linux-kernel - Re: Performance counter API review was [patch] Performance Counters for Linux, v3

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Message-ID: <20090204023236.GH6913@localdomain>
Date:	Tue, 3 Feb 2009 20:32:36 -0600
From:	Nathan Lynch <ntl@...ox.com>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephane Eranian <eranian@...glemail.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Robert Richter <robert.richter@....com>,
	Arjan van de Veen <arjan@...radead.org>,
	Peter Anvin <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	Maynard Johnson <mpjohn@...ibm.com>, carll@...ibm.com
Subject: Re: Performance counter API review was [patch] Performance Counters
 for Linux, v3

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).
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