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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 254FFC4167B for ; Tue, 5 Dec 2023 11:31:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345011AbjLELbD (ORCPT ); Tue, 5 Dec 2023 06:31:03 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54886 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1442218AbjLELa4 (ORCPT ); Tue, 5 Dec 2023 06:30:56 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id A2D2D116 for ; Tue, 5 Dec 2023 03:31:02 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A8988139F; Tue, 5 Dec 2023 03:31:48 -0800 (PST) Received: from [10.57.73.130] (unknown [10.57.73.130]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 000CE3F766; Tue, 5 Dec 2023 03:30:57 -0800 (PST) Message-ID: Date: Tue, 5 Dec 2023 11:30:56 +0000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 01/15] mm: Batch-copy PTE ranges during fork() Content-Language: en-GB To: David Hildenbrand , Catalin Marinas , Will Deacon , Ard Biesheuvel , Marc Zyngier , Oliver Upton , James Morse , Suzuki K Poulose , Zenghui Yu , Andrey Ryabinin , Alexander Potapenko , Andrey Konovalov , Dmitry Vyukov , Vincenzo Frascino , Andrew Morton , Anshuman Khandual , Matthew Wilcox , Yu Zhao , Mark Rutland , Kefeng Wang , John Hubbard , Zi Yan , Barry Song <21cnbao@gmail.com>, Alistair Popple , Yang Shi Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20231204105440.61448-1-ryan.roberts@arm.com> <20231204105440.61448-2-ryan.roberts@arm.com> <104de2d6-ecf9-4b0c-a982-5bd8e1aea758@redhat.com> <5b8b9f8c-8e9b-42a5-b8b2-9b96903f3ada@redhat.com> From: Ryan Roberts In-Reply-To: <5b8b9f8c-8e9b-42a5-b8b2-9b96903f3ada@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/12/2023 17:27, David Hildenbrand wrote: >> >> With rmap batching from [1] -- rebased+changed on top of that -- we could turn >> that into an effective (untested): >> >>           if (page && folio_test_anon(folio)) { >> +               nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, end, >> +                                               pte, enforce_uffd_wp, &nr_dirty, >> +                                               &nr_writable); >>                   /* >>                    * If this page may have been pinned by the parent process, >>                    * copy the page immediately for the child so that we'll always >>                    * guarantee the pinned page won't be randomly replaced in the >>                    * future. >>                    */ >> -               folio_get(folio); >> -               if (unlikely(folio_try_dup_anon_rmap_pte(folio, page, >> src_vma))) { >> +               folio_ref_add(folio, nr); >> +               if (unlikely(folio_try_dup_anon_rmap_ptes(folio, page, nr, >> src_vma))) { >>                           /* Page may be pinned, we have to copy. */ >> -                       folio_put(folio); >> -                       return copy_present_page(dst_vma, src_vma, dst_pte, >> src_pte, >> -                                                addr, rss, prealloc, page); >> +                       folio_ref_sub(folio, nr); >> +                       ret = copy_present_page(dst_vma, src_vma, dst_pte, >> +                                               src_pte, addr, rss, prealloc, >> +                                               page); >> +                       return ret == 0 ? 1 : ret; >>                   } >> -               rss[MM_ANONPAGES]++; >> +               rss[MM_ANONPAGES] += nr; >>           } else if (page) { >> -               folio_get(folio); >> -               folio_dup_file_rmap_pte(folio, page); >> -               rss[mm_counter_file(page)]++; >> +               nr = folio_nr_pages_cont_mapped(folio, page, src_pte, addr, end, >> +                                               pte, enforce_uffd_wp, &nr_dirty, >> +                                               &nr_writable); >> +               folio_ref_add(folio, nr); >> +               folio_dup_file_rmap_ptes(folio, page, nr); >> +               rss[mm_counter_file(page)] += nr; >>           } >> >> >> We'll have to test performance, but it could be that we want to specialize >> more on !folio_test_large(). That code is very performance-sensitive. >> >> >> [1] https://lkml.kernel.org/r/20231204142146.91437-1-david@redhat.com > > So, on top of [1] without rmap batching but with a slightly modified version of Can you clarify what you mean by "without rmap batching"? I thought [1] implicitly adds rmap batching? (e.g. folio_dup_file_rmap_ptes(), which you've added in the code snippet above). > yours (that keeps the existing code structure as pointed out and e.g., updates > counter updates), running my fork() microbenchmark with a 1 GiB of memory: > > Compared to [1], with all order-0 pages it gets 13--14% _slower_ and with all > PTE-mapped THP (order-9) it gets ~29--30% _faster_. What test are you running - I'd like to reproduce if possible, since it sounds like I've got some work to do to remove the order-0 regression. > > So looks like we really want to have a completely seprate code path for > "!folio_test_large()" to keep that case as fast as possible. And "Likely" we > want to use "likely(!folio_test_large()". ;) Yuk, but fair enough. If I can repro the perf numbers, I'll have a go a reworking this. I think you're also implicitly suggesting that this change needs to depend on [1]? Which is a shame... I guess I should also go through a similar exercise for patch 2 in this series. > > Performing rmap batching on top of that code only slightly (another 1% or so) > improves performance in the PTE-mapped THP (order-9) case right now, in contrast > to other rmap batching. Reason is as all rmap code gets inlined here and we're > only doing subpage mapcount updates + PAE handling. >