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 invoking the SysRq key sequence SysRq-S.

When the BPF scheduler triggers an error, debug information is dumped to aid debugging. The debug dump is passed to and printed out by the scheduler binary. The debug dump can also be accessed through the sched_ext_dump tracepoint. The SysRq key sequence SysRq-D triggers a debug dump. This doesn’t terminate the BPF scheduler and can only be read through the tracepoint.

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. The following config options should be enabled to use sched_ext:

CONFIG_BPF=y
CONFIG_SCHED_CLASS_EXT=y
CONFIG_BPF_SYSCALL=y
CONFIG_BPF_JIT=y
CONFIG_DEBUG_INFO_BTF=y
CONFIG_BPF_JIT_ALWAYS_ON=y
CONFIG_BPF_JIT_DEFAULT_ON=y
CONFIG_PAHOLE_HAS_SPLIT_BTF=y
CONFIG_PAHOLE_HAS_BTF_TAG=y

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 the fair-class scheduler until the BPF scheduler is loaded.

When the BPF scheduler is loaded and SCX_OPS_SWITCH_PARTIAL is not set in ops->flags, all SCHED_NORMAL, SCHED_BATCH, SCHED_IDLE, and SCHED_EXT tasks are scheduled by sched_ext.

However, when the BPF scheduler is loaded and SCX_OPS_SWITCH_PARTIAL is set in ops->flags, only tasks with the SCHED_EXT policy are scheduled by sched_ext, while tasks with SCHED_NORMAL, SCHED_BATCH and SCHED_IDLE policies are scheduled by the fair-class scheduler.

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 the fair-class scheduler.

# make -j16 -C tools/sched_ext
# tools/sched_ext/build/bin/scx_simple
local=0 global=3
local=5 global=24
local=9 global=44
local=13 global=56
local=17 global=72
^CEXIT: BPF scheduler unregistered

The current status of the BPF scheduler can be determined as follows:

# cat /sys/kernel/sched_ext/state
enabled
# cat /sys/kernel/sched_ext/root/ops
simple

You can check if any BPF scheduler has ever been loaded since boot by examining this monotonically incrementing counter (a value of zero indicates that no BPF scheduler has been loaded):

# cat /sys/kernel/sched_ext/enable_seq
1

tools/sched_ext/scx_show_state.py is a drgn script which shows more detailed information:

# tools/sched_ext/scx_show_state.py
ops           : simple
enabled       : 1
switching_all : 1
switched_all  : 1
enable_state  : enabled (2)
bypass_depth  : 0
nr_rejected   : 0
enable_seq    : 1

Whether a given task is on sched_ext can be determined as follows:

# 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 ops.name which must be a valid BPF object name. All operations are optional. The following modified excerpt is from tools/sched_ext/scx_simple.bpf.c showing a minimal global FIFO scheduler.

/*
* Decide which CPU a task should be migrated to before being
* enqueued (either at wakeup, fork time, or exec time). If an
* idle core is found by the default ops.select_cpu() implementation,
* then insert the task directly into SCX_DSQ_LOCAL and skip the
* ops.enqueue() callback.
*
* Note that this implementation has exactly the same behavior as the
* default ops.select_cpu implementation. The behavior of the scheduler
* would be exactly same if the implementation just didn't define the
* simple_select_cpu() struct_ops prog.
*/
s32 BPF_STRUCT_OPS(simple_select_cpu, struct task_struct *p,
s32 prev_cpu, u64 wake_flags)
{
s32 cpu;
/* Need to initialize or the BPF verifier will reject the program */
bool direct = false;

cpu = scx_bpf_select_cpu_dfl(p, prev_cpu, wake_flags, &direct);

if (direct)
scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_DFL, 0);

return cpu;
}

/*
* Do a direct insertion of a task to the global DSQ. This ops.enqueue()
* callback will only be invoked if we failed to find a core to insert
* into in ops.select_cpu() above.
*
* Note that this implementation has exactly the same behavior as the
* default ops.enqueue implementation, which just dispatches the task
* to SCX_DSQ_GLOBAL. The behavior of the scheduler would be exactly same
* if the implementation just didn't define the simple_enqueue struct_ops
* prog.
*/
void BPF_STRUCT_OPS(simple_enqueue, struct task_struct *p, u64 enq_flags)
{
scx_bpf_dsq_insert(p, SCX_DSQ_GLOBAL, SCX_SLICE_DFL, enq_flags);
}

s32 BPF_STRUCT_OPS_SLEEPABLE(simple_init)
{
/*
* By default, all SCHED_EXT, SCHED_OTHER, SCHED_IDLE, and
* SCHED_BATCH tasks should use sched_ext.
*/
return 0;
}

void BPF_STRUCT_OPS(simple_exit, struct scx_exit_info *ei)
{
exit_type = ei->type;
}

SEC(".struct_ops")
struct sched_ext_ops simple_ops = {
.select_cpu             = (void *)simple_select_cpu,
.enqueue                = (void *)simple_enqueue,
.init                   = (void *)simple_init,
.exit                   = (void *)simple_exit,
.name                   = "simple",
};

Dispatch Queues¶

To match the impedance between the scheduler core and the BPF scheduler, sched_ext uses DSQs (dispatch queues) which can operate as both a FIFO and a priority queue. 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 DSQs using scx_bpf_create_dsq() and scx_bpf_destroy_dsq().

A CPU always executes a task from its local DSQ. A task is “inserted” into a DSQ. A task in a non-local DSQ is “move”d into the target CPU’s local DSQ.

When a CPU is looking for the next task to run, if the local DSQ is not empty, the first task is picked. Otherwise, the CPU tries to move a task from the global DSQ. If that doesn’t yield a runnable task either, ops.dispatch() is invoked.

Scheduling Cycle¶

The following briefly shows how a waking task is scheduled and executed.

When a task is waking up, ops.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 ops.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 ops.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 ops.select_cpu() judiciously can be simpler and more efficient.

A task can be immediately inserted into a DSQ from ops.select_cpu() by calling scx_bpf_dsq_insert(). If the task is inserted into SCX_DSQ_LOCAL from ops.select_cpu(), it will be inserted into the local DSQ of whichever CPU is returned from ops.select_cpu(). Additionally, inserting directly from ops.select_cpu() will cause the ops.enqueue() callback to be skipped.

Note that the scheduler core will ignore an invalid CPU selection, for example, if it’s outside the allowed cpumask of the task. 1.

Once the target CPU is selected, ops.enqueue() is invoked (unless the task was inserted directly from ops.select_cpu()). ops.enqueue() can make one of the following decisions:

Immediately insert the task into either the global or a local DSQ by calling scx_bpf_dsq_insert() with one of the following options: SCX_DSQ_GLOBAL, SCX_DSQ_LOCAL, or SCX_DSQ_LOCAL_ON | cpu.

Immediately insert the task into a custom DSQ by calling scx_bpf_dsq_insert() with a DSQ ID which is smaller than 2^63.

Queue the task on the BPF side. 1.

When a CPU is ready to schedule, it first looks at its local DSQ. If empty, it then looks at the global DSQ. If there still isn’t a task to run, ops.dispatch() is invoked which can use the following two functions to populate the local DSQ.

scx_bpf_dsq_insert() inserts a task to a DSQ. Any target DSQ can be used - SCX_DSQ_LOCAL, SCX_DSQ_LOCAL_ON | cpu, SCX_DSQ_GLOBAL or a custom DSQ. While scx_bpf_dsq_insert() currently can’t be called with BPF locks held, this is being worked on and will be supported. scx_bpf_dsq_insert() schedules insertion rather than performing them immediately. There can be up to ops.dispatch_max_batch pending tasks.

scx_bpf_move_to_local() moves a task from the specified non-local DSQ to the dispatching DSQ. This function cannot be called with any BPF locks held. scx_bpf_move_to_local() flushes the pending insertions tasks before trying to move from the specified DSQ. 1.

After ops.dispatch() returns, if there are tasks in the local DSQ, the CPU runs the first one. If empty, the following steps are taken:

Try to move from the global DSQ. If successful, run the task.

If ops.dispatch() has dispatched any tasks, retry #3.

If the previous task is an SCX task and still runnable, keep executing it (see SCX_OPS_ENQ_LAST).

Go idle.

Note that the BPF scheduler can always choose to dispatch tasks immediately in ops.enqueue() as illustrated in the above simple example. If only the built-in DSQs are used, there is no need to implement ops.dispatch() as a task is never queued on the BPF scheduler and both the local and global DSQs are executed automatically.

scx_bpf_dsq_insert() inserts the task on the FIFO of the target DSQ. Use scx_bpf_dsq_insert_vtime() for the priority queue. Internal DSQs such as SCX_DSQ_LOCAL and SCX_DSQ_GLOBAL do not support priority-queue dispatching, and must be dispatched to with scx_bpf_dsq_insert(). See the function documentation and usage in tools/sched_ext/scx_simple.bpf.c for more information.

Task Lifecycle¶

The following pseudo-code summarizes the entire lifecycle of a task managed by a sched_ext scheduler:

ops.init_task();            /* A new task is created */
ops.enable();               /* Enable BPF scheduling for the task */

while (task in SCHED_EXT) {
if (task can migrate)
ops.select_cpu();   /* Called on wakeup (optimization) */

ops.runnable();         /* Task becomes ready to run */

while (task is runnable) {
if (task is not in a DSQ && task->scx.slice == 0) {
ops.enqueue();  /* Task can be added to a DSQ */

/* Any usable CPU becomes available */

ops.dispatch(); /* Task is moved to a local DSQ */
}
ops.running();      /* Task starts running on its assigned CPU */
while (task->scx.slice > 0 && task is runnable)
ops.tick();     /* Called every 1/HZ seconds */
ops.stopping();     /* Task stops running (time slice expires or wait) */

/* Task's CPU becomes available */

ops.dispatch();     /* task->scx.slice can be refilled */
}

ops.quiescent();        /* Task releases its assigned CPU (wait) */
}

ops.disable();              /* Disable BPF scheduling for the task */
ops.exit_task();            /* Task is destroyed */

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_simple[.bpf].c: Minimal global FIFO scheduler example using a custom DSQ.

scx_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.