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

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Message-ID: <20180406090502.GI4129@hirez.programming.kicks-ass.net>
Date:   Fri, 6 Apr 2018 11:05:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dou Liyang <douly.fnst@...fujitsu.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Li RongQing <lirongqing@...du.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] genirq: only scan the present CPUs

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