From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751759AbdHRJHs (ORCPT ); Fri, 18 Aug 2017 05:07:48 -0400 Received: from mx2.suse.de ([195.135.220.15]:34083 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750729AbdHRJHq (ORCPT ); Fri, 18 Aug 2017 05:07:46 -0400 Subject: Re: [PATCH RFC] x86: enable RCU based table free when PARAVIRT To: Vitaly Kuznetsov , x86@kernel.org Cc: linux-kernel@vger.kernel.org, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , "Kirill A. Shutemov" , Peter Zijlstra , Linus Torvalds , Jork Loeser , KY Srinivasan , Stephen Hemminger , Steven Rostedt , Boris Ostrovsky , Andrew Cooper , Andy Lutomirski , xen-devel@lists.xenproject.org References: <20170817092057.18920-1-vkuznets@redhat.com> From: Juergen Gross Message-ID: <841bbc41-37d5-d9dd-54b7-20de21cc7cb2@suse.com> Date: Fri, 18 Aug 2017 11:07:39 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170817092057.18920-1-vkuznets@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: de-DE Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 17/08/17 11:20, Vitaly Kuznetsov wrote: > On x86 software page-table walkers depend on the fact that remote TLB flush > does an IPI: walk is performed lockless but with interrupts disabled and in > case the pagetable is freed the freeing CPU will get blocked as remote TLB > flush is required. On other architecture which don't require an IPI to do > remote TLB flush we have an RCU-based mechanism (see > include/asm-generic/tlb.h for more details). > > In virtualized environments we may want to override .flush_tlb_others hook > in pv_mmu_ops and use a hypercall asking the hypervisor to do remote TLB > flush for us. This breaks the assumption about IPI. Xen PV does this for > years and the upcoming remote TLB flush for Hyper-V will do it too. This > is not safe, software pagetable walkers may step on an already freed page. > > Solve the issue by enabling RCU-based table free mechanism when PARAVIRT > is selected in config. Testing with kernbench doesn't show any notable > performance impact: > > 6-CPU host: > > Average Half load -j 3 Run (std deviation): > CURRENT HAVE_RCU_TABLE_FREE > ======= =================== > Elapsed Time 400.498 (0.179679) Elapsed Time 399.909 (0.162853) > User Time 1098.72 (0.278536) User Time 1097.59 (0.283894) > System Time 100.301 (0.201629) System Time 99.736 (0.196254) > Percent CPU 299 (0) Percent CPU 299 (0) > Context Switches 5774.1 (69.2121) Context Switches 5744.4 (79.4162) > Sleeps 87621.2 (78.1093) Sleeps 87586.1 (99.7079) > > Average Optimal load -j 24 Run (std deviation): > CURRENT HAVE_RCU_TABLE_FREE > ======= =================== > Elapsed Time 219.03 (0.652534) Elapsed Time 218.959 (0.598674) > User Time 1119.51 (21.3284) User Time 1118.81 (21.7793) > System Time 100.499 (0.389308) System Time 99.8335 (0.251423) > Percent CPU 432.5 (136.974) Percent CPU 432.45 (136.922) > Context Switches 81827.4 (78029.5) Context Switches 81818.5 (78051) > Sleeps 97124.8 (9822.4) Sleeps 97207.9 (9955.04) > > 6-CPU host: I guess this is wrong information ... > > Average Half load -j 8 Run (std deviation): > CURRENT HAVE_RCU_TABLE_FREE > ======= =================== > Elapsed Time 213.538 (3.7891) Elapsed Time 212.5 (3.10939) > User Time 1306.4 (1.83399) User Time 1307.65 (1.01364) > System Time 194.59 (0.864378) System Time 195.478 (0.794588) > Percent CPU 702.6 (13.5388) Percent CPU 707 (11.1131) > Context Switches 21189.2 (1199.4) Context Switches 21288.2 (552.388) > Sleeps 89390.2 (482.325) Sleeps 89677 (277.06) > > Average Optimal load -j 64 Run (std deviation): > CURRENT HAVE_RCU_TABLE_FREE > ======= =================== > Elapsed Time 137.866 (0.787928) Elapsed Time 138.438 (0.218792) > User Time 1488.92 (192.399) User Time 1489.92 (192.135) > System Time 234.981 (42.5806) System Time 236.09 (42.8138) > Percent CPU 1057.1 (373.826) Percent CPU 1057.1 (369.114) ... as I suspect more than 100% usage per cpu are rather unlikely. :-) > Context Switches 187514 (175324) Context Switches 187358 (175060) > Sleeps 112633 (24535.5) Sleeps 111743 (23297.6) > > Suggested-by: Peter Zijlstra > Signed-off-by: Vitaly Kuznetsov Acked-by: Juergen Gross Thanks, Juergen