From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9DE16C433DB for ; Thu, 25 Feb 2021 15:18:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 628CF64E83 for ; Thu, 25 Feb 2021 15:18:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231942AbhBYPSS (ORCPT ); Thu, 25 Feb 2021 10:18:18 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:56071 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231320AbhBYPSM (ORCPT ); Thu, 25 Feb 2021 10:18:12 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1614266204; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ozeQTDLaZ00GZ1fdAE68ODxlqYJtYZh4rnRKYlI0+3s=; b=aGa2ftkWsbuTrk/80dGoc2wiTDCu4V9/ueiXG6MyBIicoZ2OWuAaQ7Wolnu7PdM1PMGtYJ QbC053f9wiXvt4qJiHKoR8IT8dg3WJFarks7uQMajJwxyGMC4RB9Tz02qvtk1M8wuP7orr o4YbHA2HI8wL77pVKx3h7og2dWsDakU= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-179-kD9_EiBkOqyof0Bqi5sK5Q-1; Thu, 25 Feb 2021 10:16:40 -0500 X-MC-Unique: kD9_EiBkOqyof0Bqi5sK5Q-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3352E84E243; Thu, 25 Feb 2021 15:16:39 +0000 (UTC) Received: from carbon (unknown [10.36.110.51]) by smtp.corp.redhat.com (Postfix) with ESMTP id 937945D9C2; Thu, 25 Feb 2021 15:16:34 +0000 (UTC) Date: Thu, 25 Feb 2021 16:16:33 +0100 From: Jesper Dangaard Brouer To: Mel Gorman Cc: linux-mm@kvack.org, chuck.lever@oracle.com, netdev@vger.kernel.org, linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, brouer@redhat.com Subject: Re: [PATCH RFC net-next 3/3] mm: make zone->free_area[order] access faster Message-ID: <20210225161633.53e5f910@carbon> In-Reply-To: <20210225112849.GM3697@techsingularity.net> References: <161419296941.2718959.12575257358107256094.stgit@firesoul> <161419301128.2718959.4838557038019199822.stgit@firesoul> <20210225112849.GM3697@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 25 Feb 2021 11:28:49 +0000 Mel Gorman wrote: > As a side-node, I didn't pick up the other patches as there is review > feedback and I didn't have strong opinions either way. Patch 3 is curious > though, it probably should be split out and sent separetly but still; > > On Wed, Feb 24, 2021 at 07:56:51PM +0100, Jesper Dangaard Brouer wrote: > > Avoid multiplication (imul) operations when accessing: > > zone->free_area[order].nr_free > > > > This was really tricky to find. I was puzzled why perf reported that > > rmqueue_bulk was using 44% of the time in an imul operation: > > > > ??? del_page_from_free_list(): > > 44,54 ??? e2: imul $0x58,%rax,%rax > > > > This operation was generated (by compiler) because the struct free_area have > > size 88 bytes or 0x58 hex. The compiler cannot find a shift operation to use > > and instead choose to use a more expensive imul, to find the offset into the > > array free_area[]. > > > > The patch align struct free_area to a cache-line, which cause the > > compiler avoid the imul operation. The imul operation is very fast on > > modern Intel CPUs. To help fast-path that decrement 'nr_free' move the > > member 'nr_free' to be first element, which saves one 'add' operation. > > > > Looking up instruction latency this exchange a 3-cycle imul with a > > 1-cycle shl, saving 2-cycles. It does trade some space to do this. > > > > Used: gcc (GCC) 9.3.1 20200408 (Red Hat 9.3.1-2) > > > > I'm having some trouble parsing this and matching it to the patch itself. > > First off, on my system (x86-64), the size of struct free area is 72, > not 88 bytes. For either size, cache-aligning the structure is a big > increase in the struct size. Yes, the increase in size is big. For the struct free_area 40 bytes for my case and 56 bytes for your case. The real problem is that this is multiplied by 11 (MAX_ORDER) and multiplied by number of zone structs (is it 5?). Thus, 56*11*5 = 3080 bytes. Thus, I'm not sure it is worth it! As I'm only saving 2-cycles, for something that depends on the compiler generating specific code. And the compiler can easily change, and "fix" this on-its-own in a later release, and then we are just wasting memory. I did notice this imul happens 45 times in mm/page_alloc.o, with this offset 0x58, but still this is likely not on hot-path. > struct free_area { > struct list_head free_list[4]; /* 0 64 */ > /* --- cacheline 1 boundary (64 bytes) --- */ > long unsigned int nr_free; /* 64 8 */ > > /* size: 72, cachelines: 2, members: 2 */ > /* last cacheline: 8 bytes */ > }; > > Are there other patches in the tree? What does pahole say? The size of size of struct free_area varies based on some CONFIG setting, as free_list[] array size is determined by MIGRATE_TYPES, which on my system is 5, and not 4 as on your system. struct list_head free_list[MIGRATE_TYPES]; CONFIG_CMA and CONFIG_MEMORY_ISOLATION both increase MIGRATE_TYPES with one. Thus, the array size can vary from 4 to 6. > With gcc-9, I'm also not seeing the imul instruction outputted like you > described in rmqueue_pcplist which inlines rmqueue_bulk. At the point > where it calls get_page_from_free_area, it's using shl for the page list > operation. This might be a compiler glitch but given that free_area is a > different size, I'm less certain and wonder if something else is going on. I think it is the size variation. > Finally, moving nr_free to the end and cache aligning it will make the > started of each free_list cache-aligned because of its location in the > struct zone so what purpose does __pad_to_align_free_list serve? The purpose of purpose of __pad_to_align_free_list is because struct list_head is 16 bytes, thus I wanted to align free_list to 16, given we already have wasted the space. Notice I added some more detailed notes in[1]: [1] https://github.com/xdp-project/xdp-project/blob/master/areas/mem/page_pool06_alloc_pages_bulk.org#micro-optimisations -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer