linux-kernel - Re: [PATCH v2 0/11] remove cpu

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Message-ID: <CAKfTPtDtv=hFtprNpAfRVMEAvWZn-55u8fcHTdJT6xT92Qdt-g@mail.gmail.com>
Date:	Tue, 18 Feb 2014 13:28:25 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Alex Shi <alex.shi@...aro.org>
Cc:	Morten Rasmussen <morten.rasmussen@....com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
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Subject: Re: [PATCH v2 0/11] remove cpu_load in rq

On 18 February 2014 13:05, Morten Rasmussen <morten.rasmussen@....com> wrote:
> On Mon, Feb 17, 2014 at 01:55:06AM +0000, Alex Shi wrote:
>> The cpu_load decays on time according past cpu load of rq. The sched_avg also decays tasks' load on time. Now we has 2 kind decay for cpu_load. That is a kind of redundancy. And increase the system load by decay calculation. This patch try to remove the cpu_load decay.
>>
>> There are 5 load_idx used for cpu_load in sched_domain. busy_idx and idle_idx are not zero usually, but newidle_idx, wake_idx and forkexec_idx are all zero on every arch. A shortcut to remove cpu_Load decay in the first patch. just one line patch for this change.
>>
>> V2,
>> 1, This version do some tuning on load bias of target load, to maximum match current code logical.
>> 2, Got further to remove the cpu_load in rq.
>> 3, Revert the patch 'Limit sd->*_idx range on sysctl' since no needs
>>
>> Any testing/comments are appreciated.
>
> Removing cpu_load completely certainly makes things simpler, my worry is
> just how much was lost by doing it. I agree that cpu_load needs a
> cleanup, but I can't convince myself that just removing it completely
> and not having any longer term view of cpu load anymore is without any
> negative side-effects.

Hi Alex,

Have you followed this thread about load_idx and the interest of using
them to use different average period ?
https://lkml.org/lkml/2014/1/6/499

Vincent

>
> {source, target}_load() are now instantaneous views of the cpu load,
> which means that they may change very frequently. That could potentially
> lead to more task migrations at all levels in the domain hierarchy as we
> no longer have the more conservative cpu_load[] indexes that were used
> at NUMA level.
>
> Maybe some of the NUMA experts have an opinion about this?
>
> In the discussions around V1 I think blocked load came up again as a
> potential replacement for the current cpu_load array. There are some
> issues that need to be solved around blocked_load first though.
>
> Morten
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