linux-kernel - Re: [PATCH] sched: Skip useless sched_balance_running acquisition if load balance is not due

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Message-ID: <331fb3d6-33f3-4656-a134-3ab3a245e477@amd.com>
Date: Fri, 18 Apr 2025 17:43:34 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Vincent Guittot <vincent.guittot@...aro.org>, Shrikanth Hegde
	<sshegde@...ux.ibm.com>, "Chen, Yu C" <yu.c.chen@...el.com>, Tim Chen
	<tim.c.chen@...ux.intel.com>, Ingo Molnar <mingo@...nel.org>, Doug Nelson
	<doug.nelson@...el.com>, Mohini Narkhede <mohini.narkhede@...el.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Skip useless sched_balance_running acquisition if
 load balance is not due

Hello Peter,

On 4/18/2025 2:58 PM, Peter Zijlstra wrote:
> On Fri, Apr 18, 2025 at 10:56:04AM +0530, K Prateek Nayak wrote:
>> Hello Peter,
>>
>> On 4/17/2025 5:31 PM, Peter Zijlstra wrote:
>>>> o Since this is a single flag across the entire system, it also implies
>>>>     CPUs cannon concurrently do load balancing across different NUMA
>>>>     domains which seems reasonable since a load balance at lower NUMA
>>>>     domain can potentially change the "nr_numa_running" and
>>>>     "nr_preferred_running" stats for the higher domain but if this is the
>>>>     case, a newidle balance at lower NUMA domain can interfere with a
>>>>     concurrent busy / newidle load balancing at higher NUMA domain.
>>>>     Is this expected? Should newidle balance be serialized too?
>>>
>>> Serializing new-idle might create too much idle time.
>>
>> In the context of busy and idle balancing, What are your thoughts on a
>> per sd "serialize' flag?
> 
> My sekret hope is that this push stuff can rid us all the idle balance
> bits. But yeah, early days on that.
> 
> Other than that, I don't quite see why we should split that, busy
> balancing is the one that runs more often and is the one that should be
> serialized to avoid too much cross node traffic and all that, no?
> 
> The idle thing is less often, why not limit that?

Makes sense. I'll add it to the set of my weekend experiment runs.

-- 
Thanks and Regards,
Prateek