Re: [PATCH v2 1/2] sched/core: Don't steal a proxy-exec donor

[John Stultz]

On Thu, May 7, 2026 at 3:42 AM Vasily Gorbik <gor@xxxxxxxxxxxxx> wrote:
>
> try_steal_cookie() avoids stealing src->core_pick and src->curr before
> moving a task with the same cookie via move_queued_task_locked().
>
> With proxy-exec, src->donor is the current scheduling context and may
> differ from src->curr. Stealing it migrates a task that the source rq
> still treats as current, leaving src's scheduler state for that task
> stale. For CFS this means cfs_rq->curr points at the stolen entity,
> and the next pick on the source rq hits the WARN_ON_ONCE in
> put_prev_entity().
>
> Commit 7de9d4f94638 ("sched: Start blocked_on chain processing in
> find_proxy_task()") tweaked the fair class logic so that the donor task
> isn't migrated away while we're running the proxy. Do it similarly for
> try_steal_cookie() and skip src->donor as well.
>
> Fixes: 7de9d4f94638 ("sched: Start blocked_on chain processing in find_proxy_task()")
> Signed-off-by: Vasily Gorbik <gor@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b905805bbcbe..8aed55592ca9 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6366,7 +6366,7 @@ static bool try_steal_cookie(int this, int that)
>                 return false;
>
>         do {
> -               if (p == src->core_pick || p == src->curr)
> +               if (p == src->core_pick || p == src->curr || p == src->donor)
>                         goto next;
>

This makes sense to me, we don't want to be migrating rq donors.
 Acked-by: John Stultz <jstultz@xxxxxxxxxx>

I'm still getting my head around the finer details of the core
scheduler, but I suspect we also should add a task_is_blocked() check
in here as well, since migrating blocked_on tasks isn't useful either.

thanks
-john