linux-kernel - Re: [PATCH 2/5] introduce CLOCK_MONOTONIC

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Message-ID: <Pine.LNX.4.64.0803150536290.1791@scrub.home>
Date:	Sat, 15 Mar 2008 05:50:02 +0100 (CET)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	john stultz <johnstul@...ibm.com>
cc:	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/5] introduce CLOCK_MONOTONIC_RAW

Hi,

On Fri, 14 Mar 2008, john stultz wrote:

> My solution is to introduce CLOCK_MONOTONIC_RAW. This exposes a
> nanosecond based time value, that increments starting at bootup and has
> no frequency adjustments made to it what so ever.

A general comment: the raw clock doesn't need any adjustments, so updates 
don't have to be done that frequently and you can move most of that logic 
into second_overflow().

> @@ -439,6 +475,7 @@ static void clocksource_adjust(s64 offset)
>  void update_wall_time(void)
>  {
>  	cycle_t offset;
> +	static u64 raw_snsec; /* shifted raw nanosecnds */
>  
>  	/* Make sure we're fully resumed: */
>  	if (unlikely(timekeeping_suspended))

IMO that's really a clock property, so this belongs in the clock 
structure.
(Some day we may want to have multiple active clocks for various purposes 
and thus export multiple raw clocks.)

> @@ -466,6 +504,11 @@ void update_wall_time(void)
>  			second_overflow();
>  		}
>  
> +		if (raw_snsec >= (u64)NSEC_PER_SEC << clock->shift) {
> +			raw_snsec -= (u64)NSEC_PER_SEC << clock->shift;
> +			monotonic_raw.tv_sec++;
> +		}
> +	

You don't really have to use clock->shift as a scale, thus simplifying the 
shifting here (e.g. by using NTP_SCALE_SHIFT).
I'm thinking about doing this for e.g. xtime_nsec as well.

bye, Roman
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