Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752076Ab0LPTmU (ORCPT ); Thu, 16 Dec 2010 14:42:20 -0500 Received: from casper.infradead.org ([85.118.1.10]:59453 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751066Ab0LPTmT convert rfc822-to-8bit (ORCPT ); Thu, 16 Dec 2010 14:42:19 -0500 Subject: Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1) From: Peter Zijlstra To: Jason Baron Cc: hpa@zytor.com, rostedt@goodmis.org, mingo@elte.hu, mathieu.desnoyers@polymtl.ca, tglx@linutronix.de, andi@firstfloor.org, roland@redhat.com, rth@redhat.com, masami.hiramatsu.pt@hitachi.com, fweisbec@gmail.com, avi@redhat.com, davem@davemloft.net, sam@ravnborg.org, ddaney@caviumnetworks.com, michael@ellerman.id.au, linux-kernel@vger.kernel.org In-Reply-To: <20101216193635.GC2856@redhat.com> References: <1292526602.2708.57.camel@laptop> <20101216192303.GB2856@redhat.com> <1292528031.2708.77.camel@laptop> <20101216193635.GC2856@redhat.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Thu, 16 Dec 2010 20:41:41 +0100 Message-ID: <1292528501.2708.80.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1507 Lines: 35 On Thu, 2010-12-16 at 14:36 -0500, Jason Baron wrote: > On Thu, Dec 16, 2010 at 08:33:51PM +0100, Peter Zijlstra wrote: > > On Thu, 2010-12-16 at 14:23 -0500, Jason Baron wrote: > > > > > > For the jump label disabled case, perf is using atomic_inc/dec and atomic_read > > > to check if enabled. While other consumers (tracepoints) are just using an > > > 'int'. I didn't want hurt the jump label disabled case for tracepoints. > > > If we can agree to use atomic ops for tracepoints, or drop atomics from > > > perf, that would simplify things. > > > > I had a quick look at the tracepoint stuff but got lost, but surely it > > has a reference count somewhere as well, it needs to know when the last > > probe goes away.. or does it check if the list is empty? > > > > Anyway, tracepoint enable/disable isn't a real fast-path, surely it > > could suffer an atomic op? > > It is the atomic_read() at the tracepoint site that I am concerned > about. Look at the implementation :-), its just wrapper foo, its a regular read for everything except some really weird archs (you really shouldn't care about). static inline int atomic_read(const atomic_t *v) { return (*(volatile int *)&(v)->counter); } The volatile simply forces a load to be emitted. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/