Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754771Ab0LPTtp (ORCPT ); Thu, 16 Dec 2010 14:49:45 -0500 Received: from mx1.redhat.com ([209.132.183.28]:26547 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753134Ab0LPTtn (ORCPT ); Thu, 16 Dec 2010 14:49:43 -0500 Date: Thu, 16 Dec 2010 14:48:53 -0500 From: Jason Baron To: Peter Zijlstra , athieu.desnoyers@polymtl.ca Cc: hpa@zytor.com, rostedt@goodmis.org, mingo@elte.hu, 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 Subject: Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1) Message-ID: <20101216194852.GD2856@redhat.com> References: <1292526602.2708.57.camel@laptop> <20101216192303.GB2856@redhat.com> <1292528031.2708.77.camel@laptop> <20101216193635.GC2856@redhat.com> <1292528501.2708.80.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1292528501.2708.80.camel@laptop> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1839 Lines: 49 On Thu, Dec 16, 2010 at 08:41:41PM +0100, Peter Zijlstra wrote: > 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 i did. > for everything except some really weird archs (you really shouldn't care > about). right, I wasn't sure how much those mattered. > > static inline int atomic_read(const atomic_t *v) > { > return (*(volatile int *)&(v)->counter); > } > > The volatile simply forces a load to be emitted. Mathieu, what do you think? Are you ok with an atomic_read() for checking if a tracepoint is enabled, when jump labels are disabled? thanks, -Jason -- 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/