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: <f02e2c1a-e65f-4078-a138-ccf734f84643@linux.ibm.com>
Date: Fri, 25 Apr 2025 14:43:18 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: Tim Chen <tim.c.chen@...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

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?

Thanks,
Madadi Vineeth Reddy

> 
> Tim