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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7E596CD4F4A for ; Mon, 18 May 2026 14:09:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:MIME-Version:References:In-Reply-To:Message-ID:Date:Subject:Cc: To:From:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=dBvZjwLe5zdrCJH73EVqsmX0tJp3K8ogpghIuJyZiG0=; b=D9PoLVHgMZwfMNYx7AVLmYPhpw VOrKVUjJ3tEh8/4YaaSOdPwVUHC0g37iHjhJjQmYJmPTE49BUTAtoqJLgR9EWn/lZR/iTCSOxr/En 78wCuT0cPdLHq21Nn4LDWvSyuWp7Fr73D4RK4l18LZo9nMCMT9YvGFyHGe3vFJ/0yhQCWEI9nqW5I e+xgtoB9FjvZ2P9HfwRTCTk6BWpq4eb08yWdiv6ngjbHCGoUBoEDCyAtT1UcIOJkjiV/hkMvWoVB9 7IJ02XteSWE5nwLziC/ZKsk1ZMXJ9zYKQv75OBt7Ze5l3KSJsVEKx12p9UkyKljG/dLMaBNqQFk0S KK0DK5tw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wOyfB-0000000FvOh-3xVY; Mon, 18 May 2026 14:09:45 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wOyf9-0000000FvMy-0PMS for linux-arm-kernel@lists.infradead.org; Mon, 18 May 2026 14:09:44 +0000 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 AC9641D6F; Mon, 18 May 2026 07:09:35 -0700 (PDT) Received: from devkitleo.cambridge.arm.com (devkitleo.cambridge.arm.com [10.1.196.90]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BF0FB3F85F; Mon, 18 May 2026 07:09:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1779113380; bh=l/Qlu3Hv8DU45+5xpV6c4Ys2pdhcs0h9f9qT0mq7CAA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=fF0Rbl+3+beUy/Z/uEmudgn3t5s4SroQO42yF9HRXheF6oLVt9/UdhuMxyo5YJYem 061Y/QZdP51yOHhLMIL9Lmv6zQ4EaLf5wxtJaWug/AmesDsc8mD/rQEkGe+uQIVYsO V2VQnzwfK/BX+w+nZ/E1KWe2awZTmPh4Q84ZHVEA= From: Leonardo Bras To: Marc Zyngier Cc: Leonardo Bras , Oliver Upton , Joey Gouly , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Fuad Tabba , Raghavendra Rao Ananta , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 0/2] Optimize S2 page splitting Date: Mon, 18 May 2026 15:09:36 +0100 Message-ID: X-Mailer: git-send-email 2.54.0 In-Reply-To: <87o6ifaf5z.wl-maz@kernel.org> References: <20260515195904.2466381-1-leo.bras@arm.com> <87o6ifaf5z.wl-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260518_070943_203086_21C2F94A X-CRM114-Status: GOOD ( 25.87 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Sat, May 16, 2026 at 10:15:36AM +0100, Marc Zyngier wrote: > On Fri, 15 May 2026 20:59:01 +0100, > Leonardo Bras wrote: > > > > While playing with dirty-bit tracking, I decided to take a look on how page > > splitting works. Found out all entries are walked, even though we can infer, > > for instance that: > > - If a level-3 entry is walked, it means the parent level-2 entry is split > > - If a split just succeeded in an table entry, it means all children nodes > > are already split > > > > So I tried to optimize it in a way that it does not break other users. > > > > My main idea is to introduce positive return values that hint to the > > pagetable walking mechanism that either siblings or children can be > > skipped. That should be contained to the visitor function, that returns > > zero if no error was detected. > > > > Numbers on above optimization are promising: > > A 1GB VM, running on the model, splitting all at the beginning > > (no manual protect): > > - Memory was already split (4k pages): -97.33% runtime (-172ms) - 20 runs > > - THP backed memory: -19.82% runtime (-153ms) - 10 runs > > - 1x1GB hugetlb memory: -20.65% runtime (-150ms) - 10 runs > > > > I haven't looked at the changes in details, but the methodology is > quite flawed. For a start, measuring anything on a software model > (QEMU or FVP) doesn't mean anything performance-wise. The trade-offs > are completely different from a HW implementation, and even the notion > of time is pretty inconsistent. > > Please run this on actual HW. I'm sure your employer can give you > access to one of these mythical arm64 toys. Ok, will use real hardware next. > Measure things from > userspace, not from the kernel, so that you have all the overheads. > Don't add console output, because that will make things far worse. > > I'm sure you can hack one of the selftests for this purpose. I think the dirty_log_perf_test should have an config I can use for that without introducing any change. Thanks! Leo