From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932469AbcHKPNX (ORCPT ); Thu, 11 Aug 2016 11:13:23 -0400 Received: from mga09.intel.com ([134.134.136.24]:7502 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932369AbcHKPNV (ORCPT ); Thu, 11 Aug 2016 11:13:21 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.28,505,1464678000"; d="scan'208";a="1012656520" From: "Huang\, Ying" To: Aaron Lu Cc: , , Alex Shi , Tomoki Sekiyama , Davidlohr Bueso , "Huang Ying" , Ingo Molnar , "Thomas Gleixner" , "\"H. Peter Anvin\"" Subject: Re: [PATCH] x86/irq: do not substract irq_tlb_count from irq_call_count References: <20160811074430.GA18163@aaronlu.sh.intel.com> Date: Thu, 11 Aug 2016 08:13:19 -0700 In-Reply-To: <20160811074430.GA18163@aaronlu.sh.intel.com> (Aaron Lu's message of "Thu, 11 Aug 2016 15:44:30 +0800") Message-ID: <87h9arz6cw.fsf@yhuang-mobile.sh.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Aaron Lu writes: > Since commit 52aec3308db8 ("x86/tlb: replace INVALIDATE_TLB_VECTOR by > CALL_FUNCTION_VECTOR"), the tlb remote shootdown is done through call > function vector. That commit didn't take care of irq_tlb_count so later > commit fd0f5869724f ("x86: Distinguish TLB shootdown interrupts from > other functions call interrupts") tried to fix it. > > The fix assumes every increase of irq_tlb_count has a corresponding > increase of irq_call_count. So the irq_call_count is always bigger than > irq_tlb_count and we could substract irq_tlb_count from irq_call_count. > > Unfortunately this is not true for the smp_call_function_single case. > The IPI is only sent if the target CPU's call_single_queue is empty when > adding a csd into it in generic_exec_single. That means if two threads > are both adding flush tlb csds to the same CPU's call_single_queue, only > one IPI is sent. In other words, the irq_call_count is incremented by 1 > but irq_tlb_count is incremented by 2. Over time, irq_tlb_count will be > bigger than irq_call_count and the substract will produce a very large > irq_call_count value due to overflow. > > Considering that: > 1 it's not worth to send more IPIs for the sake of accurate counting of > irq_call_count in generic_exec_single; > 2 it's not easy to tell if the call function interrupt is for TLB > shootdown in __smp_call_function_single_interrupt. > Not to exclude TLB shootdown from call function count seems to be the > simplest fix and this patch just did that. > > This is found by LKP's cyclic performance regression tracking recently > with the vm-scalability test suite. I have bisected to commit > 0a7ce4b5a632 ("mm/rmap: share the i_mmap_rwsem"). This commit didn't do > anything wrong but revealed the irq_call_count problem. IIUC, the commit > makes rwc->remap_one in rmap_walk_file concurrent with multiple threads. > When remap_one is try_to_unmap_one, then multiple threads could queue > flush tlb to the same CPU but only one IPI will be sent. > > Since the commit enter Linux v3.19, the counting problem only shows up > from v3.19. Considering this is a behaviour change, I'm not sure if I > should add the stable tag here. > > Signed-off-by: Aaron Lu Thanks for fix. You forget to add :) Reported-by: "Huang, Ying" Best Regards, Huang, Ying