From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f171.google.com (mail-pd0-f171.google.com [209.85.192.171]) by kanga.kvack.org (Postfix) with ESMTP id EEFCF6B00DA for ; Tue, 6 Jan 2015 12:02:33 -0500 (EST) Received: by mail-pd0-f171.google.com with SMTP id y13so30792522pdi.2 for ; Tue, 06 Jan 2015 09:02:33 -0800 (PST) Received: from smtp2.provo.novell.com (smtp2.provo.novell.com. [137.65.250.81]) by mx.google.com with ESMTPS id xr7si90050286pab.168.2015.01.06.09.02.30 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Tue, 06 Jan 2015 09:02:31 -0800 (PST) Message-ID: <1420563737.24290.7.camel@stgolabs.net> Subject: Re: [PATCH 1/2] mm/slub: optimize alloc/free fastpath by removing preemption on/off From: Davidlohr Bueso Date: Tue, 06 Jan 2015 09:02:17 -0800 In-Reply-To: <20150106080948.GA18346@js1304-P5Q-DELUXE> References: <1420421765-3209-1-git-send-email-iamjoonsoo.kim@lge.com> <1420513392.24290.2.camel@stgolabs.net> <20150106080948.GA18346@js1304-P5Q-DELUXE> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Joonsoo Kim Cc: Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jesper Dangaard Brouer , rostedt@goodmis.org, Thomas Gleixner On Tue, 2015-01-06 at 17:09 +0900, Joonsoo Kim wrote: > On Mon, Jan 05, 2015 at 07:03:12PM -0800, Davidlohr Bueso wrote: > > On Mon, 2015-01-05 at 10:36 +0900, Joonsoo Kim wrote: > > > - preempt_disable(); > > > - c = this_cpu_ptr(s->cpu_slab); > > > + do { > > > + tid = this_cpu_read(s->cpu_slab->tid); > > > + c = this_cpu_ptr(s->cpu_slab); > > > + } while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid)); > > > + barrier(); > > > > I don't see the compiler reodering the object/page stores below, since c > > is updated in the loop anyway. Is this really necessary (same goes for > > slab_free)? The generated code by gcc 4.8 looks correct without it. > > Additionally, the implied barriers for preemption control aren't really > > the same semantics used here (if that is actually the reason why you are > > using them). > > Hello, > > I'd like to use tid as a pivot so it should be fetched before fetching > anything on c. Is it impossible even if !CONFIG_PREEMPT without > barrier()? You'd need a smp_wmb() in between tid and c in the loop then, which looks quite unpleasant. All in all disabling preemption isn't really that expensive, and you should redo your performance number if you go this way. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org