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sched_ext: scx_qmap - Idle-check pinned tasks before direct dispatch

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pick_direct_dispatch_cid() sent a pinned task straight to its only cpu
without an idle check. An insert onto a time-shared cid is forced IMMED,
which may not wait on a busy cpu - the kernel bounces the task back to
ops.enqueue(), the shortcut re-inserts it, and the loop runs into the
reenqueue repeat limit.

Run pinned tasks through the same idle test as everyone else and queue them
when the cpu is busy. always_enq_immed forces IMMED on every local insert
and already skipped the shortcut for the same reason. The check was the last
reader of the always_enq_immed rodata mirror, so drop the variable. The -I
option still sets SCX_OPS_ALWAYS_ENQ_IMMED.

v2: Drop the now-unread always_enq_immed rodata mirror. (sashiko AI)

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
This commit is contained in:
Tejun Heo
2026-08-03 11:01:55 -10:00
parent 1f15b9a949
commit 6206980186
2 changed files with 0 additions and 5 deletions
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4
tools/sched_ext/scx_qmap.bpf.c
View File

@@ -66,7 +66,6 @@ const volatile bool print_msgs;
const volatile u64 sub_cgroup_id;
const volatile s32 disallow_tgid;
const volatile bool suppress_dump;
const volatile bool always_enq_immed;
const volatile u32 immed_stress_nth;
const volatile u32 max_tasks;
@@ -230,9 +229,6 @@ static s32 pick_direct_dispatch_cid(struct task_struct *p, s32 prev_cid,
s32 cid;
u32 i;
if (!always_enq_immed && p->nr_cpus_allowed == 1)
return prev_cid;
if (cmask_test(prev_cid, &qa.self_cids.mask) &&
cmask_test_and_clear(prev_cid, &qa.idle_cids.mask))
return prev_cid;

1
tools/sched_ext/scx_qmap.c
View File

@@ -315,7 +315,6 @@ int main(int argc, char **argv)
skel->struct_ops.qmap_ops->flags |= SCX_OPS_SWITCH_PARTIAL;
break;
case 'I':
skel->rodata->always_enq_immed = true;
skel->struct_ops.qmap_ops->flags |= SCX_OPS_ALWAYS_ENQ_IMMED;
break;
case 'F':