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sched_ext: Replace SCX_TASK_OFF_TASKS flag with SCX_TASK_DEAD state

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SCX_TASK_OFF_TASKS marked tasks already through sched_ext_dead() so cgroup
task iteration would skip them. This can be expressed better with a task
state. Replace the flag with SCX_TASK_DEAD.

scx_disable_and_exit_task() resets state to NONE on its way out, so
sched_ext_dead() now sets DEAD after the wrapper returns. The validation
matrix grows NONE -> DEAD, warns on DEAD -> NONE, and tightens READY's
predecessor to INIT or ENABLED so the new DEAD value cannot silently
transition to READY.

Prepares for the following enable vs dead race fix.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
This commit is contained in:
Tejun Heo
2026-05-10 10:08:16 -10:00
parent 938dd9ab2b
commit cceb8fa9cb
2 changed files with 16 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files

9
include/linux/sched/ext.h
View File

@@ -101,24 +101,25 @@ enum scx_ent_flags {
SCX_TASK_DEQD_FOR_SLEEP = 1 << 3, /* last dequeue was for SLEEP */
SCX_TASK_SUB_INIT = 1 << 4, /* task being initialized for a sub sched */
SCX_TASK_IMMED = 1 << 5, /* task is on local DSQ with %SCX_ENQ_IMMED */
SCX_TASK_OFF_TASKS = 1 << 6, /* removed from scx_tasks by sched_ext_dead() */
/*
* Bits 8 and 9 are used to carry task state:
* Bits 8 to 10 are used to carry task state:
*
* NONE ops.init_task() not called yet
* INIT ops.init_task() succeeded, but task can be cancelled
* READY fully initialized, but not in sched_ext
* ENABLED fully initialized and in sched_ext
* DEAD terminal state set by sched_ext_dead()
*/
SCX_TASK_STATE_SHIFT = 8, /* bits 8 and 9 are used to carry task state */
SCX_TASK_STATE_BITS = 2,
SCX_TASK_STATE_SHIFT = 8,
SCX_TASK_STATE_BITS = 3,
SCX_TASK_STATE_MASK = ((1 << SCX_TASK_STATE_BITS) - 1) << SCX_TASK_STATE_SHIFT,
SCX_TASK_NONE = 0 << SCX_TASK_STATE_SHIFT,
SCX_TASK_INIT = 1 << SCX_TASK_STATE_SHIFT,
SCX_TASK_READY = 2 << SCX_TASK_STATE_SHIFT,
SCX_TASK_ENABLED = 3 << SCX_TASK_STATE_SHIFT,
SCX_TASK_DEAD = 4 << SCX_TASK_STATE_SHIFT,
/*
* Bits 12 and 13 are used to carry reenqueue reason. In addition to

17
kernel/sched/ext.c
View File

@@ -723,17 +723,22 @@ static void scx_set_task_state(struct task_struct *p, u32 state)
switch (state) {
case SCX_TASK_NONE:
warn = prev_state == SCX_TASK_DEAD;
break;
case SCX_TASK_INIT:
warn = prev_state != SCX_TASK_NONE;
p->scx.flags |= SCX_TASK_RESET_RUNNABLE_AT;
break;
case SCX_TASK_READY:
warn = prev_state == SCX_TASK_NONE;
warn = !(prev_state == SCX_TASK_INIT ||
prev_state == SCX_TASK_ENABLED);
break;
case SCX_TASK_ENABLED:
warn = prev_state != SCX_TASK_READY;
break;
case SCX_TASK_DEAD:
warn = prev_state != SCX_TASK_NONE;
break;
default:
WARN_ONCE(1, "sched_ext: Invalid task state %d -> %d for %s[%d]",
prev_state, state, p->comm, p->pid);
@@ -972,11 +977,11 @@ static struct task_struct *scx_task_iter_next_locked(struct scx_task_iter *iter)
/*
* cgroup_task_dead() removes the dead tasks from cset->tasks
* after sched_ext_dead() and cgroup iteration may see tasks
* which already finished sched_ext_dead(). %SCX_TASK_OFF_TASKS
* is set by sched_ext_dead() under @p's rq lock. Test it to
* which already finished sched_ext_dead(). %SCX_TASK_DEAD is
* set by sched_ext_dead() under @p's rq lock. Test it to
* avoid visiting tasks which are already dead from SCX POV.
*/
if (p->scx.flags & SCX_TASK_OFF_TASKS) {
if (scx_get_task_state(p) == SCX_TASK_DEAD) {
__scx_task_iter_rq_unlock(iter);
continue;
}
@@ -3847,7 +3852,7 @@ void sched_ext_dead(struct task_struct *p)
* @p is off scx_tasks and wholly ours. scx_root_enable()'s READY ->
* ENABLED transitions can't race us. Disable ops for @p.
*
* %SCX_TASK_OFF_TASKS synchronizes against cgroup task iteration - see
* %SCX_TASK_DEAD synchronizes against cgroup task iteration - see
* scx_task_iter_next_locked(). NONE tasks need no marking: cgroup
* iteration is only used from sub-sched paths, which require root
* enabled. Root enable transitions every live task to at least READY.
@@ -3858,7 +3863,7 @@ void sched_ext_dead(struct task_struct *p)
rq = task_rq_lock(p, &rf);
scx_disable_and_exit_task(scx_task_sched(p), p);
p->scx.flags |= SCX_TASK_OFF_TASKS;
scx_set_task_state(p, SCX_TASK_DEAD);
task_rq_unlock(rq, p, &rf);
}
}