From f0e1a0643a59bf1f922fa209cec86a170b784f3f Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 18 Jun 2024 10:09:17 -1000 Subject: sched_ext: Implement BPF extensible scheduler class MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 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 Co-authored-by: David Vernet Acked-by: Josh Don Acked-by: Hao Luo Acked-by: Barret Rhoden Cc: Andrea Righi --- init/init_task.c | 11 +++++++++++ 1 file changed, 11 insertions(+) (limited to 'init/init_task.c') diff --git a/init/init_task.c b/init/init_task.c index eeb110c65fe2..c6804396fe12 100644 --- a/init/init_task.c +++ b/init/init_task.c @@ -6,6 +6,7 @@ #include #include #include +#include #include #include #include @@ -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), -- cgit v1.2.3 From 8a010b81b3a50b033fc3cddc613517abda586cbe Mon Sep 17 00:00:00 2001 From: David Vernet Date: Tue, 18 Jun 2024 10:09:18 -1000 Subject: sched_ext: Implement runnable task stall watchdog The most common and critical way that a BPF scheduler can misbehave is by failing to run runnable tasks for too long. This patch implements a watchdog. * All tasks record when they become runnable. * A watchdog work periodically scans all runnable tasks. If any task has stayed runnable for too long, the BPF scheduler is aborted. * scheduler_tick() monitors whether the watchdog itself is stuck. If so, the BPF scheduler is aborted. Because the watchdog only scans the tasks which are currently runnable and usually very infrequently, the overhead should be negligible. scx_qmap is updated so that it can be told to stall user and/or kernel tasks. A detected task stall looks like the following: sched_ext: BPF scheduler "qmap" errored, disabling sched_ext: runnable task stall (dbus-daemon[953] failed to run for 6.478s) scx_check_timeout_workfn+0x10e/0x1b0 process_one_work+0x287/0x560 worker_thread+0x234/0x420 kthread+0xe9/0x100 ret_from_fork+0x1f/0x30 A detected watchdog stall: sched_ext: BPF scheduler "qmap" errored, disabling sched_ext: runnable task stall (watchdog failed to check in for 5.001s) scheduler_tick+0x2eb/0x340 update_process_times+0x7a/0x90 tick_sched_timer+0xd8/0x130 __hrtimer_run_queues+0x178/0x3b0 hrtimer_interrupt+0xfc/0x390 __sysvec_apic_timer_interrupt+0xb7/0x2b0 sysvec_apic_timer_interrupt+0x90/0xb0 asm_sysvec_apic_timer_interrupt+0x1b/0x20 default_idle+0x14/0x20 arch_cpu_idle+0xf/0x20 default_idle_call+0x50/0x90 do_idle+0xe8/0x240 cpu_startup_entry+0x1d/0x20 kernel_init+0x0/0x190 start_kernel+0x0/0x392 start_kernel+0x324/0x392 x86_64_start_reservations+0x2a/0x2c x86_64_start_kernel+0x104/0x109 secondary_startup_64_no_verify+0xce/0xdb Note that this patch exposes scx_ops_error[_type]() in kernel/sched/ext.h to inline scx_notify_sched_tick(). v4: - While disabling, cancel_delayed_work_sync(&scx_watchdog_work) was being called before forward progress was guaranteed and thus could lead to system lockup. Relocated. - While enabling, it was comparing msecs against jiffies without conversion leading to spurious load failures on lower HZ kernels. Fixed. - runnable list management is now used by core bypass logic and moved to the patch implementing sched_ext core. v3: - bpf_scx_init_member() was incorrectly comparing ops->timeout_ms against SCX_WATCHDOG_MAX_TIMEOUT which is in jiffies without conversion leading to spurious load failures in lower HZ kernels. Fixed. v2: - Julia Lawall noticed that the watchdog code was mixing msecs and jiffies. Fix by using jiffies for everything. Signed-off-by: David Vernet Reviewed-by: Tejun Heo Signed-off-by: Tejun Heo Acked-by: Josh Don Acked-by: Hao Luo Acked-by: Barret Rhoden Cc: Julia Lawall --- init/init_task.c | 1 + 1 file changed, 1 insertion(+) (limited to 'init/init_task.c') diff --git a/init/init_task.c b/init/init_task.c index c6804396fe12..8a44c932d10f 100644 --- a/init/init_task.c +++ b/init/init_task.c @@ -106,6 +106,7 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = { .sticky_cpu = -1, .holding_cpu = -1, .runnable_node = LIST_HEAD_INIT(init_task.scx.runnable_node), + .runnable_at = INITIAL_JIFFIES, .ddsp_dsq_id = SCX_DSQ_INVALID, .slice = SCX_SLICE_DFL, }, -- cgit v1.2.3 From 06e51be3d5e7a07aea5c9012773df8d5de01db6c Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 18 Jun 2024 10:09:21 -1000 Subject: sched_ext: Add vtime-ordered priority queue to dispatch_q's Currently, a dsq is always a FIFO. A task which is dispatched earlier gets consumed or executed earlier. While this is sufficient when dsq's are used for simple staging areas for tasks which are ready to execute, it'd make dsq's a lot more useful if they can implement custom ordering. This patch adds a vtime-ordered priority queue to dsq's. When the BPF scheduler dispatches a task with the new scx_bpf_dispatch_vtime() helper, it can specify the vtime tha the task should be inserted at and the task is inserted into the priority queue in the dsq which is ordered according to time_before64() comparison of the vtime values. A DSQ can either be a FIFO or priority queue and automatically switches between the two depending on whether scx_bpf_dispatch() or scx_bpf_dispatch_vtime() is used. Using the wrong variant while the DSQ already has the other type queued is not allowed and triggers an ops error. Built-in DSQs must always be FIFOs. This makes it very easy for the BPF schedulers to implement proper vtime based scheduling within each dsq very easy and efficient at a negligible cost in terms of code complexity and overhead. scx_simple and scx_example_flatcg are updated to default to weighted vtime scheduling (the latter within each cgroup). FIFO scheduling can be selected with -f option. v4: - As allowing mixing priority queue and FIFO on the same DSQ sometimes led to unexpected starvations, DSQs now error out if both modes are used at the same time and the built-in DSQs are no longer allowed to be priority queues. - Explicit type struct scx_dsq_node added to contain fields needed to be linked on DSQs. This will be used to implement stateful iterator. - Tasks are now always linked on dsq->list whether the DSQ is in FIFO or PRIQ mode. This confines PRIQ related complexities to the enqueue and dequeue paths. Other paths only need to look at dsq->list. This will also ease implementing BPF iterator. - Print p->scx.dsq_flags in debug dump. v3: - SCX_TASK_DSQ_ON_PRIQ flag is moved from p->scx.flags into its own p->scx.dsq_flags. The flag is protected with the dsq lock unlike other flags in p->scx.flags. This led to flag corruption in some cases. - Add comments explaining the interaction between using consumption of p->scx.slice to determine vtime progress and yielding. v2: - p->scx.dsq_vtime was not initialized on load or across cgroup migrations leading to some tasks being stalled for extended period of time depending on how saturated the machine is. Fixed. Signed-off-by: Tejun Heo Reviewed-by: David Vernet --- init/init_task.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'init/init_task.c') diff --git a/init/init_task.c b/init/init_task.c index 8a44c932d10f..5726b3a0eea9 100644 --- a/init/init_task.c +++ b/init/init_task.c @@ -102,7 +102,7 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = { #endif #ifdef CONFIG_SCHED_CLASS_EXT .scx = { - .dsq_node = LIST_HEAD_INIT(init_task.scx.dsq_node), + .dsq_node.list = LIST_HEAD_INIT(init_task.scx.dsq_node.list), .sticky_cpu = -1, .holding_cpu = -1, .runnable_node = LIST_HEAD_INIT(init_task.scx.runnable_node), -- cgit v1.2.3 From d4af01c3731ff9c6e224d7183f8226a56d72b56c Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Mon, 8 Jul 2024 14:30:55 -1000 Subject: sched_ext: Take out ->priq and ->flags from scx_dsq_node struct scx_dsq_node contains two data structure nodes to link the containing task to a DSQ and a flags field that is protected by the lock of the associated DSQ. One reason why they are grouped into a struct is to use the type independently as a cursor node when iterating tasks on a DSQ. However, when iterating, the cursor only needs to be linked on the FIFO list and the rb_node part ends up inflating the size of the iterator data structure unnecessarily making it potentially too expensive to place it on stack. Take ->priq and ->flags out of scx_dsq_node and put them in sched_ext_entity as ->dsq_priq and ->dsq_flags, respectively. scx_dsq_node is renamed to scx_dsq_list_node and the field names are renamed accordingly. This will help implementing DSQ task iterator that can be allocated on stack. No functional change intended. Signed-off-by: Tejun Heo Suggested-by: Alexei Starovoitov Acked-by: Alexei Starovoitov Cc: David Vernet --- init/init_task.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'init/init_task.c') diff --git a/init/init_task.c b/init/init_task.c index 5726b3a0eea9..e222722e790b 100644 --- a/init/init_task.c +++ b/init/init_task.c @@ -102,7 +102,7 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = { #endif #ifdef CONFIG_SCHED_CLASS_EXT .scx = { - .dsq_node.list = LIST_HEAD_INIT(init_task.scx.dsq_node.list), + .dsq_list.node = LIST_HEAD_INIT(init_task.scx.dsq_list.node), .sticky_cpu = -1, .holding_cpu = -1, .runnable_node = LIST_HEAD_INIT(init_task.scx.runnable_node), -- cgit v1.2.3