Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752345AbcKRKe3 (ORCPT <rfc822;w@1wt.eu>);
        Fri, 18 Nov 2016 05:34:29 -0500
Received: from merlin.infradead.org ([205.233.59.134]:45518 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751685AbcKRKe2 (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 18 Nov 2016 05:34:28 -0500
Date: Fri, 18 Nov 2016 11:34:25 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Chris Metcalf <cmetcalf@mellanox.com>
Cc: John Stultz <john.stultz@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>, Salman Qazi <sqazi@google.com>,
        Paul Turner <pjt@google.com>, Tony Lindgren <tony@atomide.com>,
        Steven Miao <realmz6@gmail.com>, lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute
 cycle count
Message-ID: <20161118103425.GL3117@twins.programming.kicks-ass.net>
References: <bf36c76f-b26f-8d17-b951-f6b919845172@mellanox.com>
 <1479324933-8161-1-git-send-email-cmetcalf@mellanox.com>
 <CALAqxLWemyrZPjERPW0vMQ+iti0zfX+nLrvgyj4vW76MW2yTig@mail.gmail.com>
 <c656758a-8299-0b96-6d87-23f9beafc481@mellanox.com>
 <20161117095343.GF3142@twins.programming.kicks-ass.net>
 <d7a8a75d-ddc3-a408-dc6c-c0225c824dc2@mellanox.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <d7a8a75d-ddc3-a408-dc6c-c0225c824dc2@mellanox.com>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 1142
Lines: 22

On Thu, Nov 17, 2016 at 03:00:14PM -0500, Chris Metcalf wrote:
> On 11/17/2016 4:53 AM, Peter Zijlstra wrote:
> >On Wed, Nov 16, 2016 at 03:16:59PM -0500, Chris Metcalf wrote:
> >>PeterZ (cc'ed) then improved it to use __int128 math via
> >>mul_u64_u32_shr(), but that doesn't help tile; we only do one multiply
> >>instead of two, but the multiply is handled by an out-of-line call to
> >>__multi3, and the sched_clock() function ends up about 2.5x slower as
> >>a result.
> >Well, only if you set CONFIG_ARCH_SUPPORTS_INT128, otherwise it reduces
> >to 2 32x23->64 multiplications, of which one if conditional on there
> >actually being bits set in the high word of the u64 argument.
> 
> I didn't notice that.  It took me down an interesting rathole.
> 
> Obviously the branch optimization won't help on cycle counter values,
> since we blow out of the low 32 bits in the first few seconds of
> uptime.  So the conditional test won't help, but the 32x32
> multiply optimizations should.

Now, I don't quite remember things, but isn't it the idea to convert
cycle deltas and accumulate in ns? That way you most always convert
small values.