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Message-ID: <0d7fa00e-587e-4ac8-90d0-115f30fdf0ac@linux.ibm.com>
Date: Tue, 1 Apr 2025 01:47:26 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: "Chen, Yu C" <yu.c.chen@...el.com>
Cc: mingo@...nel.org, gautham.shenoy@....com, kprateek.nayak@....com,
juri.lelli@...hat.com, vincent.guittot@...aro.org,
dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org,
tim.c.chen@...ux.intel.com, tglx@...utronix.de,
Peter Zijlstra <peterz@...radead.org>,
Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [RFC][PATCH] sched: Cache aware load-balancing
Hi Chen Yu,
On 27/03/25 16:44, Chen, Yu C wrote:
> Hi Madadi,
>
> On 3/27/2025 10:43 AM, Madadi Vineeth Reddy wrote:
>> Hi Peter,
>>
>> On 25/03/25 17:39, Peter Zijlstra wrote:
>>> Hi all,
>>>
>>> One of the many things on the eternal todo list has been finishing the
>>> below hackery.
>>>
>>> It is an attempt at modelling cache affinity -- and while the patch
>>> really only targets LLC, it could very well be extended to also apply to
>>> clusters (L2). Specifically any case of multiple cache domains inside a
>>> node.
>>>
>>> Anyway, I wrote this about a year ago, and I mentioned this at the
>>> recent OSPM conf where Gautham and Prateek expressed interest in playing
>>> with this code.
>>>
>>> So here goes, very rough and largely unproven code ahead :-)
>>>
>>> It applies to current tip/master, but I know it will fail the __percpu
>>> validation that sits in -next, although that shouldn't be terribly hard
>>> to fix up.
>>>
>>> As is, it only computes a CPU inside the LLC that has the highest recent
>>> runtime, this CPU is then used in the wake-up path to steer towards this
>>> LLC and in task_hot() to limit migrations away from it.
>>>
>>> More elaborate things could be done, notably there is an XXX in there
>>> somewhere about finding the best LLC inside a NODE (interaction with
>>> NUMA_BALANCING).
>>
>> Tested the patch on a 12-core, 96-thread Power10 system using a real-life
>> workload, DayTrader.
>
> Do all the Cores share the same LLC within 1 node? If this is the case,
> the regression might be due to over-migration/task stacking within 1 LLC/node. This patch should be modified that cache aware load balancing/wakeup will not be triggered if there is only 1 LLC within the node IMO.
Are you asking whether LLC is shared at the node level?
In Power10, the LLC is at the small core level, covering 4 threads.
In my test setup, there were 4 nodes, each with 24 CPUs, meaning there
were 6 LLCs per node.
Went through the patch in more detail and will check if task stacking
is an issue using micro-benchmarks.
Thanks for your feedback.
Thanks,
Madadi Vineeth Reddy
>
> thanks,
> Chenyu
>
>>
>> Here is a summary of the runs:
>>
>> Users | Instances | Throughput vs Base | Avg Resp. Time vs Base
>> --------------------------------------------------------------
>> 30 | 1 | -25.3% | +50%
>> 60 | 1 | -25.1% | +50%
>> 30 | 3 | -22.8% | +33%
>>
>> As of now, the patch negatively impacts performance both in terms of
>> throughput and latency.
>>
>> I will conduct more extensive testing with both microbenchmarks and
>> real-life workloads.
>>
>> Thanks,
>> Madadi Vineeth Reddy
>>
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