Re: [XEN PATCH] xen/credit2: let the tickled pCPU pick up the unit it was tickled for

[George Dunlap <gwd@xxxxxxxxxxxxxx>]

On Tue, Sep 29, 2026 at 3:07 PM Jiaqing Zhao <Zhao.Jiaqing@xxxxxxx> wrote:
>
> runq_tickle() picks an idle pCPU without taking CSCHED2_MIGRATE_RESIST
> into account, but runq_candidate() on that pCPU still applies it. With
> the unit's credit within CSCHED2_MIGRATE_RESIST of CSCHED2_CREDIT_MIN,
> the tickled pCPU declines the unit because it is not its master, while
> the master has already skipped it as deferred to the tickled pCPU. Both
> go idle and the unit stays runnable on the runqueue with nobody to run
> it.
>
> Seen as a permanent PVH dom0 stall under nested virtualization, where
> dom0 overruns its slice far enough to be clamped at CSCHED2_CREDIT_MIN.
>
> Don't resist migration to the pCPU that was tickled for the unit.
>
> Fixes: 069cf39fb171 ("xen: credit2: make tickling more deterministic")
> Signed-off-by: Jiaqing Zhao <Zhao.Jiaqing@xxxxxxx>
> ---
>  xen/common/sched/credit2.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/xen/common/sched/credit2.c b/xen/common/sched/credit2.c
> index 4949606881..3791a8d4cd 100644
> --- a/xen/common/sched/credit2.c
> +++ b/xen/common/sched/credit2.c
> @@ -3517,6 +3517,7 @@ runq_candidate(struct csched2_runqueue_data *rqd,
>           * its credit is at least CSCHED2_MIGRATE_RESIST higher.
>           */
>          if ( sched_unit_master(svc->unit) != cpu
> +             && svc->tickled_cpu != cpu

Thanks for this.  This is the right fix; the only issue is that the
comment above this is stale; it needs to be expanded to include the
extra condition you've added here.

With that addressed:

Reviewed-by: George Dunlap <gwd@xxxxxxxxxxxxxx>