From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f197.google.com (mail-pf0-f197.google.com [209.85.192.197]) by kanga.kvack.org (Postfix) with ESMTP id E4DC16B0263 for ; Wed, 13 Jul 2016 11:47:53 -0400 (EDT) Received: by mail-pf0-f197.google.com with SMTP id y134so47188481pfg.1 for ; Wed, 13 Jul 2016 08:47:53 -0700 (PDT) Received: from blackbird.sr71.net (www.sr71.net. [198.145.64.142]) by mx.google.com with ESMTP id dz9si4719317pab.5.2016.07.13.08.47.53 for ; Wed, 13 Jul 2016 08:47:53 -0700 (PDT) Subject: Re: [PATCH 2/4] x86, pagetable: ignore A/D bits in pte/pmd/pud_none() References: <20160708001909.FB2443E2@viggo.jf.intel.com> <20160708001912.5216F89C@viggo.jf.intel.com> <20160713152145.GC20693@dhcp22.suse.cz> From: Dave Hansen Message-ID: <578662A7.3040409@sr71.net> Date: Wed, 13 Jul 2016 08:47:51 -0700 MIME-Version: 1.0 In-Reply-To: <20160713152145.GC20693@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: linux-kernel@vger.kernel.org, x86@kernel.org, linux-mm@kvack.org, torvalds@linux-foundation.org, akpm@linux-foundation.org, bp@alien8.de, ak@linux.intel.com, dave.hansen@intel.com, dave.hansen@linux.intel.com On 07/13/2016 08:21 AM, Michal Hocko wrote: >> > This adds a tiny amount of overhead to all pte_none() checks. >> > I doubt we'll be able to measure it anywhere. > It would be better to introduce the overhead only for the affected > cpu models but I guess this is also acceptable. Would it be too > complicated to use alternatives for that? The patch as it stands ends up doing a one-instruction change in pte_none(). It goes from 64c8: 48 85 ff test %rdi,%rdi to 64a8: 48 f7 c7 9f ff ff ff test $0xffffffffffffff9f,%rdi So it essentially eats 4 bytes of icache more than it did before. But, it's the same number of instructions, and I can't imagine that the CPU will have any more trouble with a test against an immediate than a test against 0. We could theoretically do alternatives for this, but we would at *best* end up with 4 bytes of noops. So, unless the processor likes decoding 4 noops better than 4 bytes of immediate as part of an instruction, we'll not win anything. *Plus* the ugliness of the assembly that we'll need to have the compiler guarantee that the PTE ends up in %rdi. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org