From 0c4a59df370bea245695c00aaae6ae75747139bd Mon Sep 17 00:00:00 2001 From: Changwoo Min Date: Thu, 2 Apr 2026 11:31:50 +0900 Subject: sched_ext: Fix is_bpf_migration_disabled() false negative on non-PREEMPT_RCU Since commit 8e4f0b1ebcf2 ("bpf: use rcu_read_lock_dont_migrate() for trampoline.c"), the BPF prolog (__bpf_prog_enter) calls migrate_disable() only when CONFIG_PREEMPT_RCU is enabled, via rcu_read_lock_dont_migrate(). Without CONFIG_PREEMPT_RCU, the prolog never touches migration_disabled, so migration_disabled == 1 always means the task is truly migration-disabled regardless of whether it is the current task. The old unconditional p == current check was a false negative in this case, potentially allowing a migration-disabled task to be dispatched to a remote CPU and triggering scx_error in task_can_run_on_remote_rq(). Only apply the p == current disambiguation when CONFIG_PREEMPT_RCU is enabled, where the ambiguity with the BPF prolog still exists. Fixes: 8e4f0b1ebcf2 ("bpf: use rcu_read_lock_dont_migrate() for trampoline.c") Cc: stable@vger.kernel.org # v6.18+ Link: https://lore.kernel.org/lkml/20250821090609.42508-8-dongml2@chinatelecom.cn/ Signed-off-by: Changwoo Min Reviewed-by: Andrea Righi Signed-off-by: Tejun Heo --- kernel/sched/ext_idle.c | 31 +++++++++++++++++++------------ 1 file changed, 19 insertions(+), 12 deletions(-) diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c index 0ae93cd64004..44c3a50c542c 100644 --- a/kernel/sched/ext_idle.c +++ b/kernel/sched/ext_idle.c @@ -860,25 +860,32 @@ static bool check_builtin_idle_enabled(struct scx_sched *sch) * code. * * We can't simply check whether @p->migration_disabled is set in a - * sched_ext callback, because migration is always disabled for the current - * task while running BPF code. + * sched_ext callback, because the BPF prolog (__bpf_prog_enter) may disable + * migration for the current task while running BPF code. * - * The prolog (__bpf_prog_enter) and epilog (__bpf_prog_exit) respectively - * disable and re-enable migration. For this reason, the current task - * inside a sched_ext callback is always a migration-disabled task. + * Since the BPF prolog calls migrate_disable() only when CONFIG_PREEMPT_RCU + * is enabled (via rcu_read_lock_dont_migrate()), migration_disabled == 1 for + * the current task is ambiguous only in that case: it could be from the BPF + * prolog rather than a real migrate_disable() call. * - * Therefore, when @p->migration_disabled == 1, check whether @p is the - * current task or not: if it is, then migration was not disabled before - * entering the callback, otherwise migration was disabled. + * Without CONFIG_PREEMPT_RCU, the BPF prolog never calls migrate_disable(), + * so migration_disabled == 1 always means the task is truly + * migration-disabled. + * + * Therefore, when migration_disabled == 1 and CONFIG_PREEMPT_RCU is enabled, + * check whether @p is the current task or not: if it is, then migration was + * not disabled before entering the callback, otherwise migration was disabled. * * Returns true if @p is migration-disabled, false otherwise. */ static bool is_bpf_migration_disabled(const struct task_struct *p) { - if (p->migration_disabled == 1) - return p != current; - else - return p->migration_disabled; + if (p->migration_disabled == 1) { + if (IS_ENABLED(CONFIG_PREEMPT_RCU)) + return p != current; + return true; + } + return p->migration_disabled; } static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p, -- cgit v1.2.3 From 7e0ffb72de8aa3b25989c2d980e81b829c577010 Mon Sep 17 00:00:00 2001 From: Andrea Righi Date: Fri, 3 Apr 2026 08:57:20 +0200 Subject: sched_ext: Fix stale direct dispatch state in ddsp_dsq_id @p->scx.ddsp_dsq_id can be left set (non-SCX_DSQ_INVALID) triggering a spurious warning in mark_direct_dispatch() when the next wakeup's ops.select_cpu() calls scx_bpf_dsq_insert(), such as: WARNING: kernel/sched/ext.c:1273 at scx_dsq_insert_commit+0xcd/0x140 The root cause is that ddsp_dsq_id was only cleared in dispatch_enqueue(), which is not reached in all paths that consume or cancel a direct dispatch verdict. Fix it by clearing it at the right places: - direct_dispatch(): cache the direct dispatch state in local variables and clear it before dispatch_enqueue() on the synchronous path. For the deferred path, the direct dispatch state must remain set until process_ddsp_deferred_locals() consumes them. - process_ddsp_deferred_locals(): cache the dispatch state in local variables and clear it before calling dispatch_to_local_dsq(), which may migrate the task to another rq. - do_enqueue_task(): clear the dispatch state on the enqueue path (local/global/bypass fallbacks), where the direct dispatch verdict is ignored. - dequeue_task_scx(): clear the dispatch state after dispatch_dequeue() to handle both the deferred dispatch cancellation and the holding_cpu race, covering all cases where a pending direct dispatch is cancelled. - scx_disable_task(): clear the direct dispatch state when transitioning a task out of the current scheduler. Waking tasks may have had the direct dispatch state set by the outgoing scheduler's ops.select_cpu() and then been queued on a wake_list via ttwu_queue_wakelist(), when SCX_OPS_ALLOW_QUEUED_WAKEUP is set. Such tasks are not on the runqueue and are not iterated by scx_bypass(), so their direct dispatch state won't be cleared. Without this clear, any subsequent SCX scheduler that tries to direct dispatch the task will trigger the WARN_ON_ONCE() in mark_direct_dispatch(). Fixes: 5b26f7b920f7 ("sched_ext: Allow SCX_DSQ_LOCAL_ON for direct dispatches") Cc: stable@vger.kernel.org # v6.12+ Cc: Daniel Hodges Cc: Patrick Somaru Signed-off-by: Andrea Righi Signed-off-by: Tejun Heo --- kernel/sched/ext.c | 49 +++++++++++++++++++++++++++++++++++-------------- 1 file changed, 35 insertions(+), 14 deletions(-) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index d5bdcdb3f700..064eaa76be4b 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -1109,15 +1109,6 @@ static void dispatch_enqueue(struct scx_sched *sch, struct scx_dispatch_q *dsq, dsq_mod_nr(dsq, 1); p->scx.dsq = dsq; - /* - * scx.ddsp_dsq_id and scx.ddsp_enq_flags are only relevant on the - * direct dispatch path, but we clear them here because the direct - * dispatch verdict may be overridden on the enqueue path during e.g. - * bypass. - */ - p->scx.ddsp_dsq_id = SCX_DSQ_INVALID; - p->scx.ddsp_enq_flags = 0; - /* * We're transitioning out of QUEUEING or DISPATCHING. store_release to * match waiters' load_acquire. @@ -1283,12 +1274,34 @@ static void mark_direct_dispatch(struct scx_sched *sch, p->scx.ddsp_enq_flags = enq_flags; } +/* + * Clear @p direct dispatch state when leaving the scheduler. + * + * Direct dispatch state must be cleared in the following cases: + * - direct_dispatch(): cleared on the synchronous enqueue path, deferred + * dispatch keeps the state until consumed + * - process_ddsp_deferred_locals(): cleared after consuming deferred state, + * - do_enqueue_task(): cleared on enqueue fallbacks where the dispatch + * verdict is ignored (local/global/bypass) + * - dequeue_task_scx(): cleared after dispatch_dequeue(), covering deferred + * cancellation and holding_cpu races + * - scx_disable_task(): cleared for queued wakeup tasks, which are excluded by + * the scx_bypass() loop, so that stale state is not reused by a subsequent + * scheduler instance + */ +static inline void clear_direct_dispatch(struct task_struct *p) +{ + p->scx.ddsp_dsq_id = SCX_DSQ_INVALID; + p->scx.ddsp_enq_flags = 0; +} + static void direct_dispatch(struct scx_sched *sch, struct task_struct *p, u64 enq_flags) { struct rq *rq = task_rq(p); struct scx_dispatch_q *dsq = find_dsq_for_dispatch(sch, rq, p->scx.ddsp_dsq_id, p); + u64 ddsp_enq_flags; touch_core_sched_dispatch(rq, p); @@ -1329,8 +1342,10 @@ static void direct_dispatch(struct scx_sched *sch, struct task_struct *p, return; } - dispatch_enqueue(sch, dsq, p, - p->scx.ddsp_enq_flags | SCX_ENQ_CLEAR_OPSS); + ddsp_enq_flags = p->scx.ddsp_enq_flags; + clear_direct_dispatch(p); + + dispatch_enqueue(sch, dsq, p, ddsp_enq_flags | SCX_ENQ_CLEAR_OPSS); } static bool scx_rq_online(struct rq *rq) @@ -1439,6 +1454,7 @@ enqueue: */ touch_core_sched(rq, p); refill_task_slice_dfl(sch, p); + clear_direct_dispatch(p); dispatch_enqueue(sch, dsq, p, enq_flags); } @@ -1610,6 +1626,7 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int deq_flags sub_nr_running(rq, 1); dispatch_dequeue(rq, p); + clear_direct_dispatch(p); return true; } @@ -2293,13 +2310,15 @@ static void process_ddsp_deferred_locals(struct rq *rq) struct task_struct, scx.dsq_list.node))) { struct scx_sched *sch = scx_root; struct scx_dispatch_q *dsq; + u64 dsq_id = p->scx.ddsp_dsq_id; + u64 enq_flags = p->scx.ddsp_enq_flags; list_del_init(&p->scx.dsq_list.node); + clear_direct_dispatch(p); - dsq = find_dsq_for_dispatch(sch, rq, p->scx.ddsp_dsq_id, p); + dsq = find_dsq_for_dispatch(sch, rq, dsq_id, p); if (!WARN_ON_ONCE(dsq->id != SCX_DSQ_LOCAL)) - dispatch_to_local_dsq(sch, rq, dsq, p, - p->scx.ddsp_enq_flags); + dispatch_to_local_dsq(sch, rq, dsq, p, enq_flags); } } @@ -3015,6 +3034,8 @@ static void scx_disable_task(struct task_struct *p) lockdep_assert_rq_held(rq); WARN_ON_ONCE(scx_get_task_state(p) != SCX_TASK_ENABLED); + clear_direct_dispatch(p); + if (SCX_HAS_OP(sch, disable)) SCX_CALL_OP_TASK(sch, SCX_KF_REST, disable, rq, p); scx_set_task_state(p, SCX_TASK_READY); -- cgit v1.2.3