Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752423AbdFSWC5 (ORCPT ); Mon, 19 Jun 2017 18:02:57 -0400 Received: from mail.kernel.org ([198.145.29.99]:39016 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750903AbdFSWC4 (ORCPT ); Mon, 19 Jun 2017 18:02:56 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 501B823A0B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=luto@kernel.org MIME-Version: 1.0 In-Reply-To: <740B1D51-B801-48C9-A4C9-F31B34A09AEF@gmail.com> References: <35264bd304c93f6d3cfff2329e3e01b084598ea1.1497415951.git.luto@kernel.org> <740B1D51-B801-48C9-A4C9-F31B34A09AEF@gmail.com> From: Andy Lutomirski Date: Mon, 19 Jun 2017 15:02:33 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2 10/10] x86/mm: Try to preserve old TLB entries using PCID To: Nadav Amit Cc: Andy Lutomirski , X86 ML , "linux-kernel@vger.kernel.org" , Borislav Petkov , Linus Torvalds , Andrew Morton , Mel Gorman , "linux-mm@kvack.org" , Rik van Riel , Dave Hansen , Arjan van de Ven , Peter Zijlstra Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1439 Lines: 31 On Sat, Jun 17, 2017 at 11:26 PM, Nadav Amit wrote: > >> On Jun 13, 2017, at 9:56 PM, Andy Lutomirski wrote: >> >> PCID is a "process context ID" -- it's what other architectures call >> an address space ID. Every non-global TLB entry is tagged with a >> PCID, only TLB entries that match the currently selected PCID are >> used, and we can switch PGDs without flushing the TLB. x86's >> PCID is 12 bits. >> >> This is an unorthodox approach to using PCID. x86's PCID is far too >> short to uniquely identify a process, and we can't even really >> uniquely identify a running process because there are monster >> systems with over 4096 CPUs. To make matters worse, past attempts >> to use all 12 PCID bits have resulted in slowdowns instead of >> speedups. >> >> This patch uses PCID differently. We use a PCID to identify a >> recently-used mm on a per-cpu basis. An mm has no fixed PCID >> binding at all; instead, we give it a fresh PCID each time it's >> loaded except in cases where we want to preserve the TLB, in which >> case we reuse a recent value. >> >> In particular, we use PCIDs 1-3 for recently-used mms and we reserve >> PCID 0 for swapper_pg_dir and for PCID-unaware CR3 users (e.g. EFI). >> Nothing ever switches to PCID 0 without flushing PCID 0 non-global >> pages, so PCID 0 conflicts won't cause problems. > > Is this commit message outdated? Yes, it's old. Will fix.