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Re: [PATCH] xen/sched: Fix the sorted insertion in sched_domain_insert_unit()
- To: Jan Beulich <jbeulich@xxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxx>
- From: Jürgen Groß <jgross@xxxxxxxx>
- Date: Mon, 26 Oct 2026 14:15:41 +0100
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- Cc: dfaggioli@xxxxxxxx, gwd@xxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 26 Oct 2026 13:15:57 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 26.10.26 13:46, Jan Beulich wrote:
On 21.10.2026 00:35, Stefano Stabellini wrote:
The loop links the new unit in front of the unit whose successor has a
higher unit_id, rather than after it: inserting 1 into {0, 2} gives
{1, 0, 2}. Stop at the first unit with a higher unit_id instead.
I think it usually works because guest vCPUs are typically inserted in
order.
Plus I can't seem to be able to spot anything which depends on any
particular ordering (except for sched_domain_insert_unit() itself).
sched_move_domain() relies on the sequence in a subtle way:
if ( unit->unit_id + gran == v->vcpu_id )
unit = unit->next_in_list;
--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -489,8 +489,7 @@ static void sched_domain_insert_unit(struct sched_unit
*unit, struct domain *d)
for ( prev_unit = &d->sched_unit_list; *prev_unit;
prev_unit = &(*prev_unit)->next_in_list )
- if ( (*prev_unit)->next_in_list &&
- (*prev_unit)->next_in_list->unit_id > unit->unit_id )
+ if ( (*prev_unit)->unit_id > unit->unit_id )
break;
unit->next_in_list = *prev_unit;
Part of the problem may also be that "prev_unit" doesn't look to
properly describe the purpose of the variable: We insert before
prev_unit, when the name (to me at least) suggests we mean to
insert after it. IOW I think a rename (together with the correction
of the condition) may help here.
I agree. Maybe "next_unit"?
Juergen
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