linux-kernel - Re: [PATCH v2] tile: avoid using clocksource

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Message-ID: <20161117095343.GF3142@twins.programming.kicks-ass.net>
Date:   Thu, 17 Nov 2016 10:53:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Chris Metcalf <cmetcalf@...lanox.com>
Cc:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Salman Qazi <sqazi@...gle.com>, Paul Turner <pjt@...gle.com>,
        Tony Lindgren <tony@...mide.com>,
        Steven Miao <realmz6@...il.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute
 cycle count

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.