linux-kernel - Re: [PATCH V2] sched: Improve load balancing in the presence of idle CPUs

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Date:	Fri, 27 Mar 2015 10:03:21 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
CC:	peterz@...radead.org, mingo@...nel.org, riel@...hat.com,
	daniel.lezcano@...aro.org, 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, jason.low2@...com
Subject: Re: [PATCH V2] sched: Improve load balancing in the presence of idle
 CPUs

Hi Wanpeng

On 03/27/2015 07:42 AM, Wanpeng Li wrote:
> Hi Preeti,
> On Thu, Mar 26, 2015 at 06:32:44PM +0530, Preeti U Murthy wrote:
>>
>> 1. An ILB CPU was chosen from the first numa domain to trigger nohz idle
>> load balancing [Given the experiment, upto 6 CPUs per core could be
>> potentially idle in this domain.]
>>
>> 2. However the ILB CPU would call load_balance() on itself before
>> initiating nohz idle load balancing.
>>
>> 3. Given cores are SMT8, the ILB CPU had enough opportunities to pull
>> tasks from its sibling cores to even out load.
>>
>> 4. Now that the ILB CPU was no longer idle, it would abort nohz idle
>> load balancing
> 
> I don't see abort nohz idle load balancing when ILB CPU was no longer idle 
> in nohz_idle_balance(), could you explain more in details?

When the ILB CPU pulls load in rebalance_domains(), its idle state
is set to CPU_NOT_IDLE.

""
idle = idle_cpu(cpu) ? CPU_IDLE : CPU_NOT_IDLE;

And,

When nohz_idle_balance() is called, the state of idle of ILB CPU is
checked before proceeding with load balancing on idle CPUs.

""

if (idle != CPU_IDLE ||
             !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
                 goto end;

You see that nohz idle balancing gets aborted.

Regards
Preeti U Murthy



> 
> Regards,
> Wanpeng Li 
> 

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