Re: [RFC PATCH] sched/fair: scale wake_wide() threshold by SMT width

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 4/7/26 8:08 PM, Dietmar Eggemann wrote:
> On 07.04.26 08:39, Zhang Qiao wrote:
>> wake_wide() uses sd_llc_size as the spreading threshold to detect wide
>> waker/wakee relationships and to disable wake_affine() for those cases.
>>
>> On SMT systems, sd_llc_size counts logical CPUs rather than physical
>> cores. This inflates the wake_wide() threshold, allowing wake_affine()
>> to pack more tasks into one LLC domain than the actual compute capacity
>> of its physical cores can sustain. The resulting SMT interference may
>> cost more than the cache-locality benefit wake_affine() intends to gain.
>>
>> Scale the factor by the SMT width of the current CPU so that it
>> approximates the number of independent physical cores in the LLC domain,
>> making wake_wide() more likely to kick in before SMT interference
>> becomes significant. On non-SMT systems the SMT width is 1 and behaviour
>> is unchanged.
>>
>> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
>> ---
>>   kernel/sched/fair.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index f07df8987a5ef..4896582c6e904 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -7334,6 +7334,11 @@ static int wake_wide(struct task_struct *p)
>>   	unsigned int slave = p->wakee_flips;
>>   	int factor = __this_cpu_read(sd_llc_size);
>>   
>> +	/* Scale factor to physical-core count to account for SMT interference. */
>> +	if (sched_smt_active())
>> +		factor = DIV_ROUND_UP(factor,
>> +				cpumask_weight(cpu_smt_mask(smp_processor_id())));
>> +
>>   	if (master < slave)
>>   		swap(master, slave);
>>   	if (slave < factor || master < slave * factor)
> 
> I assume not a lot of people care since this needs:

wake_affine machinery needs SD_WAKE_AFFINE. No?

> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 5847b83d9d55..596c5d590532 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1691,7 +1691,7 @@ sd_init(struct sched_domain_topology_level *tl,
>                  .flags                  = 1*SD_BALANCE_NEWIDLE
>                                          | 1*SD_BALANCE_EXEC
>                                          | 1*SD_BALANCE_FORK
> -                                       | 0*SD_BALANCE_WAKE
> +                                       | 1*SD_BALANCE_WAKE
>                                          | 1*SD_WAKE_AFFINE
>                                          | 0*SD_SHARE_CPUCAPACITY
>                                          | 0*SD_SHARE_LLC
> 
> And then it's a trade-off between one busy thread per core vs. wakeup cost.