Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765786AbZFOTAx (ORCPT ); Mon, 15 Jun 2009 15:00:53 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759727AbZFOTAp (ORCPT ); Mon, 15 Jun 2009 15:00:45 -0400 Received: from casper.infradead.org ([85.118.1.10]:33634 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751635AbZFOTAo (ORCPT ); Mon, 15 Jun 2009 15:00:44 -0400 Subject: Re: [tip:perfcounters/core] x86: Add NMI types for kmap_atomic From: Peter Zijlstra To: Ingo Molnar Cc: Hugh Dickins , linux-kernel@vger.kernel.org, mingo@redhat.com, hpa@zytor.com, paulus@samba.org, acme@redhat.com, efault@gmx.de, npiggin@suse.de, tglx@linutronix.de, linux-tip-commits@vger.kernel.org, Linus Torvalds , Andrew Morton In-Reply-To: <20090615185259.GK11248@elte.hu> References: <1245077165.6800.497.camel@laptop> <1245080486.6800.561.camel@laptop> <1245089065.13761.19316.camel@twins> <20090615181555.GA11248@elte.hu> <1245089943.13761.19334.camel@twins> <20090615182549.GD11248@elte.hu> <1245090608.13761.19349.camel@twins> <20090615184217.GG11248@elte.hu> <1245091674.6741.180.camel@laptop> <20090615185259.GK11248@elte.hu> Content-Type: text/plain Date: Mon, 15 Jun 2009 21:00:33 +0200 Message-Id: <1245092433.6741.201.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2194 Lines: 56 On Mon, 2009-06-15 at 20:52 +0200, Ingo Molnar wrote: > * Peter Zijlstra wrote: > > > On Mon, 2009-06-15 at 20:42 +0200, Ingo Molnar wrote: > > > * Peter Zijlstra wrote: > > > > > > > On Mon, 2009-06-15 at 20:25 +0200, Ingo Molnar wrote: > > > > > * Peter Zijlstra wrote: > > > > > > > > > but ... look at the APIs i propose above. We dont need _any_ > > > > > 'types'. > > > > > > > > > > That type enumeration is basically an open-coded allocator. If we do > > > > > a _real_ allocator (a balanced stack of atomic kmaps) we dont need > > > > > any of those indices, and all the potential for mismatch goes away > > > > > as well - a stack nests trivially with IRQ and NMI and arbitrary > > > > > other contexts. > > > > > > > > You want types because: > > > > - they encode the intent, and can be verified > > > > - they help keep track of the max nesting depth > > > > > > > > In the proposed implementation all type code basically falls away > > > > no ! CONFIG_DEBUG_VM, but is kept around for robustness. > > > > > > But much of the fragility of the types (and their clumsiness - for > > > example in highpte ops we have to know at which level of the > > > pagetables we are, and use the right kind of index) is _precisely_ > > > because we have the types ... > > > > How will you manage the max depth? > > if (++depth == MAX_DEPTH) { > print_all_entries_and_nasty_warning(); > /* hope we'll live long enough for the syslog to touch disk */ > depth = 0; > } That will only trigger if we hit it, which will be _very_ rare. > unbalanced kmap is a bad bug - the easier we make it to catch, the > better. The system wouldnt survive anyway. My proposed patch validates strict balance of types. But I can easily add the above as well. By removing the types it becomes very difficult to verify the max depth. I really don't like removing them. -- 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/