From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail115-79.sinamail.sina.com.cn (mail115-79.sinamail.sina.com.cn [218.30.115.79]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E5F4F1EE01B for ; Thu, 20 Feb 2025 11:59:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=218.30.115.79 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740052755; cv=none; b=bamuiNzNmq28aNqc6ji068bM4pYjGWOLWnBt+6O6B40Xby2tNpQoWMKLRpoqast13x4cGVtrtZGz3LiByru8zDLvnc3q3dad5+FHRosm0iAb22mT0dJKqj4nfnmEo0uGPJHIi93w36wcIxBBJYiqSZ6y8xKLOBKJ+PSpGY/earg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740052755; c=relaxed/simple; bh=VnBygHI1QJwOIYQ/g2J+eJivC5SOxDBSn4PU3NWh0V0=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=DRGpG8VtsGWwXGg3PzpM+6m3XDa7fBiYAvcINSo8MyqunzWCoYU7FGKsnlURCO/rUBxHUZNYTIqokSS+6MpvtkUpWBNDvci/fiWSyNOg8+VJwwGt83riXdWdWp3oV4ReU3+x2+4UsinQIJGrcMU109feBfV3p9QFPgBiDwOwb3U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=sina.com; spf=pass smtp.mailfrom=sina.com; arc=none smtp.client-ip=218.30.115.79 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=sina.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=sina.com X-SMAIL-HELO: localhost.localdomain Received: from unknown (HELO localhost.localdomain)([113.88.51.172]) by sina.com (10.185.250.22) with ESMTP id 67B716C700007EBD; Thu, 20 Feb 2025 19:49:31 +0800 (CST) X-Sender: hdanton@sina.com X-Auth-ID: hdanton@sina.com Authentication-Results: sina.com; spf=none smtp.mailfrom=hdanton@sina.com; dkim=none header.i=none; dmarc=none action=none header.from=hdanton@sina.com X-SMAIL-MID: 5901547602708 X-SMAIL-UIID: EDD8DF5FFF434C329343F466FEF693FA-20250220-194931-1 From: Hillf Danton To: Byungchul Park Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel_team@skhynix.com Subject: Re: [RFC PATCH v12 00/26] LUF(Lazy Unmap Flush) reducing tlb numbers over 90% Date: Thu, 20 Feb 2025 19:49:19 +0800 Message-ID: <20250220114920.2383-1-hdanton@sina.com> In-Reply-To: <20250220110935.GA64704@system.software.com> References: <20250220052027.58847-1-byungchul@sk.com> <20250220103223.2360-1-hdanton@sina.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Thu, 20 Feb 2025 20:09:35 +0900 Byungchul Park wrote: > On Thu, Feb 20, 2025 at 06:32:22PM +0800, Hillf Danton wrote: > > On Thu, 20 Feb 2025 14:20:01 +0900 Byungchul Park > > > To check luf's stability, I ran a heavy LLM inference workload consuming > > > 210GiB over 7 days on a machine with 140GiB memory, and decided it's > > > stable enough. > > > > > > I'm posting the latest version so that anyone can try luf mechanism if > > > wanted by any chance. However, I tagged RFC again because there are > > > still issues that should be resolved to merge to mainline: > > > > > > 1. Even though system wide total cpu time for TLB shootdown is > > > reduced over 95%, page allocation paths should take additional cpu > > > time shifted from page reclaim to perform TLB shootdown. > > > > > > 2. We need luf debug feature to detect when luf goes wrong by any > > > chance. I implemented just a draft version that checks the sanity > > > on mkwrite(), kmap(), and so on. I need to gather better ideas > > > to improve the debug feature. > > > > > > --- > > > > > > Hi everyone, > > > > > > While I'm working with a tiered memory system e.g. CXL memory, I have > > > been facing migration overhead esp. tlb shootdown on promotion or > > > demotion between different tiers. Yeah.. most tlb shootdowns on > > > migration through hinting fault can be avoided thanks to Huang Ying's > > > work, commit 4d4b6d66db ("mm,unmap: avoid flushing tlb in batch if PTE > > > is inaccessible"). > > > > > > However, it's only for migration through hinting fault. I thought it'd > > > be much better if we have a general mechanism to reduce all the tlb > > > numbers that we can apply to any unmap code, that we normally believe > > > tlb flush should be followed. > > > > > > I'm suggesting a new mechanism, LUF(Lazy Unmap Flush), that defers tlb > > > flush until folios that have been unmapped and freed, eventually get > > > allocated again. It's safe for folios that had been mapped read-only > > > and were unmapped, as long as the contents of the folios don't change > > > while staying in pcp or buddy so we can still read the data through the > > > stale tlb entries. > > > > > Given pcp or buddy, you are opening window for use after free which makes > > no sense in 99% cases. > > Just in case that I don't understand what you meant and for better > understanding, can you provide a simple and problematic example from > the u-a-f? > Tell us if it is illegal to commit rape without pregnancy in your home town? PS defering flushing tlb [1,2] is no go. Subject: Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ_FULL CPUs [1] https://lore.kernel.org/lkml/20250127155146.GB25757@willie-the-truck/ [2] https://lore.kernel.org/lkml/xhsmhwmdwihte.mognet@vschneid-thinkpadt14sgen2i.remote.csb/