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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3BE45C433EF for ; Thu, 11 Nov 2021 12:20:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1CB766128E for ; Thu, 11 Nov 2021 12:20:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232778AbhKKMX2 (ORCPT ); Thu, 11 Nov 2021 07:23:28 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:52192 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230358AbhKKMX0 (ORCPT ); Thu, 11 Nov 2021 07:23:26 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1636633237; 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=aoQBLYfwfih7osFlStGXH7gQBAYiwFFQHNuh+X/UmEw=; b=QVuC9dkVcwm9upZpWmD/dWBFtfjGwQyiDDc14oQF9U3IFpMzjMAT/LPlykgMP9zrmSbhM8 hmQ0PTQKq4PnNUw+WeTpAH1t1Vvi9tLJhj1kkjZsyqZZnbU2wbgbYkBDiyc5U66GxKdQto 3S2GROkk2zkC9IxGHqUoyA+ni5VFScY= Received: from mail-wm1-f70.google.com (mail-wm1-f70.google.com [209.85.128.70]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-51-fnZAZconPfqITL84HaVbBg-1; Thu, 11 Nov 2021 07:20:36 -0500 X-MC-Unique: fnZAZconPfqITL84HaVbBg-1 Received: by mail-wm1-f70.google.com with SMTP id 201-20020a1c04d2000000b003335bf8075fso1760505wme.0 for ; Thu, 11 Nov 2021 04:20:36 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:to:cc:references:from:organization:in-reply-to :content-transfer-encoding; bh=aoQBLYfwfih7osFlStGXH7gQBAYiwFFQHNuh+X/UmEw=; b=e3n+bapSnrb5yAwhHSOSfMD4nOvC52v37shzpdFWD32HAz8Zz6hWXjtphO9UYmI9Qv TOmPPOaSZRLq3prrMp+Hjq8saN3y/wDYlT+6Y+mZ5cypbwmqAq9coQy3lnwcJyIuEfnT IYkY9PqcLLLO8GALbyA3VHKUaSQ9nCrdim4XbSTAD/j92jY1Udy0gVabvHLOa8I5YjPH z1lMjuJD9BSPgYtJJY6lZnRBTxm6bmWH6EJii9yikJuK3FKZQED4ehrAysgF6us4N6xd H+4eVMK+ZLAempNP9OWoTijPrt92i0w21Ua720w7eDx+LepMSsIfhxpg/xaw1jThznXi SDcQ== X-Gm-Message-State: AOAM531exwvDG8kjfDtErtbiini2dLP2WfL43EJSckKpPE7hDBZ8CJRO CDRcbwh9d2VDhNmB0D1pSBRMd/08j9DgAYIAm32iqrKPNIgpk8OJ+ckE96vDBm5lwo9fjG2QbMm /sfKwKmNCKM9A6vFtJkin1V9N X-Received: by 2002:a5d:6091:: with SMTP id w17mr8195241wrt.65.1636633235122; Thu, 11 Nov 2021 04:20:35 -0800 (PST) X-Google-Smtp-Source: ABdhPJwDkfbLgbUXf179DhzC9TAAmFX7axgjafNXhQWqRtuPLGX+trncNPcR9VOyRq9W7S+kA3uCyw== X-Received: by 2002:a5d:6091:: with SMTP id w17mr8195205wrt.65.1636633234860; Thu, 11 Nov 2021 04:20:34 -0800 (PST) Received: from [192.168.3.132] (p4ff23ee8.dip0.t-ipconnect.de. [79.242.62.232]) by smtp.gmail.com with ESMTPSA id d8sm2868226wrm.76.2021.11.11.04.20.33 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Nov 2021 04:20:34 -0800 (PST) Message-ID: <1489f02f-d024-b9ec-2ab6-e6efc8a022f1@redhat.com> Date: Thu, 11 Nov 2021 13:20:33 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH v3 00/15] Free user PTE page table pages Content-Language: en-US To: Qi Zheng , Jason Gunthorpe Cc: akpm@linux-foundation.org, tglx@linutronix.de, kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, songmuchun@bytedance.com, zhouchengming@bytedance.com References: <20211110105428.32458-1-zhengqi.arch@bytedance.com> <20211110125601.GQ1740502@nvidia.com> <8d0bc258-58ba-52c5-2e0d-a588489f2572@redhat.com> <20211110143859.GS1740502@nvidia.com> <6ac9cc0d-7dea-0e19-51b3-625ec6561ac7@redhat.com> <20211110163925.GX1740502@nvidia.com> <7c97d86f-57f4-f764-3e92-1660690a0f24@redhat.com> <60515562-5f93-11cd-6c6a-c7cc92ff3bf8@bytedance.com> <9ee06b52-4844-7996-fa34-34fc7d4fdc10@bytedance.com> <27d73395-70b4-fe4a-4c8d-415b43ff9c1f@redhat.com> <2e19ad1b-15f3-7508-c5d5-6c31765f26d3@bytedance.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <2e19ad1b-15f3-7508-c5d5-6c31765f26d3@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11.11.21 13:00, Qi Zheng wrote: > > > On 11/11/21 7:19 PM, David Hildenbrand wrote: >> On 11.11.21 12:08, Qi Zheng wrote: >>> >>> >>> On 11/11/21 5:22 PM, David Hildenbrand wrote: >>>> On 11.11.21 04:58, Qi Zheng wrote: >>>>> >>>>> >>>>> On 11/11/21 1:37 AM, David Hildenbrand wrote: >>>>>>>> It would still be a fairly coarse-grained locking, I am not sure if that >>>>>>>> is a step into the right direction. If you want to modify *some* page >>>>>>>> table in your process you have exclude each and every page table walker. >>>>>>>> Or did I mis-interpret what you were saying? >>>>>>> >>>>>>> That is one possible design, it favours fast walking and penalizes >>>>>>> mutation. We could also stick a lock in the PMD (instead of a >>>>>>> refcount) and still logically be using a lock instead of a refcount >>>>>>> scheme. Remember modify here is "want to change a table pointer into a >>>>>>> leaf pointer" so it isn't an every day activity.. >>>>>> >>>>>> It will be if we somewhat frequent when reclaim an empty PTE page table >>>>>> as soon as it turns empty. This not only happens when zapping, but also >>>>>> during writeback/swapping. So while writing back / swapping you might be >>>>>> left with empty page tables to reclaim. >>>>>> >>>>>> Of course, this is the current approach. Another approach that doesn't >>>>>> require additional refcounts is scanning page tables for empty ones and >>>>>> reclaiming them. This scanning can either be triggered manually from >>>>>> user space or automatically from the kernel. >>>>> >>>>> Whether it is introducing a special rwsem or scanning an empty page >>>>> table, there are two problems as follows: >>>>> >>>>> #1. When to trigger the scanning or releasing? >>>> >>>> For example when reclaiming memory, when scanning page tables in >>>> khugepaged, or triggered by user space (note that this is the approach I >>>> originally looked into). But it certainly requires more locking thought >>>> to avoid stopping essentially any page table walker. >>>> >>>>> #2. Every time to release a 4K page table page, 512 page table >>>>> entries need to be scanned. >>>> >>>> It would happen only when actually trigger reclaim of page tables >>>> (again, someone has to trigger it), so it's barely an issue. >>>> >>>> For example, khugepaged already scans the page tables either way. >>>> >>>>> >>>>> For #1, if the scanning is triggered manually from user space, the >>>>> kernel is relatively passive, and the user does not fully know the best >>>>> timing to scan. If the scanning is triggered automatically from the >>>>> kernel, that is great. But the timing is not easy to confirm, is it >>>>> scanned and reclaimed every time zap or try_to_unmap? >>>>> >>>>> For #2, refcount has advantages. >>>>> >>>>>> >>>>>>> >>>>>>> There is some advantage with this thinking because it harmonizes well >>>>>>> with the other stuff that wants to convert tables into leafs, but has >>>>>>> to deal with complicated locking. >>>>>>> >>>>>>> On the other hand, refcounts are a degenerate kind of rwsem and only >>>>>>> help with freeing pages. It also puts more atomics in normal fast >>>>>>> paths since we are refcounting each PTE, not read locking the PMD. >>>>>>> >>>>>>> Perhaps the ideal thing would be to stick a rwsem in the PMD. read >>>>>>> means a table cannot be come a leaf. I don't know if there is space >>>>>>> for another atomic in the PMD level, and we'd have to use a hitching >>>>>>> post/hashed waitq scheme too since there surely isn't room for a waitq >>>>>>> too.. >>>>>>> >>>>>>> I wouldn't be so quick to say one is better than the other, but at >>>>>>> least let's have thought about a locking solution before merging >>>>>>> refcounts :) >>>>>> >>>>>> Yes, absolutely. I can see the beauty in the current approach, because >>>>>> it just reclaims "automatically" once possible -- page table empty and >>>>>> nobody is walking it. The downside is that it doesn't always make sense >>>>>> to reclaim an empty page table immediately once it turns empty. >>>>>> >>>>>> Also, it adds complexity for something that is only a problem in some >>>>>> corner cases -- sparse memory mappings, especially relevant for some >>>>>> memory allocators after freeing a lot of memory or running VMs with >>>>>> memory ballooning after inflating the balloon. Some of these use cases >>>>>> might be good with just triggering page table reclaim manually from user >>>>>> space. >>>>>> >>>>> >>>>> Yes, this is indeed a problem. Perhaps some flags can be introduced so >>>>> that the release of page table pages can be delayed in some cases. >>>>> Similar to the lazyfree mechanism in MADV_FREE? >>>> >>>> The issue AFAIU is that once your refcount hits 0 (no more references, >>>> no more entries), the longer you wait with reclaim, the longer others >>>> have to wait for populating a fresh page table because the "page table >>>> to be reclaimed" is still stuck around. You'd have to keep the refcount >>>> increased for a while, and only drop it after a while. But when? And >>>> how? IMHO it's not trivial, but maybe there is an easy way to achieve it. >>>> >>> >>> For running VMs with memory ballooning after inflating the balloon, is >>> this a hot behavior? Even if it is, it is already facing the release and >>> reallocation of physical pages. The overhead after introducing >>> pte_refcount is that we need to release and re-allocate page table page. >>> But 2MB physical pages only corresponds to 4KiB of PTE page table page. >>> So maybe the overhead is not big. >> >> The cases that come to my mind are >> >> a) Swapping on shared memory with concurrent access >> b) Reclaim on file-backed memory with concurrent access >> c) Free page reporting as implemented by virtio-balloon >> >> In all of these cases, you can have someone immediately re-access the >> page table and re-populate it. > > In the performance test shown on the cover, we repeatedly performed > touch and madvise(MADV_DONTNEED) actions, which simulated the case > you said above. > > We did find a small amount of performance regression, but I think it is > acceptable, and no new perf hotspots have been added. That test always accesses 2MiB and does it from a single thread. Things might (IMHO will) look different when only accessing individual pages and doing the access from one/multiple separate threads (that's what a),b) and c) essentially do, they don't do it in the pattern you measured. what you measured matches rather a typical memory allocator). -- Thanks, David / dhildenb