linux-kernel - Re: [PATCH v8 07/10] sched: replace capacity

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Date:	Mon, 3 Nov 2014 11:59:12 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Rik van Riel <riel@...hat.com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH v8 07/10] sched: replace capacity_factor by usage

On 3 November 2014 08:01, Wanpeng Li <kernellwp@...il.com> wrote:
> Hi Vincent,
> On 14/10/31 下午4:47, Vincent Guittot wrote:
>>
>> The scheduler tries to compute how many tasks a group of CPUs can handle
>> by
>> assuming that a task's load is SCHED_LOAD_SCALE and a CPU's capacity is
>> SCHED_CAPACITY_SCALE. group_capacity_factor divides the capacity of the
>> group
>> by SCHED_LOAD_SCALE to estimate how many task can run in the group. Then,
>> it
>> compares this value with the sum of nr_running to decide if the group is
>> overloaded or not. But the group_capacity_factor is hardly working for SMT
>>   system, it sometimes works for big cores but fails to do the right thing
>> for
>>   little cores.
>>
>> Below are two examples to illustrate the problem that this patch solves:
>>
>> 1- If the original capacity of a CPU is less than SCHED_CAPACITY_SCALE
>> (640 as an example), a group of 3 CPUS will have a max capacity_factor of
>> 2
>> (div_round_closest(3x640/1024) = 2) which means that it will be seen as
>> overloaded even if we have only one task per CPU.
>>
>> 2 - If the original capacity of a CPU is greater than SCHED_CAPACITY_SCALE
>> (1512 as an example), a group of 4 CPUs will have a capacity_factor of 4
>> (at max and thanks to the fix [0] for SMT system that prevent the
>> apparition
>
>
> What's the story of 'fix [0] for SMT system' here?

The [0] url has disappeared. please find it below:
[0] https://lkml.org/lkml/2013/8/28/194

Let me know if your need more explanation than my answer of your
previous question for the cover letter

Regards,
Vincent

>
> Regards,
> Wanpeng Li
>
>
>> of ghost CPUs) but if one CPU is fully used by rt tasks (and its capacity
>> is
>> reduced to nearly nothing), the capacity factor of the group will still be
>> 4
>> (div_round_closest(3*1512/1024) = 5 which is cap to 4 with [0]).
>>

[snip]
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