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

[Tim Chen]

On Tue, 2025-12-09 at 12:12 +0100, Peter Zijlstra wrote:
> 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.

Okay. Got it.

Tim

> 
> > +
> > +	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);
> > +	}
> > +}