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

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

Hi.

On 4/7/26 12:09 PM, 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.
>

Isn't load balance to move it out? What does the workload do?

> 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.
> 

There are systems where LLC_SIZE == SMT_SIZE. i.e one core in the LLC.
This would effectively disable wake_affine feature in such systems.

Power10 being a major example.

> 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)