This repository was archived by the owner on Apr 13, 2023. It is now read-only.
Commit
This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
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.
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 <[email protected]>
Co-authored-by: David Vernet <[email protected]>
Acked-by: Josh Don <[email protected]>
Acked-by: Hao Luo <[email protected]>
Acked-by: Barret Rhoden <[email protected]>- Loading branch information
