linux-kernel - Re: [RFC patch v3 07/20] sched: Add helper function to decide whether to allow cache aware scheduling

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Message-ID: <7f4f1684-fbe6-4148-bbe4-fc2e138abb98@oracle.com>
Date: Mon, 7 Jul 2025 17:41:43 -0700
From: Libo Chen <libo.chen@...cle.com>
To: Tim Chen <tim.c.chen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>
Cc: 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>,
        Tim Chen <tim.c.chen@...el.com>,
        Vincent Guittot
 <vincent.guittot@...aro.org>,
        Abel Wu <wuyun.abel@...edance.com>,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
        Hillf Danton <hdanton@...a.com>, Len Brown <len.brown@...el.com>,
        linux-kernel@...r.kernel.org, Chen Yu <yu.c.chen@...el.com>
Subject: Re: [RFC patch v3 07/20] sched: Add helper function to decide whether
 to allow cache aware scheduling

Hi Tim and Chenyu,


On 6/18/25 11:27, Tim Chen wrote:
> Cache-aware scheduling is designed to aggregate threads into their
> preferred LLC, either via the task wake up path or the load balancing
> path. One side effect is that when the preferred LLC is saturated,
> more threads will continue to be stacked on it, degrading the workload's
> latency. A strategy is needed to prevent this aggregation from going too
> far such that the preferred LLC is too overloaded.
> 
> Introduce helper function _get_migrate_hint() to implement the LLC
> migration policy:
> 
> 1) A task is aggregated to its preferred LLC if both source/dest LLC
>    are not too busy (<50% utilization, tunable), or the preferred
>    LLC will not be too out of balanced from the non preferred LLC
>    (>20% utilization, tunable, close to imbalance_pct of the LLC
>    domain).
> 2) Allow a task to be moved from the preferred LLC to the
>    non-preferred one if the non-preferred LLC will not be too out
>    of balanced from the preferred prompting an aggregation task
>    migration later.  We are still experimenting with the aggregation
>    and migration policy. Some other possibilities are policy based
>    on LLC's load or average number of tasks running.  Those could
>    be tried out by tweaking _get_migrate_hint().
> 
> The function _get_migrate_hint() returns migration suggestions for the upper-le
> +__read_mostly unsigned int sysctl_llc_aggr_cap       = 50;
> +__read_mostly unsigned int sysctl_llc_aggr_imb       = 20;
> +


I think this patch has a great potential.

Since _get_migrate_hint() is tied to an individual task anyway, why not add a
per-task llc_aggr_imb which defaults to the sysctl one? Tasks have different
preferences for llc stacking, they can all be running in the same system at the
same time. This way you can offer a greater deal of optimization without much
burden to others.

Also with sysctl_llc_aggr_imb, do we really need SCHED_CACHE_WAKE? Does setting
sysctl_llc_aggr_imb to 0 basically say no preference for either LLC, no?

Thanks,
Libo

> +static enum llc_mig_hint _get_migrate_hint(int src_cpu, int dst_cpu,
> +					   unsigned long tsk_util,
> +					   bool to_pref)
> +{
> +	unsigned long src_util, dst_util, src_cap, dst_cap;
> +
> +	if (cpus_share_cache(src_cpu, dst_cpu))
> +		return mig_allow;
> +
> +	if (!get_llc_stats(src_cpu, &src_util, &src_cap) ||
> +	    !get_llc_stats(dst_cpu, &dst_util, &dst_cap))
> +		return mig_allow;
> +
> +	if (!fits_llc_capacity(dst_util, dst_cap) &&
> +	    !fits_llc_capacity(src_util, src_cap))
> +		return mig_ignore;
> +
> +	src_util = src_util < tsk_util ? 0 : src_util - tsk_util;
> +	dst_util = dst_util + tsk_util;
> +	if (to_pref) {
> +		/*
> +		 * sysctl_llc_aggr_imb is the imbalance allowed between
> +		 * preferred LLC and non-preferred LLC.
> +		 * Don't migrate if we will get preferred LLC too
> +		 * heavily loaded and if the dest is much busier
> +		 * than the src, in which case migration will
> +		 * increase the imbalance too much.
> +		 */
> +		if (!fits_llc_capacity(dst_util, dst_cap) &&
> +		    util_greater(dst_util, src_util))
> +			return mig_forbid;
> +	} else {
> +		/*
> +		 * Don't migrate if we will leave preferred LLC
> +		 * too idle, or if this migration leads to the
> +		 * non-preferred LLC falls within sysctl_aggr_imb percent
> +		 * of preferred LLC, leading to migration again
> +		 * back to preferred LLC.
> +		 */
> +		if (fits_llc_capacity(src_util, src_cap) ||
> +		    !util_greater(src_util, dst_util))
> +			return mig_forbid;
> +	}
> +	return mig_allow;
> +}