linux-kernel - Re: [RFC PATCH 4/5] sched: Inhibit cache aware scheduling if the preferred LLC is over aggregated

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Message-ID: <0a574da2c69e6bd607e20c5865d3b66fb9329e6d.camel@linux.intel.com>
Date: Fri, 25 Apr 2025 10:29:55 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: e45141a1a64d7dcfca2683f56735ba4da60ba19e.camel@...ux.intel.com
Cc: "Chen, Yu C" <yu.c.chen@...el.com>, Peter Zijlstra
 <peterz@...radead.org>,  Ingo Molnar <mingo@...hat.com>, K Prateek Nayak
 <kprateek.nayak@....com>, "Gautham R . Shenoy" <gautham.shenoy@....com>,
 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>, Libo Chen <libo.chen@...cle.com>,
 Abel Wu <wuyun.abel@...edance.com>, Hillf Danton <hdanton@...a.com>, 
 linux-kernel@...r.kernel.org, Len Brown <len.brown@...el.com>, Chen Yu
 <yu.chen.surf@...mail.com>, Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [RFC PATCH 4/5] sched: Inhibit cache aware scheduling if the
 preferred LLC is over aggregated

On Fri, 2025-04-25 at 14:43 +0530, Madadi Vineeth Reddy wrote:
> Hi Tim,
> 
> On 24/04/25 21:21, Tim Chen wrote:
> > On Thu, 2025-04-24 at 22:11 +0800, Chen, Yu C wrote:
> > > 
> > > > It spawns lot of threads and is CPU intensive. So, I think it's not impacted
> > > > due to the below conditions.
> > > > 
> > > > Also, in schbench numbers provided by you, there is a degradation in saturated
> > > > case. Is it due to the overhead in computing the preferred llc which is not
> > > > being used due to below conditions?
> > > 
> > > Yes, the overhead of preferred LLC calculation could be one part, and we 
> > > also suspect that the degradation might be tied to the task migrations. 
> > > We still observed more task migrations than the baseline, even when the 
> > > system was saturated (in theory, after 25% is exceeded, we should 
> > > fallback to the generic task wakeup path). We haven't dug into that yet, 
> > > and we can conduct an investigation in the following days.
> > 
> > In the saturation case it is mostly the tail latency that has regression.
> > The preferred LLC has a tendency to have higher load than the
> > other LLCs. Load balancer will try to move tasks out and wake balance will
> > try to move it back to the preferred LLC. This increases the task migrations
> > and affect tail latency.
> 
> Why would the task be moved back to the preferred LLC in wakeup path for the
> saturated case? The checks shouldn't allow it right?

The task wake ups happens very frequently in schbench and it takes a while for utilization to catch
up. The utilization of the LLC is updated at the load balance time of LLC. 

So once utilization falls below the utilization threshold, there is a window
where the woken tasks will rush into the preferred LLC until the utilization
is updated at the next load balance time. 

Tim

> 
> Thanks,
> Madadi Vineeth Reddy
> 
> > 
> > Tim
>