Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756755AbZFIQCV (ORCPT ); Tue, 9 Jun 2009 12:02:21 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754776AbZFIQCN (ORCPT ); Tue, 9 Jun 2009 12:02:13 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:36106 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753437AbZFIQCM (ORCPT ); Tue, 9 Jun 2009 12:02:12 -0400 Date: Tue, 9 Jun 2009 09:00:08 -0700 (PDT) From: Linus Torvalds X-X-Sender: torvalds@localhost.localdomain To: Nick Piggin cc: Rusty Russell , Ingo Molnar , Jeremy Fitzhardinge , "H. Peter Anvin" , Thomas Gleixner , Linux Kernel Mailing List , Andrew Morton , Peter Zijlstra , Avi Kivity , Arjan van de Ven Subject: Re: [benchmark] 1% performance overhead of paravirt_ops on native kernels In-Reply-To: <20090609153847.GB9211@wotan.suse.de> Message-ID: References: <4A0B62F7.5030802@goop.org> <200906032208.28061.rusty@rustcorp.com.au> <200906041554.37102.rusty@rustcorp.com.au> <20090609093918.GC16940@wotan.suse.de> <20090609153847.GB9211@wotan.suse.de> User-Agent: Alpine 2.01 (LFD 1184 2008-12-16) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1359 Lines: 32 On Tue, 9 Jun 2009, Nick Piggin wrote: > > If it's such a problem, it could be made a lot faster without too > much problem. You could just introduce a FIFO of ptes behind it > and flush them all in one go. 4K worth of ptes per CPU might > hopefully bring your overhead down to < 1%. We already have that. The regular kmap() does that. It's just not usable in atomic context. We'd need to fix the locking: right now kmap_high() uses non-irq-safe locks, and it does that whole cross-cpu flushing thing (which is why those locks _have_ to be non-irq-safe. The way to fix that, though, would be to never do any cross-cpu calls, and instead just have a cpumask saying "you need to flush before you do anything with kmap". So you'd just set that cpumask inside the lock, and if/when some other CPU does a kmap, they'd flush their local TLB at _that_ point instead of having to have an IPI call. If we can get rid of kmap_atomic(), I'd already like HIGHMEM more. Right now I absolutely _hate_ all the different "levels" of kmap_atomic() and having to be careful about crazy nesting rules etc. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/