linux-kernel - Re: sched: Improve load balancing in the presence of idle CPUs

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Message-ID: <551B9514.80701@linux.vnet.ibm.com>
Date:	Wed, 01 Apr 2015 12:19:56 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Jason Low <jason.low2@...com>
CC:	peterz@...radead.org, mingo@...nel.org,
	Daniel Lezcano <daniel.lezcano@...aro.org>, riel@...hat.com,
	vincent.guittot@...aro.org, srikar@...ux.vnet.ibm.com,
	pjt@...gle.com, benh@...nel.crashing.org, efault@....de,
	linux-kernel@...r.kernel.org, iamjoonsoo.kim@....com,
	svaidy@...ux.vnet.ibm.com, tim.c.chen@...ux.intel.com,
	morten.rasmussen@....com
Subject: Re: sched: Improve load balancing in the presence of idle CPUs


On 04/01/2015 12:24 AM, Jason Low wrote:
> On Tue, 2015-03-31 at 14:07 +0530, Preeti U Murthy wrote:
>> Hi Jason,
>>
>> On 03/31/2015 12:25 AM, Jason Low wrote:
>>> Hi Preeti,
>>>
>>> I noticed that another commit 4a725627f21d converted the check in
>>> nohz_kick_needed() from idle_cpu() to rq->idle_balance, causing a
>>> potentially outdated value to be used if this cpu is able to pull tasks
>>> using rebalance_domains(), and nohz_kick_needed() directly returning
>>> false.
>>
>> I see that rebalance_domains() will be run at the end of the scheduler
>> tick interrupt handling. trigger_load_balance() only sets the softirq,
>> it does not call rebalance_domains() immediately. So the call graph
>> would be:
> 
> Oh right, since that only sets the softirq, this wouldn't be the issue,
> though we would need these changes if we were to incorporate any sort of
> nohz_kick_needed() logic into the nohz_idle_balance() code path correct?

I am sorry I don't quite get this. Can you please elaborate?

Regards
Preeti U Murthy
> 

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