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=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 BAE29C4361B for ; Thu, 17 Dec 2020 12:49:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5E6952389F for ; Thu, 17 Dec 2020 12:49:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727874AbgLQMtb (ORCPT ); Thu, 17 Dec 2020 07:49:31 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:39718 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725988AbgLQMt3 (ORCPT ); Thu, 17 Dec 2020 07:49:29 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1608209282; 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=8b4oiC4dSPxOwjeEBB7JqxHVb4sztDSV/987LEUNdbY=; b=M6428RSAfpXtD5HwT9VQsIyubOx30o2DMlSDJl17CrcrwA70z/+6+toJTEzblehj2pEBBw sjHWeycuIwkVSRPIttb9e2gTptTH+kKqgxXRDsdb07Xjyd14zngthSWf8itgd6+c6+Aohd tbYqGhmZz4m3s0v1QZ+GPJqpThohV48= 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-374-A0_NjNNcMIKKcm9terGWqQ-1; Thu, 17 Dec 2020 07:48:00 -0500 X-MC-Unique: A0_NjNNcMIKKcm9terGWqQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id CF6D1801AC1; Thu, 17 Dec 2020 12:47:59 +0000 (UTC) Received: from [10.36.113.93] (ovpn-113-93.ams2.redhat.com [10.36.113.93]) by smtp.corp.redhat.com (Postfix) with ESMTP id C755660C47; Thu, 17 Dec 2020 12:47:58 +0000 (UTC) Subject: Re: [PATCH 00/25] Page folios To: "Matthew Wilcox (Oracle)" , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org References: <20201216182335.27227-1-willy@infradead.org> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <9e764222-a274-0a99-5e41-7cfa9ea15b86@redhat.com> Date: Thu, 17 Dec 2020 13:47:57 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <20201216182335.27227-1-willy@infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16.12.20 19:23, Matthew Wilcox (Oracle) wrote: > One of the great things about compound pages is that when you try to > do various operations on a tail page, it redirects to the head page and > everything Just Works. One of the awful things is how much we pay for > that simplicity. Here's an example, end_page_writeback(): > > if (PageReclaim(page)) { > ClearPageReclaim(page); > rotate_reclaimable_page(page); > } > get_page(page); > if (!test_clear_page_writeback(page)) > BUG(); > > smp_mb__after_atomic(); > wake_up_page(page, PG_writeback); > put_page(page); > > That all looks very straightforward, but if you dive into the disassembly, > you see that there are four calls to compound_head() in this function > (PageReclaim(), ClearPageReclaim(), get_page() and put_page()). It's > all for nothing, because if anyone does call this routine with a tail > page, wake_up_page() will VM_BUG_ON_PGFLAGS(PageTail(page), page). > > I'm not really a CPU person, but I imagine there's some kind of dependency > here that sucks too: > > 1fd7: 48 8b 57 08 mov 0x8(%rdi),%rdx > 1fdb: 48 8d 42 ff lea -0x1(%rdx),%rax > 1fdf: 83 e2 01 and $0x1,%edx > 1fe2: 48 0f 44 c7 cmove %rdi,%rax > 1fe6: f0 80 60 02 fb lock andb $0xfb,0x2(%rax) > > Sure, it's going to be cache hot, but that cmove has to execute before > the lock andb. > > I would like to introduce a new concept that I call a Page Folio. > Or just struct folio to its friends. Here it is, > struct folio { > struct page page; > }; > > A folio is a struct page which is guaranteed not to be a tail page. > So it's either a head page or a base (order-0) page. That means > we don't have to call compound_head() on it and we save massively. > end_page_writeback() reduces from four calls to compound_head() to just > one (at the beginning of the function) and it shrinks from 213 bytes > to 126 bytes (using distro kernel config options). I think even that one > can be eliminated, but I'm going slowly at this point and taking the > safe route of transforming a random struct page pointer into a struct > folio pointer by calling page_folio(). By the end of this exercise, > end_page_writeback() will become end_folio_writeback(). > > This is going to be a ton of work, and massively disruptive. It'll touch > every filesystem, and a good few device drivers! But I think it's worth > it. Not every routine benefits as much as end_page_writeback(), but it > makes everything a little better. At 29 bytes per call to lock_page(), > unlock_page(), put_page() and get_page(), that's on the order of 60kB of > text for allyesconfig. More when you add on all the PageFoo() calls. > With the small amount of work I've done here, mm/filemap.o shrinks its > text segment by over a kilobyte from 33687 to 32318 bytes (and also 192 > bytes of data). Just wondering, as the primary motivation here is "minimizing CPU work", did you run any benchmarks that revealed a visible performance improvement? Otherwise, we're left with a concept that's hard to grasp first (folio - what?!) and "a ton of work, and massively disruptive", saving some kb of code - which does not sound too appealing to me. (I like the idea of abstracting which pages are actually worth looking at directly instead of going via a tail page - tail pages act somewhat like a proxy for the head page when accessing flags) -- Thanks, David / dhildenb