Re: [PATCH v2 4/7] sched/fair: Fix lag clamp

[Vincent Guittot]

On Thu, 19 Feb 2026 at 09:10, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Vincent reported that he was seeing undue lag clamping in a mixed
> slice workload. Implement the max_slice tracking as per the todo
> comment.
>
> Fixes: 147f3efaa241 ("sched/fair: Implement an EEVDF-like scheduling policy")
> Reported-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Tested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Tested-by: Shubhang Kaushik <shubhang@xxxxxxxxxxxxxxxxxxxxxx>
> Link: https://patch.msgid.link/20250422101628.GA33555@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

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


> ---
>  include/linux/sched.h |    1 +
>  kernel/sched/fair.c   |   39 +++++++++++++++++++++++++++++++++++----
>  2 files changed, 36 insertions(+), 4 deletions(-)
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -574,6 +574,7 @@ struct sched_entity {
>         u64                             deadline;
>         u64                             min_vruntime;
>         u64                             min_slice;
> +       u64                             max_slice;
>
>         struct list_head                group_node;
>         unsigned char                   on_rq;
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -748,6 +748,8 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
>         return cfs_rq->zero_vruntime;
>  }
>
> +static inline u64 cfs_rq_max_slice(struct cfs_rq *cfs_rq);
> +
>  /*
>   * lag_i = S - s_i = w_i * (V - v_i)
>   *
> @@ -761,17 +763,16 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
>   * EEVDF gives the following limit for a steady state system:
>   *
>   *   -r_max < lag < max(r_max, q)
> - *
> - * XXX could add max_slice to the augmented data to track this.
>   */
>  static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  {
> +       u64 max_slice = cfs_rq_max_slice(cfs_rq) + TICK_NSEC;
>         s64 vlag, limit;
>
>         WARN_ON_ONCE(!se->on_rq);
>
>         vlag = avg_vruntime(cfs_rq) - se->vruntime;
> -       limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se);
> +       limit = calc_delta_fair(max_slice, se);
>
>         se->vlag = clamp(vlag, -limit, limit);
>  }
> @@ -829,6 +830,21 @@ static inline u64 cfs_rq_min_slice(struc
>         return min_slice;
>  }
>
> +static inline u64 cfs_rq_max_slice(struct cfs_rq *cfs_rq)
> +{
> +       struct sched_entity *root = __pick_root_entity(cfs_rq);
> +       struct sched_entity *curr = cfs_rq->curr;
> +       u64 max_slice = 0ULL;
> +
> +       if (curr && curr->on_rq)
> +               max_slice = curr->slice;
> +
> +       if (root)
> +               max_slice = max(max_slice, root->max_slice);
> +
> +       return max_slice;
> +}
> +
>  static inline bool __entity_less(struct rb_node *a, const struct rb_node *b)
>  {
>         return entity_before(__node_2_se(a), __node_2_se(b));
> @@ -853,6 +869,15 @@ static inline void __min_slice_update(st
>         }
>  }
>
> +static inline void __max_slice_update(struct sched_entity *se, struct rb_node *node)
> +{
> +       if (node) {
> +               struct sched_entity *rse = __node_2_se(node);
> +               if (rse->max_slice > se->max_slice)
> +                       se->max_slice = rse->max_slice;
> +       }
> +}
> +
>  /*
>   * se->min_vruntime = min(se->vruntime, {left,right}->min_vruntime)
>   */
> @@ -860,6 +885,7 @@ static inline bool min_vruntime_update(s
>  {
>         u64 old_min_vruntime = se->min_vruntime;
>         u64 old_min_slice = se->min_slice;
> +       u64 old_max_slice = se->max_slice;
>         struct rb_node *node = &se->run_node;
>
>         se->min_vruntime = se->vruntime;
> @@ -870,8 +896,13 @@ static inline bool min_vruntime_update(s
>         __min_slice_update(se, node->rb_right);
>         __min_slice_update(se, node->rb_left);
>
> +       se->max_slice = se->slice;
> +       __max_slice_update(se, node->rb_right);
> +       __max_slice_update(se, node->rb_left);
> +
>         return se->min_vruntime == old_min_vruntime &&
> -              se->min_slice == old_min_slice;
> +              se->min_slice == old_min_slice &&
> +              se->max_slice == old_max_slice;
>  }
>
>  RB_DECLARE_CALLBACKS(static, min_vruntime_cb, struct sched_entity,
>
>