From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B565CC4724C for ; Thu, 30 Apr 2020 16:45:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 924BF2073E for ; Thu, 30 Apr 2020 16:45:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588265121; bh=8Ei3F4eOoRqpaa7fs/fC4JcNYYhRrZ7GhNIqqYVS8hA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=lFw6Hag6e2OWoKHR1vJpsxrwtBz0Dq4ZSAyVrGwcTb7P7NL5vKpYO07ETzUjH8D1s lSmyG8VCurCMlSB2bmc2ke9uiX/hdpYFWhHMM89tqpCww0fjrZpegqIkYHSy1Y7wBn 6HP1uGbznY7cuhGWEz+CkC5b4y3mh8tudNN6Rtbw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726455AbgD3QpU (ORCPT ); Thu, 30 Apr 2020 12:45:20 -0400 Received: from mail.kernel.org ([198.145.29.99]:43292 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726318AbgD3QpU (ORCPT ); Thu, 30 Apr 2020 12:45:20 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 4C4092070B; Thu, 30 Apr 2020 16:45:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588265120; bh=8Ei3F4eOoRqpaa7fs/fC4JcNYYhRrZ7GhNIqqYVS8hA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=liXRB4Ez5/7mwCxGidfN1vCb/UYI28M67EfS64G5GcjqtN5CEcAZ6uS0PKnMRBhO/ cwiQXgViU+P+1Cfosg7/pAqtWIBbj2VOueaq9p33lZ6bORmHC4o7GR4xeOQraeH/bo 1/Zgh5ToR19V2SwKj18fE+/mTf6+oKw5CcQggp64= Date: Thu, 30 Apr 2020 17:45:14 +0100 From: Will Deacon To: Peter Zijlstra Cc: Leo Yan , Mark Rutland , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Catalin Marinas , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Mathieu Poirier , Mike Leach , Al Grant , James Clark , maz@kernel.org, tglx@linutronix.de Subject: Re: [PATCH] arm64: perf_event: Fix time_offset for arch timer Message-ID: <20200430164513.GG25258@willie-the-truck> References: <20200320093545.28227-1-leo.yan@linaro.org> <20200430145823.GA25258@willie-the-truck> <20200430162750.GD13575@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200430162750.GD13575@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 30, 2020 at 06:27:50PM +0200, Peter Zijlstra wrote: > On Thu, Apr 30, 2020 at 03:58:24PM +0100, Will Deacon wrote: > > On Fri, Mar 20, 2020 at 05:35:45PM +0800, Leo Yan wrote: > > > + /* > > > + * Since arch timer is enabled ealier than sched clock registration, > > > + * compuate the delta (in nanosecond unit) between the arch timer > > > + * counter and sched clock, assign the delta to time_offset and > > > + * perf tool can use it for timestamp calculation. > > > + * > > > + * The formula for conversion arch timer cycle to ns is: > > > + * quot = (cyc >> time_shift); > > > + * rem = cyc & ((1 << time_shift) - 1); > > > + * ns = quot * time_mult + ((rem * time_mult) >> time_shift); > > > + */ > > > + count = arch_timer_read_counter(); > > > + quot = count >> shift; > > > + rem = count & ((1 << shift) - 1); > > > + ns = quot * userpg->time_mult + ((rem * userpg->time_mult) >> shift); > > > + userpg->time_offset = now - ns; > > > > Hmm, reading the counter and calculating the delta feels horribly > > approximate to me. It would be much better if we could get hold of the > > initial epoch cycles from the point at which sched_clock was initialised > > using the counter. This represents the true cycle delta between the counter > > and what sched_clock uses for 0 ns. > > > > Unfortunately, I can't see a straightforward way to grab that information. > > It looks like x86 pulls this directly from the TSC driver. > > Yeah, and I'm thinking you should do the same. IIRC ARM uses this > kernel/time/sched_clock.c thing, and if I read that right, the struct > clock_data there has all the bits you need here. > > So I'm thinking that you might want to add a helper function here to get > you the good stuff. Thanks, Peter. Leo -- do you think you could look at implementing this as part of a v2, please? Will