linux-kernel - Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers and writers of load

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Date:	Wed, 02 Dec 2015 13:48:31 -0500
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>,
	Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Yuyang Du <yuyang.du@...el.com>
Subject: Re: [RFC PATCH 3/3] sched/fair: Use different cachelines for readers
 and writers of load_avg

On 12/01/2015 03:49 AM, Peter Zijlstra wrote:
> On Mon, Nov 30, 2015 at 10:55:02PM -0500, Waiman Long wrote:
>> On 11/30/2015 05:09 PM, Peter Zijlstra wrote:
>>> On Mon, Nov 30, 2015 at 02:13:32PM -0500, Waiman Long wrote:
>>>>> This begs the question tough; why are you running a global load in a
>>>>> cgroup; and do we really need to update this for the root cgroup? It
>>>>> seems to me we don't need calc_tg_weight() for the root cgroup, it
>>>>> doesn't need to normalize its weight numbers.
>>>>>
>>>>> That is; isn't this simply a problem we should avoid?
>>>> I didn't use any cgroup in my test setup. Autogroup was enabled, though.
>>>> Booting up a 4.4-rc2 kernel caused sched_create_group() to be called 56
>>>> times.
>>> Yeah, can you kill autogroup and see if that helps? If not, we probably
>>> should add some code to avoid calculating things for the root group.
>> I will try that out tomorrow. However, SCHED_AUTOGROUP was enabled in the
>> distribution kernels. So we still need to look at that with autogroup
>> enabled.
> Meh, or just tell the people that have stupid large machines to use
> noautogroup on boot (its of questionable benefit in the first place imo,
> esp. on servers).

Yes, I was able to recover most of the lost performance by disabling 
autogroup. I did send out a new patch to disable load_avg update for 
root_task_group. I need that for backporting to earlier kernels which 
was forced to update load_avg for every clock tick even for root_task_group.

Cheers,
Longman
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