #endif /* CONFIG_FAIR_GROUP_SCHED */
 };
 
+#ifdef CONFIG_SCHED_CLASS_EXT
+struct scx_rq {
+	struct scx_dispatch_q	local_dsq;
+	struct list_head	runnable_list;		/* runnable tasks on this rq */
+	unsigned long		ops_qseq;
+	u64			extra_enq_flags;	/* see move_task_to_local_dsq() */
+	u32			nr_running;
+};
+#endif /* CONFIG_SCHED_CLASS_EXT */
+
 static inline int rt_bandwidth_enabled(void)
 {
 	return sysctl_sched_rt_runtime >= 0;
@@ -1066,6 +1080,9 @@ struct rq {
 	struct cfs_rq		cfs;
 	struct rt_rq		rt;
 	struct dl_rq		dl;
+#ifdef CONFIG_SCHED_CLASS_EXT
+	struct scx_rq		scx;
+#endif
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	/* list of leaf cfs_rq on this CPU: */

kernel/sched/syscalls.c

@@ -1622,6 +1622,7 @@ SYSCALL_DEFINE1(sched_get_priority_max, int, policy)
 	case SCHED_NORMAL:
 	case SCHED_BATCH:
 	case SCHED_IDLE:
+	case SCHED_EXT:
 		ret = 0;
 		break;
 	}
@@ -1649,6 +1650,7 @@ SYSCALL_DEFINE1(sched_get_priority_min, int, policy)
 	case SCHED_NORMAL:
 	case SCHED_BATCH:
 	case SCHED_IDLE:
+	case SCHED_EXT:
 		ret = 0;
 	}
 	return ret;