Commit 2fa0fbeb authored by Tejun Heo's avatar Tejun Heo
Browse files

sched_ext: Implement auto local dispatching of migration disabled tasks 



Migration disabled tasks are special and pinned to their previous CPUs. They
tripped up some unsuspecting BPF schedulers as their ->nr_cpus_allowed may
not agree with the bits set in ->cpus_ptr. Make it easier for BPF schedulers
by automatically dispatching them to the pinned local DSQs by default. If a
BPF scheduler wants to handle migration disabled tasks explicitly, it can
set SCX_OPS_ENQ_MIGRATION_DISABLED.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarAndrea Righi <arighi@nvidia.com>
parent 029b6ce7

kernel/sched/ext.c

+23 −0
Original line number Diff line number Diff line
@@ -122,6 +122,19 @@ enum scx_ops_flags { 
	 */

	SCX_OPS_SWITCH_PARTIAL	= 1LLU << 3,



	/*

	 * A migration disabled task can only execute on its current CPU. By

	 * default, such tasks are automatically put on the CPU's local DSQ with

	 * the default slice on enqueue. If this ops flag is set, they also go

	 * through ops.enqueue().

	 *

	 * A migration disabled task never invokes ops.select_cpu() as it can

	 * only select the current CPU. Also, p->cpus_ptr will only contain its

	 * current CPU while p->nr_cpus_allowed keeps tracking p->user_cpus_ptr

	 * and thus may disagree with cpumask_weight(p->cpus_ptr).

	 */

	SCX_OPS_ENQ_MIGRATION_DISABLED = 1LLU << 4,



	/*

	 * CPU cgroup support flags

	 */
@@ -130,6 +143,7 @@ enum scx_ops_flags { 
	SCX_OPS_ALL_FLAGS	= SCX_OPS_KEEP_BUILTIN_IDLE |

				  SCX_OPS_ENQ_LAST |

				  SCX_OPS_ENQ_EXITING |

				  SCX_OPS_ENQ_MIGRATION_DISABLED |

				  SCX_OPS_SWITCH_PARTIAL |

				  SCX_OPS_HAS_CGROUP_WEIGHT,

};
@@ -882,6 +896,7 @@ static bool scx_warned_zero_slice; 


static DEFINE_STATIC_KEY_FALSE(scx_ops_enq_last);

static DEFINE_STATIC_KEY_FALSE(scx_ops_enq_exiting);

static DEFINE_STATIC_KEY_FALSE(scx_ops_enq_migration_disabled);

static DEFINE_STATIC_KEY_FALSE(scx_ops_cpu_preempt);

static DEFINE_STATIC_KEY_FALSE(scx_builtin_idle_enabled);
@@ -2014,6 +2029,11 @@ static void do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags, 
	    unlikely(p->flags & PF_EXITING))

		goto local;



	/* see %SCX_OPS_ENQ_MIGRATION_DISABLED */

	if (!static_branch_unlikely(&scx_ops_enq_migration_disabled) &&

	    is_migration_disabled(p))

		goto local;



	if (!SCX_HAS_OP(enqueue))

		goto global;
@@ -5052,6 +5072,7 @@ static void scx_ops_disable_workfn(struct kthread_work *work) 
		static_branch_disable(&scx_has_op[i]);

	static_branch_disable(&scx_ops_enq_last);

	static_branch_disable(&scx_ops_enq_exiting);

	static_branch_disable(&scx_ops_enq_migration_disabled);

	static_branch_disable(&scx_ops_cpu_preempt);

	static_branch_disable(&scx_builtin_idle_enabled);

	synchronize_rcu();
@@ -5661,6 +5682,8 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link) 


	if (ops->flags & SCX_OPS_ENQ_EXITING)

		static_branch_enable(&scx_ops_enq_exiting);

	if (ops->flags & SCX_OPS_ENQ_MIGRATION_DISABLED)

		static_branch_enable(&scx_ops_enq_migration_disabled);

	if (scx_ops.cpu_acquire || scx_ops.cpu_release)

		static_branch_enable(&scx_ops_cpu_preempt);