Re: [RFC PATCH] sched/fair: dynamically scale the period of cache work

[Chen, Yu C]

Hi Jianyong,

On 4/13/2026 3:23 PM, Jianyong Wu wrote:
> When a preferred LLC is selected and remains stable, task_cache_work does
> not need to run frequently. Because it scans all system CPUs for
> computation, high-frequency execution hurts performance. We thus reduce
> the scan rate in such cases.
>
> On the other hand, if the preferred node becomes suboptimal, we should
> increase the scan frequency to quickly find a better placement. The scan
> period is therefore dynamically adjusted.
>
> Signed-off-by: Jianyong Wu <wujianyong@xxxxxxxx>
>
> ---
> Hi ChenYu, Tim, Gengkun,
>
> I have another approach to address this issue, based on the observation
> that the scan work can be canceled if the preferred node is stable.This
> patch merely demonstrates the idea, but still needs more testing to
> verify its functionality. I'm sending it out early to gather feedback and
> opinions.

Thanks for providing this patch.

>   	if (work->next == work) {
> @@ -1728,7 +1734,7 @@ static void task_cache_work(struct callback_head *work)
>   	struct task_struct *p = current, *cur;
>   	unsigned long curr_m_a_occ = 0;
>   	struct mm_struct *mm = p->mm;
> -	unsigned long m_a_occ = 0;
> +	unsigned long m_a_occ = 0, need_scan = 0, now;
>   	cpumask_var_t cpus;
>   	u64 t0, scan_cost;
>   
> @@ -1753,6 +1759,12 @@ static void task_cache_work(struct callback_head *work)
>   
>   	t0 = sched_clock_cpu(curr_cpu);
>   
> +	now = jiffies;
> +	if (time_before(now, READ_ONCE(mm->sc_stat.next_scan)))
> +		return;
> +

I agree that limiting the scan rate would be useful,
and your above change is actually similar to what NUMA balancing
did in task_numa_work(). It allows only one thread within the
same process to perform the statistics calculation, which avoids
redundant computation.

> +	WRITE_ONCE(mm->sc_stat.next_scan, (now + mm->sc_stat.scan_period));
> +

I suppose the above should be try_cmpxchg()?

That is to say, with your above change, we have already limited the scan
ratio for multi-threaded processes significantly. There appears to be no
need to perform adaptive adjustment of scan_period - the benefit of 
introducing
an adaptive scan_period may not offset the overhead of frequent writing 
to the
"global" mm->sc_stat.scan_period due to c2c?

thanks,
Chenyu