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sched_ext: Implement auto local dispatching of migration disabled tasks

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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: Tejun Heo <tj@kernel.org>
Acked-by: Andrea Righi <arighi@nvidia.com>
This commit is contained in:
Tejun Heo 2025-02-07 10:58:23 -10:00
parent 029b6ce733
commit 2fa0fbeb69
1 changed files with 23 additions and 0 deletions
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23
kernel/sched/ext.c
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@ -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);