Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752555AbaFJQMd (ORCPT ); Tue, 10 Jun 2014 12:12:33 -0400 Received: from cdptpa-outbound-snat.email.rr.com ([107.14.166.226]:7777 "EHLO cdptpa-oedge-vip.email.rr.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751771AbaFJQMc (ORCPT ); Tue, 10 Jun 2014 12:12:32 -0400 Date: Tue, 10 Jun 2014 12:12:30 -0400 From: Steven Rostedt To: Namhyung Kim Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton Subject: Re: [for-next][PATCH 3/6] tracing: Only calculate stats of tracepoint benchmarks for 2^32 times Message-ID: <20140610121230.368a62a9@gandalf.local.home> In-Reply-To: <20140610094206.51ab5c02@gandalf.local.home> References: <20140606163034.516780857@goodmis.org> <20140606172104.552734596@goodmis.org> <87sindbawt.fsf@sejong.aot.lge.com> <20140610094206.51ab5c02@gandalf.local.home> X-Mailer: Claws Mail 3.9.3 (GTK+ 2.24.23; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-RR-Connecting-IP: 107.14.168.142:25 X-Cloudmark-Score: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 10 Jun 2014 09:42:06 -0400 Steven Rostedt wrote: > On Tue, 10 Jun 2014 14:17:54 +0900 > Namhyung Kim wrote: > > > Hi Steve, > > > > On Fri, 06 Jun 2014 12:30:37 -0400, Steven Rostedt wrote: > > > From: "Steven Rostedt (Red Hat)" > > > > > > When calculating the average and standard deviation, it is required that > > > the count be less than UINT_MAX, otherwise the do_div() will get > > > undefined results. After 2^32 counts of data, the average and standard > > > deviation should pretty much be set anyway. > > > > [SNIP] > > > if (bm_cnt > 1) { > > > /* > > > * Apply Welford's method to calculate standard deviation: > > > * s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2) > > > */ > > > stddev = (u64)bm_cnt * bm_totalsq - bm_total * bm_total; > > > - do_div(stddev, bm_cnt); > > > - do_div(stddev, bm_cnt - 1); > > > + do_div(stddev, (u32)bm_cnt); > > > + do_div(stddev, (u32)bm_cnt - 1); > > > } else > > > stddev = 0; > > > > I also think that this if-else can go as it checks bm_cnt == 1 above. > > > > I noticed this too, but just to keep gcc from complaining, I left it in. > Oh, and you are talking about the entire if () else block. Yeah, I thought about that as well, but I'm paranoid about doing a divide by zero ;-) It's code for testing purposes only. A fringe of the kernel base. Not a hot path. Slowness isn't an issue here, but correctness is. Thanks, -- Steve -- 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/