linux-kernel - Re: [PATCH 02/19] sched/fair: Record per-LLC utilization to guide cache-aware scheduling decisions

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ea646ffe-908e-4730-8b3a-69c9318ad5d0@intel.com>
Date: Mon, 27 Oct 2025 22:07:52 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: K Prateek Nayak <kprateek.nayak@....com>
CC: 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>, 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>, Ingo Molnar <mingo@...hat.com>, Adam Li
	<adamli@...amperecomputing.com>, Tim Chen <tim.c.chen@...el.com>,
	<linux-kernel@...r.kernel.org>, Tim Chen <tim.c.chen@...ux.intel.com>, "Peter
 Zijlstra" <peterz@...radead.org>, "Gautham R . Shenoy"
	<gautham.shenoy@....com>
Subject: Re: [PATCH 02/19] sched/fair: Record per-LLC utilization to guide
 cache-aware scheduling decisions

Hi Prateek,

On 10/27/2025 1:01 PM, K Prateek Nayak wrote:
> Hello Tim,
> 
> On 10/11/2025 11:54 PM, Tim Chen wrote:
>> +#ifdef CONFIG_SCHED_CACHE
>> +/*
>> + * Record the statistics for this scheduler group for later
>> + * use. These values guide load balancing on aggregating tasks
>> + * to a LLC.
>> + */
>> +static void record_sg_llc_stats(struct lb_env *env,
>> +				struct sg_lb_stats *sgs,
>> +				struct sched_group *group)
>> +{
>> +	/*
>> +	 * Find the child domain on env->dst_cpu. This domain
>> +	 * is either the domain that spans this group(if the
>> +	 * group is a local group), or the sibling domain of
>> +	 * this group.
>> +	 */
>> +	struct sched_domain *sd = env->sd->child;
> 
> Was this intentionally done to limit the update to sg_llc_stats to the
> load balancing period of "sd_llc->parent"?
> 
> Can't this be done with update_idle_cpu_scan()? I believe it is more
> frequent, "sds->total_capacity" from caller gives you the equivalent of
> "group_capacity", and "group_util" is already calculated as "sum_util".
> 
> Checking "sd_llc->parent" there should be sufficient to check if there
> are multiple LLC domains or not. Thoughts?
> 

The original idea was to calculate the statistics for the CPUs within
one LLC, and set the tag for that sched group as well as its sg_lb_stats
(but not at the sched domain scope). With this flag set in that sched group,
we can perform some comparisons in update_sd_pick_busiest() to determine if
that sched group has any tasks that need to be moved to other LLC sched 
groups.
If we do this in update_idle_cpu_scan(), might it be a bit late for
update_sd_pick_busiest()?

thanks,
Chenyu

>> +	struct sched_domain_shared *sd_share;
>> +
>> +	if (!sched_feat(SCHED_CACHE) || env->idle == CPU_NEWLY_IDLE)
>> +		return;
>> +
>> +	/* only care about sched domains spanning a LLC */
>> +	if (sd != rcu_dereference(per_cpu(sd_llc, env->dst_cpu)))
>> +		return;
>> +
>> +	/*
>> +	 * At this point we know this group spans a LLC domain.
>> +	 * Record the statistic of this group in its corresponding
>> +	 * shared LLC domain.
>> +	 */
>> +	sd_share = rcu_dereference(per_cpu(sd_llc_shared,
>> +					   cpumask_first(sched_group_span(group))));
>> +	if (!sd_share)
>> +		return;
>> +
>> +	if (READ_ONCE(sd_share->util_avg) != sgs->group_util)
>> +		WRITE_ONCE(sd_share->util_avg, sgs->group_util);
>> +
>> +	if (unlikely(READ_ONCE(sd_share->capacity) != sgs->group_capacity))
>> +		WRITE_ONCE(sd_share->capacity, sgs->group_capacity);
>> +}
>