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

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Message-ID: <707b1bcd-5c66-47d6-a485-e571ef983642@nvidia.com>
Date: Sat, 15 Mar 2025 19:17:20 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Andrea Righi <arighi@...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



On 3/15/2025 1:56 PM, Andrea Righi wrote:
> 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.

Thanks for pointing this out, I will add rationale for the balance as you mentioned.

 - Joel