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Subject: Re: [PATCH] genirq: only scan the present CPUs
To:     Peter Zijlstra <peterz@infradead.org>
CC:     Thomas Gleixner <tglx@linutronix.de>,
        Li RongQing <lirongqing@baidu.com>,
        LKML <linux-kernel@vger.kernel.org>
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From:   Dou Liyang <douly.fnst@cn.fujitsu.com>
Message-ID: <465754fe-8b49-c77c-9425-e077f6faf4df@cn.fujitsu.com>
Date:   Mon, 9 Apr 2018 14:33:43 +0800
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Hi Peter,

At 04/06/2018 05:05 PM, Peter Zijlstra wrote:
> On Fri, Apr 06, 2018 at 11:02:28AM +0200, Peter Zijlstra wrote:
>> On Fri, Apr 06, 2018 at 04:42:14PM +0800, Dou Liyang wrote:
>>> 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.
> 
> Ah, did you mean to day "I can _NOT_ do it" ? Still I don't see the
                                 ^----------- Oops, yes.

> point of frobbing random users if the whole thing is buggered.
> 

If ACPI subsystem can be initialized earlier, we can get the accurate
number of possible CPUs from the ACPI namespace. then, we can reset the
_cpu_possible_mask_ as the prefill_possible_map() does. So, it can
forbid random users.

But, It needs the memory to be initialized first, so it can't be called 
earlier setup_percpu_area() which is evoked earlier than mem_init().

and you are right:

"So if you see it enumerates a gazillion empty spots but the system does
  not in fact support physical hotplug, we should discard those."

I will think it more carefully.

Thanks,

	dou
> 
>