linux-kernel - Re: [patch v4 01/18] sched: set SD_PREFER_SIBLING on MC domain to reduce a domain level

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Date:	Wed, 13 Feb 2013 21:22:26 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	torvalds@...ux-foundation.org, mingo@...hat.com,
	tglx@...utronix.de, akpm@...ux-foundation.org,
	arjan@...ux.intel.com, bp@...en8.de, pjt@...gle.com,
	namhyung@...nel.org, efault@....de, vincent.guittot@...aro.org,
	gregkh@...uxfoundation.org, preeti@...ux.vnet.ibm.com,
	viresh.kumar@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [patch v4 01/18] sched: set SD_PREFER_SIBLING on MC domain to
 reduce a domain level

On 02/12/2013 06:11 PM, Peter Zijlstra wrote:
> On Thu, 2013-01-24 at 11:06 +0800, Alex Shi wrote:
>> The domain flag SD_PREFER_SIBLING was set both on MC and CPU domain at
>> frist commit b5d978e0c7e79a, and was removed uncarefully when clear up
>> obsolete power scheduler. Then commit 6956dc568 recover the flag on CPU
>> domain only. It works, but it introduces a extra domain level since this
>> cause MC/CPU different.
>>
>> So, recover the the flag in MC domain too to remove a domain level in
>> x86 platform.

Peter, I am very very happy to see you again! :)
> 
> This fails to clearly state why its desirable.. I'm guessing its because
> we should use sibling cache domains before sibling threads, right?

No, the flags set on MC/CPU domain, but is checked in their parents
balancing, like in NUMA domain.
Without the flag, will cause NUMA domain imbalance. like on my 2 sockets
NHM EP: 3 of 4 tasks were assigned on socket 0(lcpu, 10, 12, 14)

In this case, update_sd_pick_busiest() need a reduced group_capacity to
return true:
	if (sgs->sum_nr_running > sgs->group_capacity)
		return true;
then numa domain balancing get chance to start.

---------
05:00:28 AM  CPU    %usr   %nice      %idle
05:00:29 AM  all   25.00    0.00      74.94
05:00:29 AM    0    0.00    0.00      99.00
05:00:29 AM    1    0.00    0.00     100.00
05:00:29 AM    2    0.00    0.00     100.00
05:00:29 AM    3    0.00    0.00     100.00
05:00:29 AM    4    0.00    0.00     100.00
05:00:29 AM    5    0.00    0.00     100.00
05:00:29 AM    6    0.00    0.00     100.00
05:00:29 AM    7    0.00    0.00     100.00
05:00:29 AM    8    0.00    0.00     100.00
05:00:29 AM    9    0.00    0.00     100.00
05:00:29 AM   10  100.00    0.00       0.00
05:00:29 AM   11    0.00    0.00     100.00
05:00:29 AM   12  100.00    0.00       0.00
05:00:29 AM   13    0.00    0.00     100.00
05:00:29 AM   14  100.00    0.00       0.00
05:00:29 AM   15  100.00    0.00       0.00



-- 
Thanks
    Alex
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