Re: [PATCH v6 12/24] x86/resctrl: Add cpumask_any_housekeeping() for limbo/overflow

James Morse
2023-10-05T17:16:28Z

Hi Reinette,

On 03/10/2023 22:15, Reinette Chatre wrote:
> On 9/14/2023 10:21 AM, James Morse wrote:
>> The limbo and overflow code picks a CPU to use from the domain's list
>> of online CPUs. Work is then scheduled on these CPUs to maintain
>> the limbo list and any counters that may overflow.
>>
>> cpumask_any() may pick a CPU that is marked nohz_full, which will
>> either penalise the work that CPU was dedicated to, or delay the
>> processing of limbo list or counters that may overflow. Perhaps
>> indefinitely. Delaying the overflow handling will skew the bandwidth
>> values calculated by mba_sc, which expects to be called once a second.
>>
>> Add cpumask_any_housekeeping() as a replacement for cpumask_any()
>> that prefers housekeeping CPUs. This helper will still return
>> a nohz_full CPU if that is the only option. The CPU to use is
>> re-evaluated each time the limbo/overflow work runs. This ensures
>> the work will move off a nohz_full CPU once a housekeeping CPU is
>> available.

>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>> index 0bbed8c62d42..993837e46db1 100644
>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c

>> @@ -793,8 +793,10 @@ void cqm_handle_limbo(struct work_struct *work)
>>
>> __check_limbo(d, false);
>>
>> - if (has_busy_rmid(d))
>> + if (has_busy_rmid(d)) {
>> + cpu = cpumask_any_housekeeping(&d->cpu_mask);
>> schedule_delayed_work_on(cpu, &d->cqm_limbo, delay);
>> + }
>>
>
> ok - but if you do change the CPU the worker is running on then
> I also expect d->cqm_work_cpu to be updated. Otherwise the offline
> code will not be able to determine if the worker needs to move.

Good point - I missed this.

Thanks,

James