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 9222EC433EF for ; Thu, 11 Nov 2021 12:32:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 696F1610D2 for ; Thu, 11 Nov 2021 12:32:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233316AbhKKMfj (ORCPT ); Thu, 11 Nov 2021 07:35:39 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57786 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233274AbhKKMfi (ORCPT ); Thu, 11 Nov 2021 07:35:38 -0500 Received: from mail-pf1-x436.google.com (mail-pf1-x436.google.com [IPv6:2607:f8b0:4864:20::436]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 637AEC061766 for ; Thu, 11 Nov 2021 04:32:49 -0800 (PST) Received: by mail-pf1-x436.google.com with SMTP id z6so5489202pfe.7 for ; Thu, 11 Nov 2021 04:32:49 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bytedance-com.20210112.gappssmtp.com; s=20210112; h=message-id:date:mime-version:user-agent:subject:to:cc:references :from:in-reply-to:content-transfer-encoding; bh=N9eWrGMaTS7mIY9Ow3VHOpPVUK2E0L1tukxlTfZow/I=; b=0DCmcveGGlFkJ6nj1QVG7Pj/jIAVjn7kPov9weG3UvqT8F0hvEstw+o0v12mpnlitN n8scT97+kJmkNEdr/1/PG6GcF1illpYNK8gGUe3KIJSlXid/dAfIjlzjizeqRLrU76fx JJDPQJumxFsIAXPN8jjlFER29TRiwr2YUrrXCEX7egZUdjm5l/iI3vPZCTpRkmTfGaIa 7AhYSvPMi4BQ4NOV1LiU0rjZ9Qq1CavRL0l18hI6fIOp27/KcuCv1mTJLMnw0iuaGPPQ XnH1ROgwnLtx8bcbkasc/TeLPH9jUEqevAuIuK9z6mar7eYpn10wFLVlHXMH4eeoNhSb cGrg== 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 :to:cc:references:from:in-reply-to:content-transfer-encoding; bh=N9eWrGMaTS7mIY9Ow3VHOpPVUK2E0L1tukxlTfZow/I=; b=V8/Cg4/oCyWS56B7ROGfvc1tk086+TgYsYiBYSAYljm0/oTgmtkzlhGrEJbREYOUIU KOeYaxxinaSPH9dvKlIOaSKtp4zUeEOhYtpGasj1KpBNGZlDAhBTZef+k80zMxs9y00t LcS6uqc469gAuQIWbHTswYci/SvNKGpVYAHHL+Sor990a1W0Lgbjso7aNTx8fu/P+WwI pJG2bD+I0Dx2FY1lIgq+ZwpFeDTRuljNelE78KgWNPngWdUHpqH67/8zoj0kMx3CsIYP av27ST4W5+AcEDZrAVejvcDGIMfrtjKyaMt5CCIHF76X3zJrWySa2f/d/2xPpRBVRU6l ngQw== X-Gm-Message-State: AOAM5312c1FzEg/MTua5dcUFq9+ESQ3B0HEgg8+9CRmodlNGGQbIXb7P Bw/ZCCDpKndsVt/VfrJd/P0Y/dNyra+bNQ== X-Google-Smtp-Source: ABdhPJyiZWEFxYBGLynrCfVHBE7mGR89TDDZMx2f07TTTneXkPh/suRRqCYTj58MFolNEg3awLnFTA== X-Received: by 2002:a05:6a00:1a8d:b0:49f:de63:d9c0 with SMTP id e13-20020a056a001a8d00b0049fde63d9c0mr6089092pfv.79.1636633968816; Thu, 11 Nov 2021 04:32:48 -0800 (PST) Received: from [10.254.173.217] ([139.177.225.248]) by smtp.gmail.com with ESMTPSA id j17sm3327330pfj.55.2021.11.11.04.32.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 11 Nov 2021 04:32:48 -0800 (PST) Message-ID: <791ddf94-5ad1-b431-85a1-db9a07579057@bytedance.com> Date: Thu, 11 Nov 2021 20:32:39 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Thunderbird/91.3.0 Subject: Re: [PATCH v3 00/15] Free user PTE page table pages To: David Hildenbrand , 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> <1489f02f-d024-b9ec-2ab6-e6efc8a022f1@redhat.com> From: Qi Zheng In-Reply-To: <1489f02f-d024-b9ec-2ab6-e6efc8a022f1@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/11/21 8:20 PM, David Hildenbrand wrote: > 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 No, it includes multi-threading: while (1) { char *c; char *start = mmap_area[cpu]; char *end = mmap_area[cpu] + FAULT_LENGTH; pthread_barrier_wait(&barrier); //printf("fault into %p-%p\n",start, end); for (c = start; c < end; c += PAGE_SIZE) *c = 0; pthread_barrier_wait(&barrier); for (i = 0; cpu==0 && i < num; i++) madvise(mmap_area[i], FAULT_LENGTH, MADV_DONTNEED); pthread_barrier_wait(&barrier); } Thread on cpu0 will use madvise(MADV_DONTNEED) to release the physical memory of threads on other cpu. > 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). > >