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Re: [PATCH] xen/sched: Fix the sorted insertion in sched_domain_insert_unit()
- To: Stefano Stabellini <stefano.stabellini@xxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 26 Oct 2026 13:46:42 +0100
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- Cc: dfaggioli@xxxxxxxx, jgross@xxxxxxxx, gwd@xxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 26 Oct 2026 12:46:50 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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).
> --- 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.
Jan
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