From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f169.google.com (mail-pf1-f169.google.com [209.85.210.169]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CA785628 for ; Fri, 13 Jan 2023 03:42:57 +0000 (UTC) Received: by mail-pf1-f169.google.com with SMTP id s3so12758042pfd.12 for ; Thu, 12 Jan 2023 19:42:57 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=4Xtd0yvioh58Ye+tRzyHugBT38/BBqw05Bz+SnnNowI=; b=fWNz1tk/5zycuFu/grWRS1t90X+SaGc2Wh3baOpYcrkSHE1YU41eznUDNBRKboV1km md12F8z0ly6VJR6bF9Gpt9EskoXK7vFBGa7uD05jIt2yv5sR7SixQ4+oVDZksLlM1H3h HCUOCS0+tmOCuULORoj/DUjpbt/5oT8evxJxM7/FErxdCHBJxFrOCctn4h+fe1eBNruo DLPj3UOZGWKAHtZcEm2wgLa0Xg/OAXrKTU+I8P39bt+UtBfKAdg3kQpgE3YkrRZquTij 9RtwDUy5Kir0hlyko8SRJZZZ7re0qBB8zXJrsCd4oA8aD9RuMjLmH0xdIIU2IN9QkyIq mVcA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=4Xtd0yvioh58Ye+tRzyHugBT38/BBqw05Bz+SnnNowI=; b=NfFPBh1WXio965R+mecczYu/aW8oOVl96OMP76jdhCdGwcdsL26BM+TPp786mhP02U /IvarMPmNu5xSXz1wkOR8i0UDaHT3kQbt9EIooYcZIX6n6abtA3v4qkkbg7F0vad0wEp OacqoVNeD3q3pI+TY/z4sTetVEtnFIqXvbXK2AVA5bIWFNYGlmXHMhkDf30P3kVqPY9Y aLszrHLMW8UjAU3/4Scwnd5VBBpF4QcA4dMaU4MIx0C0QLDYYxCmhPCjXjV9Uz0M1vfp k7Eu20g+nNTLme8SHb/U8rFSBvct8EQ3GL7QVw2x2EXLoc6bqr4Wcy+xfqcV/97euvx7 2YAw== X-Gm-Message-State: AFqh2krHKTgJ78Sig4gpJOoXSZcvj8jTrNAVpyeGfXAP/QYigIYHnrV9 BM0qim+WVCmeTtQ8b66H1bs+vQ== X-Google-Smtp-Source: AMrXdXsfHWFCsnl0TlcNERzD11sqDYuTKKUnd2zMn16BJ2CGKBjt2rA+cp26y/YEl+NjxFfBekXwbg== X-Received: by 2002:a05:6a00:a87:b0:582:13b5:d735 with SMTP id b7-20020a056a000a8700b0058213b5d735mr1168802pfl.0.1673581376966; Thu, 12 Jan 2023 19:42:56 -0800 (PST) Received: from google.com (220.181.82.34.bc.googleusercontent.com. [34.82.181.220]) by smtp.gmail.com with ESMTPSA id u14-20020a63ef0e000000b0046feca0883fsm10777204pgh.64.2023.01.12.19.42.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 12 Jan 2023 19:42:56 -0800 (PST) Date: Thu, 12 Jan 2023 19:42:52 -0800 From: Ricardo Koller To: Oliver Upton Cc: pbonzini@redhat.com, maz@kernel.org, dmatlack@google.com, qperret@google.com, catalin.marinas@arm.com, andrew.jones@linux.dev, seanjc@google.com, alexandru.elisei@arm.com, suzuki.poulose@arm.com, eric.auger@redhat.com, gshan@redhat.com, reijiw@google.com, rananta@google.com, bgardon@google.com, kvmarm@lists.linux.dev, ricarkol@gmail.com, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org Subject: Re: [RFC PATCH 00/12] KVM: arm64: Eager huge-page splitting for dirty-logging Message-ID: References: <20221112081714.2169495-1-ricarkol@google.com> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Nov 14, 2022 at 06:42:36PM +0000, Oliver Upton wrote: > Hi Ricardo, > > On Sat, Nov 12, 2022 at 08:17:02AM +0000, Ricardo Koller wrote: > > Hi, > > > > I'm sending this RFC mainly to get some early feedback on the approach used > > for implementing "Eager Page Splitting" on ARM. "Eager Page Splitting" > > improves the performance of dirty-logging (used in live migrations) when > > guest memory is backed by huge-pages. It's an optimization used in Google > > Cloud since 2016 on x86, and for the last couple of months on ARM. > > > > I tried multiple ways of implementing this optimization on ARM: from > > completely reusing the stage2 mapper, to implementing a new walker from > > scratch, and some versions in between. This RFC is one of those in > > between. They all have similar performance benefits, based on some light > > performance testing (mainly dirty_log_perf_test). > > > > Background and motivation > > ========================= > > Dirty logging is typically used for live-migration iterative copying. KVM > > implements dirty-logging at the PAGE_SIZE granularity (will refer to 4K > > pages from now on). It does it by faulting on write-protected 4K pages. > > Therefore, enabling dirty-logging on a huge-page requires breaking it into > > 4K pages in the first place. KVM does this breaking on fault, and because > > it's in the critical path it only maps the 4K page that faulted; every > > other 4K page is left unmapped. This is not great for performance on ARM > > for a couple of reasons: > > > > - Splitting on fault can halt vcpus for milliseconds in some > > implementations. Splitting a block PTE requires using a broadcasted TLB > > invalidation (TLBI) for every huge-page (due to the break-before-make > > requirement). Note that x86 doesn't need this. We observed some > > implementations that take millliseconds to complete broadcasted TLBIs > > when done in parallel from multiple vcpus. And that's exactly what > > happens when doing it on fault: multiple vcpus fault at the same time > > triggering TLBIs in parallel. > > > > - Read intensive guest workloads end up paying for dirty-logging. Only > > mapping the faulting 4K page means that all the other pages that were > > part of the huge-page will now be unmapped. The effect is that any > > access, including reads, now has to fault. > > > > Eager Page Splitting (on ARM) > > ============================= > > Eager Page Splitting fixes the above two issues by eagerly splitting > > huge-pages when enabling dirty logging. The goal is to avoid doing it while > > faulting on write-protected pages. This is what the TDP MMU does for x86 > > [0], except that x86 does it for different reasons: to avoid grabbing the > > MMU lock on fault. Note that taking care of write-protection faults still > > requires grabbing the MMU lock on ARM, but not on x86 (with the > > fast_page_fault path). > > > > An additional benefit of eagerly splitting huge-pages is that it can be > > done in a controlled way (e.g., via an IOCTL). This series provides two > > knobs for doing it, just like its x86 counterpart: when enabling dirty > > logging, and when using the KVM_CLEAR_DIRTY_LOG ioctl. The benefit of doing > > it on KVM_CLEAR_DIRTY_LOG is that this ioctl takes ranges, and not complete > > memslots like when enabling dirty logging. This means that the cost of > > splitting (mainly broadcasted TLBIs) can be throttled: split a range, wait > > for a bit, split another range, etc. The benefits of this approach were > > presented by Oliver Upton at KVM Forum 2022 [1]. > > > > Implementation > > ============== > > Patches 1-4 add a pgtable utility function for splitting huge block PTEs: > > kvm_pgtable_stage2_split(). Patches 5-6 add support for not doing > > break-before-make on huge-page breaking when FEAT_BBM level 2 is supported. > > I would suggest you split up FEAT_BBM=2 and eager page splitting into > two separate series, if possible. IMO, the eager page split is easier to > reason about if it follows the existing pattern of break-before-make. Dropping these changes in v1. > > -- > Thanks, > Oliver