linux-kernel - Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math

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Date:   Fri, 9 Dec 2016 06:26:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Liav Rehana <liavr@...lanox.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Richard Cochran <richardcochran@...il.com>,
        Parit Bhargava <prarit@...hat.com>,
        Laurent Vivier <lvivier@...hat.com>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>
Subject: Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math

On Thu, Dec 08, 2016 at 08:49:39PM -0000, Thomas Gleixner wrote:

> +static inline u64 timekeeping_delta_to_ns(struct tk_read_base *tkr, u64 delta)
> +{
> +	u32 dh, dl;
> +	u64 nsec;
> +
> +	dl = delta;
> +	dh = delta >> 32;
> +
> +	nsec = ((u64)dl * tkr->mult) + tkr->xtime_nsec;
> +	nsec >>= tkr->shift;
> +	if (unlikely(dh))
> +		nsec += ((u64)dh * tkr->mult) << (32 - tkr->shift);
> +	return nsec;
> +}

Just for giggles, on tilegx the branch is actually slower than doing the
mult unconditionally.

The problem is that the two multiplies would otherwise completely
pipeline, whereas with the conditional you serialize them.

(came to light while talking about why the mul_u64_u32_shr() fallback
didn't work right for them, which was a combination of the above issue
and the fact that their compiler 'lost' the fact that these are
32x32->64 mults and did 64x64 ones instead).