Re: [PATCH RFC 3/8] sched/ext: Add a DL server for sched_ext tasks

[Joel Fernandes <joelagnelf@nvidia.com>]

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