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 079D9C07CB1 for ; Wed, 29 Nov 2023 20:39:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234051AbjK2Uiw (ORCPT ); Wed, 29 Nov 2023 15:38:52 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34708 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234010AbjK2Uiu (ORCPT ); Wed, 29 Nov 2023 15:38:50 -0500 Received: from mail-ua1-x932.google.com (mail-ua1-x932.google.com [IPv6:2607:f8b0:4864:20::932]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6FFDFD67 for ; Wed, 29 Nov 2023 12:38:56 -0800 (PST) Received: by mail-ua1-x932.google.com with SMTP id a1e0cc1a2514c-7c51c5bc44fso43068241.3 for ; Wed, 29 Nov 2023 12:38:56 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1701290335; x=1701895135; darn=vger.kernel.org; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=06AuxLZ/sNf65GGJNy2PvCvvtYPHOzAkgOG09b2VhAE=; b=PZe42By/RdS/NF++H3WTxzeJlaPpRPBd10wcxkIWFMvrLiRiJPIuJD41eZ6VNFPLap er8P31G7DhjEzZN2xU+IOtTnL9Dc9zo4z3LV3EoOVKLcE73llYhpMv8AtpyWhJ4UryBa cmbQoG6mg53qBzjv10n0eog0x0aN8+w9fBL2PC1FI1XgwmdhZW/5gmUHrMPIMe0OyRsX weuOAI87GXElpJAG9ZUam7w3mUzSDMIS1iqhNXYG/pC4bdCMTjD3I6vOGVaTK7NTyCxD ZEZ+jMtg2EaysAoIhKuzO/6/0GOII3UTUss6FFptNWwm28aoZbt1IMGDQFMInEjEQ689 6//A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1701290335; x=1701895135; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=06AuxLZ/sNf65GGJNy2PvCvvtYPHOzAkgOG09b2VhAE=; b=MKV2kcThuA9YkR+A8BhMffIeRvcItYFSTAVpgpLUYETDazS10XSHpUOfUIgRYHc1Ob WBaah7NesAX3MNN1cAONtI9J1T5UudT7LLyrg95KGb3MdxoeAiBAhNsgicmXwrVqkJaG rb/tLi3uB+g8Z8zENJBk5zQDC7+iMN3Wl1AG9o8UiielxUOOCT2Ob3DpH6f1pXVBnThq uRtVXrE/pq6aAKVcmlH6v2Z0nt5XUgFhpS2+rtZcz5NdifKZM/pEbj4zIPqAuz4d5h7/ weXmxFpCNUHd3DrPfQdkP5Phngbe2s4VAXWEFvKbSbZcTXBfRZOFB1HdfrtXqUz8tTOw 3Kgw== X-Gm-Message-State: AOJu0YyINLB9SN+/zdcnFvadA7S2NnC4oAqdKm6HIPa+icDF/BZwVpTh 5tn9heA+nIhAs3Q7YqH2HWCCpWYzk3rU4TffOhw= X-Google-Smtp-Source: AGHT+IEHVJGZHYun74LZbxEhoCO6kpuna43oPePL5nhtRxMZFhLhhWLWRaeJHOjUaYFutQhMxF00gbRdMAnr8IfvXmM= X-Received: by 2002:a67:b34d:0:b0:464:5109:a338 with SMTP id b13-20020a67b34d000000b004645109a338mr1562541vsm.35.1701290335350; Wed, 29 Nov 2023 12:38:55 -0800 (PST) MIME-Version: 1.0 References: <20231025144546.577640-1-ryan.roberts@arm.com> <20231129074741.15682-1-v-songbaohua@oppo.com> In-Reply-To: From: Barry Song <21cnbao@gmail.com> Date: Thu, 30 Nov 2023 09:38:43 +1300 Message-ID: Subject: Re: [PATCH v3 0/4] Swap-out small-sized THP without splitting To: Ryan Roberts Cc: akpm@linux-foundation.org, david@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.com, shy828301@gmail.com, wangkefeng.wang@huawei.com, willy@infradead.org, xiang@kernel.org, ying.huang@intel.com, yuzhao@google.com, hanchuanhua@oppo.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 30, 2023 at 1:06=E2=80=AFAM Ryan Roberts = wrote: > > On 29/11/2023 07:47, Barry Song wrote: > >> Hi All, > >> > >> This is v3 of a series to add support for swapping out small-sized THP= without > >> needing to first split the large folio via __split_huge_page(). It clo= sely > >> follows the approach already used by PMD-sized THP. > >> > >> "Small-sized THP" is an upcoming feature that enables performance impr= ovements > >> by allocating large folios for anonymous memory, where the large folio= size is > >> smaller than the traditional PMD-size. See [3]. > >> > >> In some circumstances I've observed a performance regression (see patc= h 2 for > >> details), and this series is an attempt to fix the regression in advan= ce of > >> merging small-sized THP support. > >> > >> I've done what I thought was the smallest change possible, and as a re= sult, this > >> approach is only employed when the swap is backed by a non-rotating bl= ock device > >> (just as PMD-sized THP is supported today). Discussion against the RFC= concluded > >> that this is probably sufficient. > >> > >> The series applies against mm-unstable (1a3c85fa684a) > >> > >> > >> Changes since v2 [2] > >> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > >> > >> - Reuse scan_swap_map_try_ssd_cluster() between order-0 and order > 0 > >> allocation. This required some refactoring to make everything work = nicely > >> (new patches 2 and 3). > >> - Fix bug where nr_swap_pages would say there are pages available but= the > >> scanner would not be able to allocate them because they were reserv= ed for the > >> per-cpu allocator. We now allow stealing of order-0 entries from th= e high > >> order per-cpu clusters (in addition to exisiting stealing from orde= r-0 > >> per-cpu clusters). > >> > >> Thanks to Huang, Ying for the review feedback and suggestions! > >> > >> > >> Changes since v1 [1] > >> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > >> > >> - patch 1: > >> - Use cluster_set_count() instead of cluster_set_count_flag() in > >> swap_alloc_cluster() since we no longer have any flag to set. I = was unable > >> to kill cluster_set_count_flag() as proposed against v1 as other= call > >> sites depend explicitly setting flags to 0. > >> - patch 2: > >> - Moved large_next[] array into percpu_cluster to make it per-cpu > >> (recommended by Huang, Ying). > >> - large_next[] array is dynamically allocated because PMD_ORDER is= not > >> compile-time constant for powerpc (fixes build error). > >> > >> > >> Thanks, > >> Ryan > > > >> P.S. I know we agreed this is not a prerequisite for merging small-siz= ed THP, > >> but given Huang Ying had provided some review feedback, I wanted to pr= ogress it. > >> All the actual prerequisites are either complete or being worked on by= others. > >> > > > > Hi Ryan, > > > > this is quite important to a phone and a must-have component, so is lar= ge-folio > > swapin, as i explained to you in another email. > > Yes understood; the "prerequisites" are just the things that must be merg= ed > *before* small-sized THP to ensure we don't regress existing behaviour or= to > ensure that small-size THP is correct/robust when enabled. Performance > improvements can be merged after the initial small-sized series. I completely agree. I didn't mean small-THP swap out as a whole should be a prerequisite for small-THP initial patchset, just describing how importan= t it is to a phone :-) And actually we have done much further than this on phones by optimizing zsmalloc/zram and allow a large folio compressed and decompressed as a whole, we have seen compressing/decompressing a whole large folio can significantly improve compression ratio and decrease CPU consumption. so that means large folios can not only save memory but also decrease CPU consumption. > > > Luckily, we are having Chuanhua Han(Cc-ed) to prepare a patchset of lar= gefolio > > swapin on top of your this patchset, probably a port and cleanup of our > > do_swap_page[1] againest yours. > > That's great to hear - welcome aboard, Chuanhua Han! Feel free to reach o= ut if > you have questions. > > I would guess that any large swap-in changes would be independent of this > swap-out patch though? Wouldn't you just be looking for contiguous swap e= ntries > in the page table to determine a suitable folio order, then swap-in each = of > those entries into the folio? And if they happen to have contiguous swap = offsets > (enabled by this swap-out series) then you potentially get a batched disk= access > benefit. I agree. Maybe we still need to check if the number of contiguous swap entr= ies is one of those supported large folio sizes? > > That's just a guess though, perhaps you can describe your proposed approa= ch? we have an ugly hack if we are swapping in from the dedicated zRAM for large folios, we assume we have a chance to swapin as a whole, but we do also handle corn= er cases in which some entries might have been zap_pte_range()-ed. My current proposal is as below, A1. we get the number of contiguous swap entries with PTL and find it is a valid large folio size A2. we allocate large folio without PTL A3. after getting PTL again, we re-check PTEs if the situation in A1 have been changed, if no other threads change those PTEs, we set_ptes and finish the swap-in but we have a chance to fail in A2, so in this case we still need to fall back to basepage. considering the MTE thread[1] I am handling, and MTE tag life cycle is the same with swap entry life cycle. it seems we will still need a page-level arch_swap_restore even after we support large folio swap-in for the below two reasons 1. contiguous PTEs might be partially dropped by madvise(DONTNEED) etc 2. we can still fall back to basepage for swap-in if we fail to get large folio even PTEs are all contiguous swap entries Of course, if we succeed in setting all PTEs for a large folio in A3, we can have a folio-level arch_swap_restore. To me, an universal folio-level arch_swap_restore seems not sensible to handle all kinds of complex cases. [1] [RFC V3 PATCH] arm64: mm: swap: save and restore mte tags for large fol= ios https://lore.kernel.org/linux-mm/20231114014313.67232-1-v-songbaohua@oppo.c= om/ > > > > > Another concern is that swapslots can be fragmented, if we place small/= large folios > > in a swap device, since large folios always require contiguous swapslot= , we can > > result in failure of getting slots even we still have many free slots w= hich are not > > contiguous. > > This series tries to mitigate that problem by reserving a swap cluster pe= r > order. That works well until we run out of swap clusters; a cluster can't= be > freed until all contained swap entries are swapped back in and deallocate= d. > > But I think we should start with the simple approach first, and only solv= e the > problems as they arise through real testing. I agree. > > To avoid this, [2] dynamic hugepage solution have two swap devices, > > one for basepage, the other one for CONTPTE. we have modified the prior= ity-based > > selection of swap devices to choose swap devices based on small/large f= olios. > > i realize this approache is super ugly and might be very hard to find a= way to > > upstream though, it seems not universal especially if you are a linux s= erver (-_-) > > > > two devices are not a nice approach though it works well for a real pro= duct, > > we might still need some decent way to address this problem while the p= roblem > > is for sure not a stopper of your patchset. > > I guess that approach works for your case because A) you only have 2 size= s, and > B) your swap device is zRAM, which dynamically allocate RAM as it needs i= t. > > The upstream small-sized THP solution can support multiple sizes, so you = would > need a swap device per size (I think 13 is the limit at the moment - PMD = size > for 64K base page). And if your swap device is a physical block device, y= ou > can't dynamically parition it the way you can with zRAM. Nether of those = things > scale particularly well IMHO. right. > > > > > [1] https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/blob/on= eplus/sm8550_u_14.0.0_oneplus11/mm/memory.c#L4648 > > [2] https://github.com/OnePlusOSS/android_kernel_oneplus_sm8550/blob/on= eplus/sm8550_u_14.0.0_oneplus11/mm/swapfile.c#L1129 > > > >> > >> [1] https://lore.kernel.org/linux-mm/20231010142111.3997780-1-ryan.rob= erts@arm.com/ > >> [2] https://lore.kernel.org/linux-mm/20231017161302.2518826-1-ryan.rob= erts@arm.com/ > >> [3] https://lore.kernel.org/linux-mm/15a52c3d-9584-449b-8228-1335e0753= b04@arm.com/ > >> > >> > >> Ryan Roberts (4): > >> mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags > >> mm: swap: Remove struct percpu_cluster > >> mm: swap: Simplify ssd behavior when scanner steals entry > >> mm: swap: Swap-out small-sized THP without splitting > >> > >> include/linux/swap.h | 31 +++--- > >> mm/huge_memory.c | 3 - > >> mm/swapfile.c | 232 ++++++++++++++++++++++++------------------= - > >> mm/vmscan.c | 10 +- > >> 4 files changed, 149 insertions(+), 127 deletions(-) > > Thanks Barry