Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753348AbbHCMsn (ORCPT ); Mon, 3 Aug 2015 08:48:43 -0400 Received: from casper.infradead.org ([85.118.1.10]:48315 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752403AbbHCMsl (ORCPT ); Mon, 3 Aug 2015 08:48:41 -0400 Date: Mon, 3 Aug 2015 14:48:34 +0200 From: Peter Zijlstra To: Chris Metcalf Cc: Frederic Weisbecker , LKML , Thomas Gleixner , Preeti U Murthy , Christoph Lameter , Ingo Molnar , Viresh Kumar , Rik van Riel Subject: Re: [PATCH 05/10] nohz: New tick dependency mask Message-ID: <20150803124834.GI19282@twins.programming.kicks-ass.net> References: <1437669735-8786-1-git-send-email-fweisbec@gmail.com> <1437669735-8786-6-git-send-email-fweisbec@gmail.com> <55B26E07.1020005@ezchip.com> <20150724171655.GA20925@lerouge> <55B2794A.8040707@ezchip.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <55B2794A.8040707@ezchip.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1759 Lines: 44 On Fri, Jul 24, 2015 at 01:43:38PM -0400, Chris Metcalf wrote: > On 07/24/2015 01:16 PM, Frederic Weisbecker wrote: > >On Fri, Jul 24, 2015 at 12:55:35PM -0400, Chris Metcalf wrote: > >>On 07/23/2015 12:42 PM, Frederic Weisbecker wrote: > >>>+unsigned long __tick_nohz_set_tick_dependency(enum tick_dependency_bit bit, > >>>+ unsigned long *dep) > >>>+{ > >>>+ unsigned long prev; > >>>+ unsigned long old = *dep; > >>>+ unsigned long mask = BIT_MASK(bit); > >>>+ > >>>+ while ((prev = cmpxchg(dep, old, old | mask)) != old) { > >>>+ old = prev; > >>>+ cpu_relax(); > >>>+ } > >>>+ > >>>+ return prev; > >>>+} > >>Why not use set_bit() here? It is suitably atomic. > >Because I don't want to send an IPI if the CPU already had bits set in > >the dependency. > > > >Ideally I need something like test_and_set_bit() but which returns the > >whole previous value and not just the previous value of the bit. > > Ah, of course. Peter, maybe we need atomic_or_return() as part > of your new atomic_or/_and/_xor series? Certainly on tilegx, and > likely other architectures, we can do better than Frederic's > cmpxchg() loop. No, atomic_or_return() would return the new value and is entirely pointless for the logic ops since they're not reversible (with the exception of xor). What you'd need is atomic_fetch_or(), but we don't have any fetch_$op primitives at all. Introducing them might make sense, but it'll have to be a separate series. Note that I have a fetch_or() macro in kernel/sched/core.c. -- 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/