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Date:   Fri, 6 Apr 2018 11:02:28 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dou Liyang <douly.fnst@cn.fujitsu.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Li RongQing <lirongqing@baidu.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] genirq: only scan the present CPUs
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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.
> 
> 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 anything you should fix up the enumeration; not random users after
the fact.

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