Re: [PATCH 1/5] xen/sched: rtds: add global-EDF utilization admission control

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.09.2026 11:10, Juergen Gross wrote:
> On 26.08.26 06:57, Furkan Caliskan wrote:
>> @@ -635,6 +656,54 @@ replq_reinsert(const struct scheduler *ops, struct 
>> rt_unit *svc)
>>           set_timer(&rt_priv(ops)->repl_timer, rearm_svc->cur_deadline);
>>   }
>>   
>> +/*
>> + * budget << RTDS_UTIL_SHIFT can't overflow: rt_validate_params()
>> + * caps budget at RTDS_MAX_BUDGET. period == 0 means "no
>> + * reservation" (a unit being removed), not an error.
>> + */
>> +static uint64_t
>> +rt_unit_utilization(s_time_t period, s_time_t budget)
>> +{
>> +    if ( period <= 0 )
>> +        return 0;
>> +
>> +    return ((uint64_t)budget << RTDS_UTIL_SHIFT) / (uint64_t)period;
>> +}
>> +
>> +/*
>> + * Utilization capacity of the cpupool domain d resides in.
>> + */
>> +static uint64_t
>> +rt_utilization_cap(const struct domain *d)
>> +{
>> +    unsigned int cpus = cpumask_weight(cpupool_domain_master_cpumask(d));
>> +
>> +    return (uint64_t)cpus * RTDS_UTIL_SCALE * RTDS_UTIL_CAP_PCT / 100;
>> +}
>> +
>> +/*
>> + * Replaces a unit's reservation and updates prv->utilization
>> + * to match. Growth that would push utilization over the
>> + * cpupool's cap is refused. Removing a unit or shrinking
>> + * a unit's reservation always succeed.
>> + */
>> +static int
>> +rt_admission_test(struct rt_private *prv, const struct domain *d,
>> +               s_time_t old_period, s_time_t old_budget,
>> +               s_time_t new_period, s_time_t new_budget)
> 
> I think the function name isn't appropriate. The function doesn't test only, 
> it
> is setting the utilization, too.
> 
> What about rt_try_set_utilization() instead? And make the return type bool
> (true on success).
> 
>> +{
>> +    uint64_t old_util = rt_unit_utilization(old_period, old_budget);
>> +    uint64_t new_util = rt_unit_utilization(new_period, new_budget);
>> +    uint64_t total    = prv->utilization - old_util + new_util;
>> +
>> +    if ( new_util > old_util && total > rt_utilization_cap(d) )
>> +        return -EINVAL;
>> +
>> +    prv->utilization = total;
>> +
>> +    return 0;
>> +}
>> +
>>   /*
>>    * Pick a valid resource for the unit vc
>>    * Valid resource of an unit is intesection of unit's affinity
>> @@ -892,8 +983,19 @@ rt_alloc_udata(const struct scheduler *ops, struct 
>> sched_unit *unit, void *dd)
>>   static void cf_check
>>   rt_free_udata(const struct scheduler *ops, void *priv)
>>   {
>> +    struct rt_private *prv = rt_priv(ops);
>>       struct rt_unit *svc = priv;
>>   
>> +    if ( svc && !is_idle_unit(svc->unit) )
>> +    {
>> +        unsigned long flags;
>> +
>> +        spin_lock_irqsave(&prv->lock, flags);
>> +        rt_admission_test(prv, svc->unit->domain,
>> +                           svc->period, svc->budget, 0, 0);
> 
> This use case clearly shows you are not only testing. :-)

Yet at the same time ignoring possible errors.

Jan