Re: [PATCH v2 01/23] sched/cache: Introduce infrastructure for cache-aware load balancing

[Peter Zijlstra]

On Wed, Dec 03, 2025 at 03:07:20PM -0800, Tim Chen wrote:

>        Minor fix in task_tick_cache() to use
>        if (mm->mm_sched_epoch >= rq->cpu_epoch)
>        to avoid mm_sched_epoch going backwards.

> +static void task_tick_cache(struct rq *rq, struct task_struct *p)
> +{
> +	struct callback_head *work = &p->cache_work;
> +	struct mm_struct *mm = p->mm;
> +
> +	if (!sched_cache_enabled())
> +		return;
> +
> +	if (!mm || !mm->pcpu_sched)
> +		return;
> +
> +	/* avoid moving backwards */
> +	if (mm->mm_sched_epoch >= rq->cpu_epoch)
> +		return;

IIRC this was supposed to be able to wrap; which then means you should
write it like:

	if ((mm->mm_sched_epoch - rq->cpu_epoch) >= 0)
		return;

or somesuch.

> +
> +	guard(raw_spinlock)(&mm->mm_sched_lock);
> +
> +	if (work->next == work) {
> +		task_work_add(p, work, TWA_RESUME);
> +		WRITE_ONCE(mm->mm_sched_epoch, rq->cpu_epoch);
> +	}
> +}