From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753014AbdDJV0T (ORCPT ); Mon, 10 Apr 2017 17:26:19 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:57032 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751585AbdDJV0S (ORCPT ); Mon, 10 Apr 2017 17:26:18 -0400 Date: Mon, 10 Apr 2017 14:26:16 -0700 From: Andrew Morton To: Mel Gorman Cc: brouer@redhat.com, willy@infradead.org, peterz@infradead.org, pagupta@redhat.com, ttoukan.linux@gmail.com, tariqt@mellanox.com, netdev@vger.kernel.org, saeedm@mellanox.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] mm, page_alloc: re-enable softirq use of per-cpu page allocator Message-Id: <20170410142616.6d37a11904dd153298cf7f3b@linux-foundation.org> In-Reply-To: <20170410150821.vcjlz7ntabtfsumm@techsingularity.net> References: <20170410150821.vcjlz7ntabtfsumm@techsingularity.net> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 10 Apr 2017 16:08:21 +0100 Mel Gorman wrote: > IRQ context were excluded from using the Per-Cpu-Pages (PCP) lists caching > of order-0 pages in commit 374ad05ab64d ("mm, page_alloc: only use per-cpu > allocator for irq-safe requests"). > > This unfortunately also included excluded SoftIRQ. This hurt the performance > for the use-case of refilling DMA RX rings in softirq context. Out of curiosity: by how much did it "hurt"? Tariq found: : I disabled the page-cache (recycle) mechanism to stress the page : allocator, and see a drastic degradation in BW, from 47.5 G in v4.10 to : 31.4 G in v4.11-rc1 (34% drop). then with this patch he found : It looks very good! I get line-rate (94Gbits/sec) with 8 streams, in : comparison to less than 55Gbits/sec before. Can I take this to mean that the page allocator's per-cpu-pages feature ended up doubling the performance of this driver? Better than the driver's private page recycling? I'd like to believe that, but am having trouble doing so ;) > This patch re-allow softirq context, which should be safe by disabling > BH/softirq, while accessing the list. PCP-lists access from both hard-IRQ > and NMI context must not be allowed. Peter Zijlstra says in_nmi() code > never access the page allocator, thus it should be sufficient to only test > for !in_irq(). > > One concern with this change is adding a BH (enable) scheduling point at > both PCP alloc and free. If further concerns are highlighted by this patch, > the result wiill be to revert 374ad05ab64d and try again at a later date > to offset the irq enable/disable overhead.