From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with ESMTP id B60CC6B0078 for ; Sun, 8 Nov 2009 06:37:06 -0500 (EST) Date: Sun, 8 Nov 2009 12:36:54 +0100 From: Ingo Molnar Subject: Re: [PATCH 02/11] Add "handle page fault" PV helper. Message-ID: <20091108113654.GO11372@elte.hu> References: <1257076590-29559-1-git-send-email-gleb@redhat.com> <1257076590-29559-3-git-send-email-gleb@redhat.com> <20091102092214.GB8933@elte.hu> <20091102160410.GF27911@redhat.com> <20091102161248.GB15423@elte.hu> <20091102162234.GH27911@redhat.com> <20091102162941.GC14544@elte.hu> <20091102174208.GJ27911@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20091102174208.GJ27911@redhat.com> Sender: owner-linux-mm@kvack.org To: Gleb Natapov Cc: kvm@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "H. Peter Anvin" , Thomas Gleixner , Fr??d??ric Weisbecker List-ID: * Gleb Natapov wrote: > On Mon, Nov 02, 2009 at 05:29:41PM +0100, Ingo Molnar wrote: > > > > * Gleb Natapov wrote: > > > > > On Mon, Nov 02, 2009 at 05:12:48PM +0100, Ingo Molnar wrote: > > > > > > > > * Gleb Natapov wrote: > > > > > > > > > On Mon, Nov 02, 2009 at 10:22:14AM +0100, Ingo Molnar wrote: > > > > > > > > > > > > * Gleb Natapov wrote: > > > > > > > > > > > > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > > > > > > > index f4cee90..14707dc 100644 > > > > > > > --- a/arch/x86/mm/fault.c > > > > > > > +++ b/arch/x86/mm/fault.c > > > > > > > @@ -952,6 +952,9 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code) > > > > > > > int write; > > > > > > > int fault; > > > > > > > > > > > > > > + if (arch_handle_page_fault(regs, error_code)) > > > > > > > + return; > > > > > > > + > > > > > > > > > > > > This patch is not acceptable unless it's done cleaner. Currently we > > > > > > already have 3 callbacks in do_page_fault() (kmemcheck, mmiotrace, > > > > > > notifier), and this adds a fourth one. Please consolidate them into a > > > > > > single callback site, this is a hotpath on x86. > > > > > > > > > > > This call is patched out by paravirt patching mechanism so overhead > > > > > should be zero for non paravirt cases. [...] > > > > > > > > arch_handle_page_fault() isnt upstream yet - precisely what is the > > > > instruction sequence injected into do_page_fault() in the patched-out > > > > case? > > > > > > It is introduced by the same patch. The instruction inserted is: > > > xor %rax, %rax > > > > ok. > > > > My observations still stand: > > > > > > > [...] What do you want to achieve by consolidate them into single > > > > > callback? [...] > > > > > > > > Less bloat in a hotpath and a shared callback infrastructure. > > > > > > > > > [...] I mean the code will still exist and will have to be executed on > > > > > every #PF. Is the goal to move them out of line? > > > > > > > > The goal is to have a single callback site for all the users - which > > > > call-site is patched out ideally - on non-paravirt too if needed. Most > > > > of these callbacks/notifier-chains have are inactive most of the time. > > > > > > > > I.e. a very low overhead 'conditional callback' facility, and a single > > > > one - not just lots of them sprinkled around the code. > > > > looks like a golden opportunity to get this right. > > > Three existing callbacks are: kmemcheck, mmiotrace, notifier. Two > of them kmemcheck, mmiotrace are enabled only for debugging, should > not be performance concern. And notifier call sites (two of them) > are deliberately, as explained by comment, not at the function entry, > so can't be unified with others. (And kmemcheck also has two different > call site BTW) We want mmiotrace to be generic distro capable so the overhead when the hook is not used is of concern. Ingo -- 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