From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752914Ab1JJM5W (ORCPT ); Mon, 10 Oct 2011 08:57:22 -0400 Received: from casper.infradead.org ([85.118.1.10]:55309 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752993Ab1JJM5V (ORCPT ); Mon, 10 Oct 2011 08:57:21 -0400 Subject: Re: [PATCH 0/3][RFC] trace_printk() using percpu buffers From: Peter Zijlstra To: Steven Rostedt Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton , Thomas Gleixner , Frederic Weisbecker In-Reply-To: <1318249909.7904.70.camel@gandalf.stny.rr.com> References: <20111008170227.792806635@goodmis.org> <1318244697.14400.18.camel@laptop> <1318249909.7904.70.camel@gandalf.stny.rr.com> Content-Type: text/plain; charset="UTF-8" Date: Mon, 10 Oct 2011 15:03:19 +0200 Message-ID: <1318251799.14400.28.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.32.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2011-10-10 at 08:31 -0400, Steven Rostedt wrote: > > > By default, it still uses the single buffer protected by a spinlock > > > and an atomic (for NMIs). The NMI case can cause dropped prints if > > > the NMI happens while a trace_printk() is processing. > > > > Why bother keeping that? > > Because very few developers debug nmi's. printk is known not to work > there. > > I still find it useful to have without having to switch on a config > option or kernel command line. But its also a massive scalability fail. There's simply no sane reason to keep the shared buffer trace_printk() implementation. > > > When trace_printk_percpu is enabled, either via the trace options or > > > the kernel command line, then two sets of percpu buffers are made, > > > one for normal and irqs (interrupts are still disabled), and the other > > > is for NMIs. These can be added or removed at anytime. > > > > So why not allocate 4, one for {task, softirq, irq, NMI} resp, then all > > you need to do is disable preemption. > > > > depending on tracing/options/trace_printk ? > > Preemption still needs to be disabled. But if you think that's better > than disabling interrupts, I could do that too. I guess it doesn't really matter that much.. > > > The last patch adds a CONFIG_TRACE_PRINTK_PERCPU that makes trace_printk() > > > permanently use two sets of per_cpu buffers, and these can not be > > > removed. This will give the least amount of overhead for trace_printk() > > > with the sacrifice of memory overhead. This is an option I could imagine > > > you would just set and forget about. > > > > Is that one dereference really that expensive? > > It's also a compare and jump, but I added this option for you :) Ah, right :-) > That way, you could set this option and forget about it. Well, if all we have is the per-cpu option I'm fine either way.