From: Tejun Heo Date: Fri, 10 Apr 2026 17:54:06 +0000 (-1000) Subject: sched_ext: Decouple kfunc unlocked-context check from kf_mask X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=0022b328504d1055be57eecf9e02c00e2ddcb0a2;p=thirdparty%2Fkernel%2Flinux.git sched_ext: Decouple kfunc unlocked-context check from kf_mask scx_kf_allowed_if_unlocked() uses !current->scx.kf_mask as a proxy for "no SCX-tracked lock held". kf_mask is removed in a follow-up patch, so its two callers - select_cpu_from_kfunc() and scx_dsq_move() - need another basis. Add a new bool scx_rq.in_select_cpu, set across the SCX_CALL_OP_TASK_RET that invokes ops.select_cpu(), to capture the one case where SCX itself holds no lock but try_to_wake_up() holds @p's pi_lock. Together with scx_locked_rq(), it expresses the same accepted-context set. select_cpu_from_kfunc() needs a runtime test because it has to take different locking paths depending on context. Open-code as a three-way branch. The unlocked branch takes raw_spin_lock_irqsave(&p->pi_lock) directly - pi_lock alone is enough for the fields the kfunc reads, and is lighter than task_rq_lock(). scx_dsq_move() doesn't really need a runtime test - its accepted contexts could be enforced at verifier load time. But since the runtime state is already there and using it keeps the upcoming load-time filter simpler, just write it the same way: (scx_locked_rq() || in_select_cpu) && !kf_allowed(DISPATCH). scx_kf_allowed_if_unlocked() is deleted with the conversions. No semantic change. v2: s/No functional change/No semantic change/ - the unlocked path now acquires pi_lock instead of the heavier task_rq_lock() (Andrea Righi). Signed-off-by: Tejun Heo Reviewed-by: Andrea Righi --- diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index 4d793a56d965..fa266fd7afd5 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -3308,10 +3308,12 @@ static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flag WARN_ON_ONCE(*ddsp_taskp); *ddsp_taskp = p; + this_rq()->scx.in_select_cpu = true; cpu = SCX_CALL_OP_TASK_RET(sch, SCX_KF_ENQUEUE | SCX_KF_SELECT_CPU, select_cpu, NULL, p, prev_cpu, wake_flags); + this_rq()->scx.in_select_cpu = false; p->scx.selected_cpu = cpu; *ddsp_taskp = NULL; if (ops_cpu_valid(sch, cpu, "from ops.select_cpu()")) @@ -8144,7 +8146,7 @@ static bool scx_dsq_move(struct bpf_iter_scx_dsq_kern *kit, bool in_balance; unsigned long flags; - if (!scx_kf_allowed_if_unlocked() && + if ((scx_locked_rq() || this_rq()->scx.in_select_cpu) && !scx_kf_allowed(sch, SCX_KF_DISPATCH)) return false; diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c index 8c31fb65477c..f99ceeba2e56 100644 --- a/kernel/sched/ext_idle.c +++ b/kernel/sched/ext_idle.c @@ -913,8 +913,8 @@ static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p, s32 prev_cpu, u64 wake_flags, const struct cpumask *allowed, u64 flags) { - struct rq *rq; - struct rq_flags rf; + unsigned long irq_flags; + bool we_locked = false; s32 cpu; if (!ops_cpu_valid(sch, prev_cpu, NULL)) @@ -924,27 +924,22 @@ static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p, return -EBUSY; /* - * If called from an unlocked context, acquire the task's rq lock, - * so that we can safely access p->cpus_ptr and p->nr_cpus_allowed. + * Accessing p->cpus_ptr / p->nr_cpus_allowed needs either @p's rq + * lock or @p's pi_lock. Three cases: * - * Otherwise, allow to use this kfunc only from ops.select_cpu() - * and ops.select_enqueue(). + * - inside ops.select_cpu(): try_to_wake_up() holds @p's pi_lock. + * - other rq-locked SCX op: scx_locked_rq() points at the held rq. + * - truly unlocked (UNLOCKED ops, SYSCALL, non-SCX struct_ops): + * nothing held, take pi_lock ourselves. */ - if (scx_kf_allowed_if_unlocked()) { - rq = task_rq_lock(p, &rf); - } else { - if (!scx_kf_allowed(sch, SCX_KF_SELECT_CPU | SCX_KF_ENQUEUE)) - return -EPERM; - rq = scx_locked_rq(); - } - - /* - * Validate locking correctness to access p->cpus_ptr and - * p->nr_cpus_allowed: if we're holding an rq lock, we're safe; - * otherwise, assert that p->pi_lock is held. - */ - if (!rq) + if (this_rq()->scx.in_select_cpu) { lockdep_assert_held(&p->pi_lock); + } else if (!scx_locked_rq()) { + raw_spin_lock_irqsave(&p->pi_lock, irq_flags); + we_locked = true; + } else if (!scx_kf_allowed(sch, SCX_KF_ENQUEUE)) { + return -EPERM; + } /* * This may also be called from ops.enqueue(), so we need to handle @@ -963,8 +958,8 @@ static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p, allowed ?: p->cpus_ptr, flags); } - if (scx_kf_allowed_if_unlocked()) - task_rq_unlock(rq, p, &rf); + if (we_locked) + raw_spin_unlock_irqrestore(&p->pi_lock, irq_flags); return cpu; } diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h index b4f36d8b9c1d..54da08a223b7 100644 --- a/kernel/sched/ext_internal.h +++ b/kernel/sched/ext_internal.h @@ -1372,11 +1372,6 @@ static inline struct rq *scx_locked_rq(void) return __this_cpu_read(scx_locked_rq_state); } -static inline bool scx_kf_allowed_if_unlocked(void) -{ - return !current->scx.kf_mask; -} - static inline bool scx_bypassing(struct scx_sched *sch, s32 cpu) { return unlikely(per_cpu_ptr(sch->pcpu, cpu)->flags & diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index ae0783e27c1e..0b6a177fd597 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -798,6 +798,7 @@ struct scx_rq { u64 extra_enq_flags; /* see move_task_to_local_dsq() */ u32 nr_running; u32 cpuperf_target; /* [0, SCHED_CAPACITY_SCALE] */ + bool in_select_cpu; bool cpu_released; u32 flags; u32 nr_immed; /* ENQ_IMMED tasks on local_dsq */