linux-kernel - Re: [PATCH] genirq: only scan the present CPUs

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Date:   Fri, 6 Apr 2018 16:42:14 +0800
From:   Dou Liyang <douly.fnst@...fujitsu.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     Li RongQing <lirongqing@...du.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] genirq: only scan the present CPUs

Hi Thomas, Peter,

At 04/03/2018 07:23 PM, Peter Zijlstra wrote:
> On Tue, Apr 03, 2018 at 12:25:56PM +0200, Thomas Gleixner wrote:
>> On Mon, 2 Apr 2018, Li RongQing wrote:
>>
>>> lots of application will read /proc/stat, like ps and vmstat, but we
>>> find the reading time are spreading on Purley platform which has lots
>>> of possible CPUs and interrupt.
>>>
>>> To reduce the reading time, only scan the present CPUs, not all possible
>>> CPUs, which speeds the reading of /proc/stat 20 times on Purley platform
>>> which has 56 present CPUs, and 224 possible CPUs
>>
>> Why is BIOS/ACPI telling the kernel that there are 224 possible CPUs unless
>> it supports physical CPU hotplug.
> 
> BIOS is crap, news at 11. I've got boxes like that too. Use
> possible_cpu=$nr if you're bothered by it -- it's what I do.
> 

Yes, I think so. it is a manual way to reset the number.

For this situation, I am investigating to restrict the number of
possible CPUs automatically, But, due to the limitation of ACPI
subsystem, I can do it _before_ setup_percpu_area where the number will
be used.

But, I can provider an indicator to tell the system that whether the 
physical CPU hotplug is support or not later. Can we use this indicator
like that in this situation:

    if ture

	Using for_each_possible_cpu(cpu)
    else

	Using for_each_present_cpu(cpu)	



Thanks,

	dou

> 
>