linux-kernel - Re: [PATCH RFC 3/8] sched/ext: Add a DL server for sched

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Message-ID: <Z9W_UbeDXUok4D6R@gpd3>
Date: Sat, 15 Mar 2025 18:56:33 +0100
From: Andrea Righi <arighi@...dia.com>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>,
	Luigi De Matteis <ldematteis123@...il.com>, paulmck@...nel.org,
	boqun.feng@...il.com
Subject: Re: [PATCH RFC 3/8] sched/ext: Add a DL server for sched_ext tasks

Hi Joel,

On Fri, Mar 14, 2025 at 10:21:50PM -0400, Joel Fernandes wrote:
...
> @@ -7793,6 +7819,38 @@ BTF_ID_FLAGS(func, scx_bpf_task_cgroup, KF_RCU | KF_ACQUIRE)
>  BTF_ID_FLAGS(func, scx_bpf_now)
>  BTF_KFUNCS_END(scx_kfunc_ids_any)
>  
> +/*
> + * Check if ext scheduler has tasks ready to run.
> + */
> +static bool ext_server_has_tasks(struct sched_dl_entity *dl_se)
> +{
> +	return !!dl_se->rq->scx.nr_running;
> +}
> +
> +/*
> + * Select the next task to run from the ext scheduling class.
> + */
> +static struct task_struct *ext_server_pick_task(struct sched_dl_entity *dl_se,
> +						void *flags)
> +{
> +	struct rq_flags *rf = flags;
> +

It'd be nice to add a comment here to clarify that we need to call
balance_scx() before pick_task_scx(), so that we can trigger ops.dispatch()
and consume tasks that may be pending in the BPF scheduler's DSQs,
otherwise pick_task_scx() may not find any scx task to run, reducing the
effectiveness of the dl_server.

> +	balance_scx(dl_se->rq, dl_se->rq->curr, rf);
> +	return pick_task_scx(dl_se->rq, rf);
> +}

Thanks,
-Andrea