Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754135AbdFWIml (ORCPT ); Fri, 23 Jun 2017 04:42:41 -0400 Received: from mail.skyhub.de ([5.9.137.197]:37354 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754090AbdFWImj (ORCPT ); Fri, 23 Jun 2017 04:42:39 -0400 Date: Fri, 23 Jun 2017 10:42:19 +0200 From: Borislav Petkov To: Andy Lutomirski Cc: X86 ML , "linux-kernel@vger.kernel.org" , Linus Torvalds , Andrew Morton , Mel Gorman , "linux-mm@kvack.org" , Nadav Amit , Rik van Riel , Dave Hansen , Arjan van de Ven , Peter Zijlstra Subject: Re: [PATCH v3 05/11] x86/mm: Track the TLB's tlb_gen and update the flushing algorithm Message-ID: <20170623084219.k4lrorgtlshej7ri@pd.tnic> References: <91f24a6145b2077f992902891f8fa59abe5c8696.1498022414.git.luto@kernel.org> <20170621184424.eixb2jdyy66xq4hg@pd.tnic> <20170622072449.4rc4bnvucn7usuak@pd.tnic> <20170622145914.tzqdulshlssiywj4@pd.tnic> <20170622172220.wf3egiwx2kqbxbi2@pd.tnic> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1901 Lines: 51 On Thu, Jun 22, 2017 at 11:08:38AM -0700, Andy Lutomirski wrote: > Yes, I agree it's confusing. There really are three numbers. Those > numbers are: the latest generation, the generation that this CPU has > caught up to, and the generation that the requester of the flush we're > currently handling has asked us to catch up to. I don't see a way to > reduce the complexity. Yeah, can you pls put that clarification what what is, over it. It explains it nicely what the check is supposed to do. > >> The flush IPI hits after a switch_mm_irqs_off() call notices the > >> change from 1 to 2. switch_mm_irqs_off() will do a full flush and > >> increment the local tlb_gen to 2, and the IPI handler for the partial > >> flush will see local_tlb_gen == mm_tlb_gen - 1 (because local_tlb_gen > >> == 2 and mm_tlb_gen == 3) and do a partial flush. > > > > Why, the 2->3 flush has f->end == TLB_FLUSH_ALL. > > > > That's why you have this thing in addition to the tlb_gen. > > Yes. The idea is that we only do remote partial flushes when it's > 100% obvious that it's safe. So why wouldn't my simplified suggestion work then? if (f->end != TLB_FLUSH_ALL && mm_tlb_gen == local_tlb_gen + 1) 1->2 is a partial flush - gets promoted to a full one 2->3 is a full flush - it will get executed as one due to the f->end setting to TLB_FLUSH_ALL. > It could be converted to two full flushes or to just one, I think, > depending on what order everything happens in. Right. One flush at the right time would be optimal. > But this approach of using three separate tlb_gen values seems to > cover all the bases, and I don't think it's *that* bad. Sure. As I said in IRC, let's document that complexity then so that when we stumble over it in the future, we at least know why it was done this way. -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.