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From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	John Stultz <john.stultz@linaro.org>,
	Ingo Molnar <mingo@kernel.org>,
	David Gibson <david@gibson.dropbear.id.au>,
	Liav Rehana <liavr@mellanox.com>,
	Chris Metcalf <cmetcalf@mellanox.com>,
	Richard Cochran <richardcochran@gmail.com>,
	Parit Bhargava <prarit@redhat.com>,
	Laurent Vivier <lvivier@redhat.com>,
	"Christopher S. Hall" <christopher.s.hall@intel.com>
Subject: Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math
Date: Fri, 9 Dec 2016 06:26:38 +0100	[thread overview]
Message-ID: <20161209052638.GC3061@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20161208204229.005418487@linutronix.de>

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).

  parent reply	other threads:[~2016-12-09  5:26 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-08 20:49 [patch 0/6] timekeeping: Cure the signed/unsigned wreckage Thomas Gleixner
2016-12-08 20:49 ` [patch 1/6] timekeeping: Force unsigned clocksource to nanoseconds conversion Thomas Gleixner
2016-12-08 23:38   ` David Gibson
2016-12-09 11:13   ` [tip:timers/core] timekeeping_Force_unsigned_clocksource_to_nanoseconds_conversion tip-bot for Thomas Gleixner
2016-12-08 20:49 ` [patch 2/6] timekeeping: Make the conversion call chain consistently unsigned Thomas Gleixner
2016-12-08 23:39   ` David Gibson
2016-12-09 11:13   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2016-12-08 20:49 ` [patch 3/6] timekeeping: Get rid of pointless typecasts Thomas Gleixner
2016-12-08 23:40   ` David Gibson
2016-12-09 11:14   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2016-12-08 20:49 ` [patch 4/6] timekeeping: Use mul_u64_u32_shr() instead of open coding it Thomas Gleixner
2016-12-08 23:41   ` David Gibson
2016-12-09 11:14   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2016-12-08 20:49 ` [patch 5/6] [RFD] timekeeping: Provide optional 128bit math Thomas Gleixner
2016-12-09  4:08   ` Ingo Molnar
2016-12-09  4:29     ` Ingo Molnar
2016-12-09  4:39       ` John Stultz
2016-12-09  4:48     ` Peter Zijlstra
2016-12-09  5:22       ` Ingo Molnar
2016-12-09  5:41         ` Peter Zijlstra
2016-12-09  5:11   ` Peter Zijlstra
2016-12-09  6:08     ` Peter Zijlstra
2016-12-09  5:26   ` Peter Zijlstra [this message]
2016-12-09  6:38     ` Peter Zijlstra
2016-12-09  8:30       ` Peter Zijlstra
2016-12-09  9:11         ` Peter Zijlstra
2016-12-09 10:01         ` Peter Zijlstra
2016-12-09 17:32         ` Chris Metcalf
2017-01-14 12:51         ` [tip:timers/core] math64, timers: Fix 32bit mul_u64_u32_shr() and friends tip-bot for Peter Zijlstra
2016-12-09 10:18       ` [patch 5/6] [RFD] timekeeping: Provide optional 128bit math Peter Zijlstra
2016-12-09 17:20         ` Chris Metcalf
2016-12-08 20:49 ` [patch 6/6] [RFD] timekeeping: Get rid of cycle_t Thomas Gleixner
2016-12-08 23:43   ` David Gibson
2016-12-09  4:52 ` [patch 0/6] timekeeping: Cure the signed/unsigned wreckage John Stultz
2016-12-09  5:30 ` Peter Zijlstra

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