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

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Message-ID: <b69923cb8a11c76c036a6eb958bd3b363025ec43.camel@linux.intel.com>
Date: Tue, 09 Dec 2025 13:39:32 -0800
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>
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 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);
> > +	}
> > +}