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

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Message-ID: <20251209111244.GJ3707891@noisy.programming.kicks-ass.net>
Date: Tue, 9 Dec 2025 12:12:44 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Hillf Danton <hdanton@...a.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Jianyong Wu <jianyong.wu@...look.com>,
	Yangyu Chen <cyy@...self.name>,
	Tingyin Duan <tingyin.duan@...il.com>,
	Vern Hao <vernhao@...cent.com>, Vern Hao <haoxing990@...il.com>,
	Len Brown <len.brown@...el.com>, Aubrey Li <aubrey.li@...el.com>,
	Zhao Liu <zhao1.liu@...el.com>, Chen Yu <yu.chen.surf@...il.com>,
	Chen Yu <yu.c.chen@...el.com>,
	Adam Li <adamli@...amperecomputing.com>,
	Aaron Lu <ziqianlu@...edance.com>, Tim Chen <tim.c.chen@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/23] sched/cache: Introduce infrastructure for
 cache-aware load balancing

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);
> +	}
> +}