Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752637AbdFSWxY (ORCPT ); Mon, 19 Jun 2017 18:53:24 -0400 Received: from mail-pf0-f196.google.com ([209.85.192.196]:34295 "EHLO mail-pf0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752365AbdFSWxW (ORCPT ); Mon, 19 Jun 2017 18:53:22 -0400 Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 10.3 \(3273\)) Subject: Re: [PATCH v2 10/10] x86/mm: Try to preserve old TLB entries using PCID From: Nadav Amit In-Reply-To: Date: Mon, 19 Jun 2017 15:53:20 -0700 Cc: 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 Message-Id: <29636D5E-53D4-47B1-8F72-8DD0FAE58A60@gmail.com> References: <35264bd304c93f6d3cfff2329e3e01b084598ea1.1497415951.git.luto@kernel.org> <740B1D51-B801-48C9-A4C9-F31B34A09AEF@gmail.com> To: Andy Lutomirski X-Mailer: Apple Mail (2.3273) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id v5JMrSNN027044 Content-Length: 1716 Lines: 37 Andy Lutomirski wrote: > 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. Just to clarify: I asked since I don’t understand how the interaction with PCID-unaware CR3 users go. Specifically, IIUC, arch_efi_call_virt_teardown() can reload CR3 with an old PCID value. No?