Re: [PATCH 12/36] sched_ext: Implement BPF extensible scheduler class

[Andrea Righi <andrea.righi@canonical.com>]

On Fri, Nov 10, 2023 at 04:47:38PM -1000, Tejun Heo wrote:
> 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.
> 
> 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 cff9b2332ab7 ("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>

...

> +#ifdef CONFIG_SCHED_DEBUG
> +static const char *scx_ops_enable_state_str[] = {
> +	[SCX_OPS_PREPPING]	= "prepping",
> +	[SCX_OPS_ENABLING]	= "enabling",
> +	[SCX_OPS_ENABLED]	= "enabled",
> +	[SCX_OPS_DISABLING]	= "disabling",
> +	[SCX_OPS_DISABLED]	= "disabled",
> +};

We may want to move scx_ops_enable_state_str[] outside of
CONFIG_SCHED_DEBUG, because we're using it later in print_scx_info()
("sched_ext: Print sched_ext info when dumping stack"), or we make
print_scx_info() dependent of CONFIG_SCHED_DEBUG.

-Andrea