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

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Message-ID: <CALAqxLWyPOET2wqg7x+PddfNpORMUJYmCfpotN0BpxAQYOpbUw@mail.gmail.com>
Date:   Wed, 16 Nov 2016 12:31:05 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Chris Metcalf <cmetcalf@...lanox.com>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute
 cycle count

On Wed, Nov 16, 2016 at 12:29 PM, John Stultz <john.stultz@...aro.org> wrote:
> On Wed, Nov 16, 2016 at 12:16 PM, Chris Metcalf <cmetcalf@...lanox.com> wrote:
>> Change 4cecf6d401a0 results in essentially identical code for x86 as
>> this proposed change does for tile.  In fact a follow-on change by
>> Salman introduced mult_frac() and switched to using it, so it was
>> identical at that point.
>>
>> 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.
>>
>> Thanks for thinking about this!
>
> Heh. Thanks for the history lesson and apologies for my forgetfulness. :)

Oh.. and some of these details might be useful to have in the commit message!

thanks
-john