linux-kernel - Re: [PATCH 1/2] sched/fair: Record the average duration of a task

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Message-ID: <8c4f0bf2-0e8e-b94e-070b-47dbd407b70d@amd.com>
Date: Wed, 3 Jul 2024 19:10:04 +0530
From: Raghavendra K T <raghavendra.kt@....com>
To: Mike Galbraith <efault@....de>, Chen Yu <yu.c.chen@...el.com>
CC: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
	<vincent.guittot@...aro.org>, Tim Chen <tim.c.chen@...el.com>, Yujie Liu
	<yujie.liu@...el.com>, K Prateek Nayak <kprateek.nayak@....com>, "Gautham R .
 Shenoy" <gautham.shenoy@....com>, Chen Yu <yu.chen.surf@...il.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/fair: Record the average duration of a task



On 7/3/2024 5:27 PM, Mike Galbraith wrote:
> On Wed, 2024-07-03 at 14:04 +0530, Raghavendra K T wrote:
>>
>>
>> On 7/1/2024 8:27 PM, Chen Yu wrote:
>>>
>>> A thought occurred to me that one possible method to determine if the waker
>>> and wakee share data could be to leverage the NUMA balance's numa_group data structure.
>>> As numa balance periodically scans the task's VMA space and groups tasks accessing
>>> the same physical page into one numa_group, we can infer that if the waker and wakee
>>> are within the same numa_group, they are likely to share data, and it might be
>>> appropriate to place the wakee on top of the waker.
>>>
>>> CC Raghavendra here in case he has any insights.
>>>
>>
>> Agree with your thought here,
>>
>> So I imagine two possible things to explore here.
>>
>> 1) Use task1, task2 numa_group and check if they belong to same
>> numa_group, also check if there is a possibility of M:N relationship
>> by checking if t1/t2->numa_group->nr_tasks > 1 etc
>>
>> 2) Given a VMA we can use vma_numab_state pids_active[] if task1, task2
>> (threads) possibly interested in same VMA.
>> Latter one looks to be practically difficult because we don't want to
>> sweep across VMAs perhaps..
> 
> Oooh dear.. as soon as you mention threads, the question of who's
> wheelhouse is this in springs to mind, ie should the kernel be
> overriding userspace by targeting bits of threaded programs for forced
> serialization?
> 

Yes.. There is no ROI on this option (mentioned only for completeness).
also we are not looking beyond process. Rather than "Practically
  difficult" I should have rephrased as Practically not an option.

> Bah, think I'll just bugger off and let you guys have a go at making
> this stacking business do less harm than good.
> 
> 	-Mike