Re: [PATCH v3] sched/eevdf: Force propagating min_slice of cfs_rq when {en,de}queue tasks

[K Prateek Nayak <kprateek.nayak@amd.com>]

Hello Tianchen,

On 2/11/2025 12:06 PM, Tianchen Ding wrote:
> When a task is enqueued and its parent cgroup se is already on_rq, this
> parent cgroup se will not be enqueued again, and hence the root->min_slice
> leaves unchanged. The same issue happens when a task is dequeued and its
> parent cgroup se has other runnable entities, and the parent cgroup se
> will not be dequeued.
> 
> Force propagating min_slice when se doesn't need to be enqueued or
> dequeued. Ensure the se hierarchy always get the latest min_slice.
> 
> Fixes: aef6987d8954 ("sched/eevdf: Propagate min_slice up the cgroup hierarchy")
> Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
> ---
> v3:
> I modified some descriptions in commit log, and rebased to the latest
> tip branch. The old version of patch can be found in [1].
> 
> The original patchset wants to add a feature. As the 2nd patch may be
> hard to be accepted, I think at least the bugfix should be applied.
> 
> The issue about this patch was described detailly in [2].
> 
> [1]https://lore.kernel.org/all/20241031094822.30531-1-dtcccc@linux.alibaba.com/
> [2]https://lore.kernel.org/all/a903d0dc-1d88-4ae7-ac81-3eed0445654d@linux.alibaba.com/
> ---
>   kernel/sched/fair.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1e78caa21436..0d479b92633a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6970,6 +6970,8 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>   		update_cfs_group(se);
>   
>   		se->slice = slice;
> +		if (se != cfs_rq->curr)
> +			min_vruntime_cb_propagate(&se->run_node, NULL);

Should we check if old slice matches with the new slice before
propagation to avoid any unnecessary propagate call? Something like:

		if (se->slice != slice) {
			se->slice = slice;
			if (se != cfs_rq->curr)
				min_vruntime_cb_propagate(&se->run_node, NULL);
		}

Thoughts?

Other than that, the fix looks good. Feel free to add:

Reviewed-and-tested-by: K Prateek Nayak <kprateek.nayak@amd.com>

-- 
Thanks and Regards,
Prateek

>   		slice = cfs_rq_min_slice(cfs_rq);
>   
>   		cfs_rq->h_nr_runnable += h_nr_runnable;
> @@ -7099,6 +7101,8 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
>   		update_cfs_group(se);
>   
>   		se->slice = slice;
> +		if (se != cfs_rq->curr)
> +			min_vruntime_cb_propagate(&se->run_node, NULL);
>   		slice = cfs_rq_min_slice(cfs_rq);
>   
>   		cfs_rq->h_nr_runnable -= h_nr_runnable;