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

[James Morse]

Hi Ilpo,

On 21/03/2023 13:21, 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
>> 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
>> @@ -55,6 +56,28 @@
>>  /* Max event bits supported */
>>  #define MAX_EVT_CONFIG_BITS		GENMASK(6, 0)
>>  
>> +/**
>> + * 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);
>> +		if (hk_cpu < nr_cpu_ids)
>> +			cpu = hk_cpu;
>> +	}
>> +
>> +	return cpu;
>> +}
>> diff --git a/include/linux/tick.h b/include/linux/tick.h
>> index bfd571f18cfd..ae2e9019fc18 100644
>> --- a/include/linux/tick.h
>> +++ b/include/linux/tick.h
>> @@ -174,9 +174,10 @@ static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
>>  static inline void tick_nohz_idle_stop_tick_protected(void) { }
>>  #endif /* !CONFIG_NO_HZ_COMMON */
>>  
>> +extern cpumask_var_t tick_nohz_full_mask;
>> +
>>  #ifdef CONFIG_NO_HZ_FULL
>>  extern bool tick_nohz_full_running;
>> -extern cpumask_var_t tick_nohz_full_mask;
> 
> Its definition seems to also be inside #ifdef:
> 
> kernel/time/tick-sched.c-#ifdef CONFIG_NO_HZ_FULL
> kernel/time/tick-sched.c:cpumask_var_t tick_nohz_full_mask;
> kernel/time/tick-sched.c:EXPORT_SYMBOL_GPL(tick_nohz_full_mask);

Indeed, but all the uses are guarded by tick_nohz_full_cpu(), which the compiler knows is
false if CONFIG_NO_HZ_FULL is not selected.

Moving the prototype is enough to let the compiler parse the code to check its correct,
before dead-code-eliminating it. There is no need to carry around the cpumask if its never
going to be used. This would only cause a problem if someone adds a user of
tick_nohz_full_mask which isn't guarded by IS_ENABLED(). I argue that would be a bug.

All this is being done to avoid more #ifdeffery!)


Thanks,

James