From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755534AbbDOAdN (ORCPT ); Tue, 14 Apr 2015 20:33:13 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:47424 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752630AbbDOAdG (ORCPT ); Tue, 14 Apr 2015 20:33:06 -0400 Message-ID: <552DB1BC.40103@linux.vnet.ibm.com> Date: Tue, 14 Apr 2015 19:33:00 -0500 From: "Suresh E. Warrier" User-Agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: Peter Zijlstra CC: lklml , Steven Rostedt , Frederic Weisbecker , Ingo Molnar , Paul Mackerras Subject: Re: [PATCH 2/2] More precise timestamps for nested writes References: <1428459449-10664-1-git-send-email-warrier@linux.vnet.ibm.com> <552C7B77.1000607@linux.vnet.ibm.com> <552C7C0D.1090201@linux.vnet.ibm.com> <552C7D89.2080802@linux.vnet.ibm.com> <20150414171324.GE17717@twins.programming.kicks-ass.net> In-Reply-To: <20150414171324.GE17717@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15041500-0037-0000-0000-0000010674E8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/14/2015 12:13 PM, Peter Zijlstra wrote: > On Mon, Apr 13, 2015 at 09:38:01PM -0500, Suresh E. Warrier wrote: >> +static u64 *get_write_timestamp(struct ring_buffer_per_cpu *cpu_buffer, >> + unsigned long *flags) >> +{ >> + if (rb_precise_nested_write_ts()) { >> + /* >> + * Ensure that we are not preempted until after we update >> + * the write timestamp. >> + */ >> + local_irq_save(*flags); >> + return &cpu_buffer->last_stamp; > > Yeah, ever hear about NMIs? This isn't going to work. That is a good point! If a NMI can come in and start running a handler that can generate a trace event, this code is indeed broken. Some architectures like PowerPC don't have NMIs like Intel and so I hadn't thought of that. Thanks for catching that! Let me update the patch to handle NMIs - trace events from NMI code cannot be made precise (the behavior will be the same as without the patch). -suresh