[Devel] [PATCH vz10 v4 1/3] sched: Clean up vCPU handling logic

[Dmitry Sepp]

________________________________________
From: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
Sent: Thursday, 19 March 2026 14:28
To: Konstantin Khorenko <khorenko at virtuozzo.com>; Dmitry Sepp <dmitry.sepp at virtuozzo.com>
Cc: devel at openvz.org <devel at openvz.org>
Subject: Re: [PATCH vz10 v4 1/3] sched: Clean up vCPU handling logic
 


On 3/19/26 14:12, Konstantin Khorenko wrote:
> On 3/19/26 10:47, Dmitry Sepp wrote:
>> The idea behind the change is to transition from the existing spatial
>> vCPU handling approach that introduces costly modification to the
>> scheduling logic to ensure the requested CPU count is obeyed (10%+
>> performance drop in some tests) to temporal isolation that can be
>> provided by the cgroup2 cpu.max.
>>
>> Drop the legacy unneeded vCPU handling code. Remove the 'cpu.rate'
>> control in favor of the internal calculation based on 'quota' and
>> 'period' from 'cpu.max'. As 'cpu.max' is not implicitly used to set the
>> rate, do not override nr_cpus when handling writes to 'cpu.max'.
>>
>> https://virtuozzo.atlassian.net/browse/VSTOR-124385
>>
>> Signed-off-by: Dmitry Sepp <dmitry.sepp at virtuozzo.com>
>> ---
>>   include/linux/sched.h          |   6 -
>>   include/linux/sched/topology.h |   5 -
>>   kernel/sched/core.c            |  98 +-------
>>   kernel/sched/fair.c            | 408 ---------------------------------
>>   kernel/sched/sched.h           |  10 -
>>   5 files changed, 12 insertions(+), 515 deletions(-)
>>
> ...
>
>>   -static int tg_set_cpu_limit(struct task_group *tg,
>> -                unsigned long cpu_rate, unsigned int nr_cpus)
>> +static int tg_set_cpu_limit(struct task_group *tg, unsigned int nr_cpus)
>>   {
>>       int ret;
>>       unsigned long rate;
>> +    unsigned long cpu_rate = tg->cpu_rate;
>>       u64 quota = RUNTIME_INF;
>>       u64 burst = tg_get_cfs_burst(tg);
>>       u64 period = default_cfs_period();
>> @@ -10090,21 +10041,6 @@ static int tg_set_cpu_limit(struct task_group *tg,
>>       return ret;
>>   }
>>  
>
> static int tg_set_cpu_limit(struct task_group *tg, unsigned int nr_cpus)
> {
>         int ret;
>         unsigned long rate;
>         unsigned long cpu_rate = tg->cpu_rate;
>         u64 quota = RUNTIME_INF;
>         u64 burst = tg_get_cfs_burst(tg);
>         u64 period = default_cfs_period();
>
>         rate = (cpu_rate && nr_cpus) ?
>                 min_t(unsigned long, cpu_rate, nr_cpus * MAX_CPU_RATE) :
>                 max_t(unsigned long, cpu_rate, nr_cpus * MAX_CPU_RATE);
>         if (rate) {
>                 quota = div_u64(period * rate, MAX_CPU_RATE);
>                 quota = max(quota, min_cfs_quota_period);
>         }
>
>         cpus_read_lock();
>         mutex_lock(&cfs_constraints_mutex);
>         ret = __tg_set_cfs_bandwidth(tg, period, quota, burst);
>         if (!ret) {
>                 tg->cpu_rate = cpu_rate;
> // the line above is a no-op.
> // i will apply the patch as is but in case this was not intended, please fix it with an incremental patch

We use ->cpu_rate in cpu_cgroup_update_vcpustat, should not it be updated to the value of rate here?

I reused the logic of the original code here. It seems rate was only intended to calculate quota. The following would apply to both nr_cpus and cpu_rate: if we set nr_cpus, tg->cpu_rate would be passed as the rate parameter and would not be changed. And other way round, if the rate control was written the nr_cpu argument was merely tg->nr_cpu, which would not be changed while changing rate. As we now don't provide the rate control there is no way to change rate via tg_set_cpu_limit(), only through cpu.max -> tg_update_cpu_limit().

>
>
>                 tg->nr_cpus = nr_cpus;
>         }
>         mutex_unlock(&cfs_constraints_mutex);
>         cpus_read_unlock();
>
>         return ret;
> }

--
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.