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Date: Tue, 3 Feb 2009 20:32:36 -0600
From: Nathan Lynch <ntl@pobox.com>
To: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Corey Ashford <cjashfor@linux.vnet.ibm.com>,
       Andi Kleen <andi@firstfloor.org>, Ingo Molnar <mingo@elte.hu>,
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       "David S. Miller" <davem@davemloft.net>,
       Maynard Johnson <mpjohn@us.ibm.com>, carll@us.ibm.com
Subject: Re: Performance counter API review was [patch] Performance Counters
 for Linux, v3
Message-ID: <20090204023236.GH6913@localdomain>
References: <20081211155230.GA4230@elte.hu>
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Paul Mackerras wrote:
>
> By the way, how does userspace get to know about cpus being added or
> removed?  Is there a better way than continually reading
> /sys/devices/system/cpu/online?

The kernel generates uevents for cpu online and offline operations;
you can see them with udevmonitor.  Not sure how you get those events
to an arbitrary application, though.  Alternatively you can set an
inotify watch on e.g. /sys/devices/system/cpu/cpu1/online (at least it
seems to work here).