Re: [PATCH v3 08/19] x86/resctrl: Add cpumask_any_housekeeping() for limbo/overflow

[James Morse <james.morse@arm.com>]

Hi Ilpo,

On 21/03/2023 15:14, Ilpo Järvinen wrote:
> On Mon, 20 Mar 2023, 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 houskeeping CPU is
> 
> housekeeping
> 
>> available.

>> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
>> index 87545e4beb70..0b5fd5a0cda2 100644
>> --- a/arch/x86/kernel/cpu/resctrl/internal.h
>> +++ b/arch/x86/kernel/cpu/resctrl/internal.h

>> +/**
>> + * cpumask_any_housekeeping() - Chose any cpu in @mask, preferring those that
>> + *			        aren't marked nohz_full
>> + * @mask:	The mask to pick a CPU from.
>> + *
>> + * Returns a CPU in @mask. If there are houskeeping CPUs that don't use
>> + * nohz_full, these are preferred.
>> + */
>> +static inline unsigned int cpumask_any_housekeeping(const struct cpumask *mask)
>> +{
>> +	int cpu, hk_cpu;
>> +
>> +	cpu = cpumask_any(mask);
>> +	if (tick_nohz_full_cpu(cpu)) {
>> +		hk_cpu = cpumask_nth_andnot(0, mask, tick_nohz_full_mask);
> 
> Why cpumask_nth_and() is not enough here? ..._andnot() seems to alter 
> tick_nohz_full_mask which doesn't seem desirable?

tick_nohz_full_mask is the list of CPUs we should avoid. This wants to find the first cpu
set in the domain mask, and clear in tick_nohz_full_mask.

Where does cpumask_nth_andnot() modify its arguments? Its arguments are const.


Thanks,

James