Re: [PATCH] sched/fair: Use update_curr_eevdf() for the remaining root cfs_rq callers

[Vincent Guittot]

On Sat, 22 Aug 2026 at 12:59, Zhan Xusheng <zhanxusheng1024@xxxxxxxxx> wrote:
>
> From: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
>
> pick_task_fair() and yield_task_fair() call update_curr(&rq->cfs) to bring
> curr up to date before they look at the eevdf state.  With cgroups that
> does not happen: update_curr() reads ->h_curr, which on the root cfs_rq is
> the top level group entity, and returns at the !entity_is_task() check
> before touching vruntime.  Both then read ->curr, so the guard and the
> update disagree about which entity they mean.
>
> Counting how often ->h_curr and ->curr differ at pick_task_fair(), on one
> CPU for 10s with three busy tasks and one 200us-periodic task:
>
>   all tasks in the root cgroup        43321 calls,     0 no-ops
>   busy tasks in G0, periodic in G1    45211 calls, 45193 no-ops
>
> Whether that matters depends on what precedes the pick.  Since
> commit 68e37487810a ("sched/fair: Fix flat hierarchy") the tick and
> enqueue/dequeue all update curr correctly, so on the normal reschedule
> path only the microseconds between those and the pick are missing, and I
> could not measure a latency difference there.  Three paths have nothing
> before them on that rq though:
>
>   - pick_task() on the sibling rqs of a core under core scheduling
>     (kernel/sched/core.c), which updates that rq's clock first for
>     exactly this reason
>   - fair_server_pick_task()
>   - yield_task_fair(), where the stale value feeds the entity_eligible()
>     test that guards forfeiting the remaining vruntime
>
> There curr can be a full tick behind, as it was before that commit.
>
> No new behaviour for the entity being updated: without cgroups ->h_curr
> is already the task, so these two call sites already run the full
> update_curr() including update_deadline(), dl_server_update() and the
> resched_curr_lazy() at the end.  This makes the cgroup case do the same.
>
> Fixes: 85570f10a4c6 ("sched/eevdf: Move to a single runqueue")
> Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
>  kernel/sched/fair.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 001140132a7d..3ea9bb5c3b47 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10052,7 +10052,7 @@ struct task_struct *pick_task_fair(struct rq *rq, struct rq_flags *rf)
>
>         /* Might not have done put_prev_entity() */
>         if (cfs_rq->curr && cfs_rq->curr->on_rq)
> -               update_curr(cfs_rq);
> +               update_curr_eevdf(cfs_rq);
>
>         se = pick_next_entity(rq, true);
>         if (!se)
> @@ -10155,7 +10155,7 @@ static void yield_task_fair(struct rq *rq)
>         /*
>          * Update run-time statistics of the 'current'.
>          */
> -       update_curr(cfs_rq);
> +       update_curr_eevdf(cfs_rq);
>         /*
>          * Tell update_rq_clock() that we've just updated,
>          * so we don't do microscopic update in schedule()
> --
> 2.43.0
>