linux-kernel - Re: [PATCH 02/12] clocksource: Simplify logic around clocksource wrapping saftey margins

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Date:	Sat, 7 Mar 2015 10:22:15 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Cochran <richardcochran@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 02/12] clocksource: Simplify logic around clocksource
 wrapping saftey margins


* John Stultz <john.stultz@...aro.org> wrote:

> The clocksource logic has a number of places where we try to
> include a safety margin. Most of these are 12% safety margins,
> but they are inconsistently applied and sometimes are applied
> on top of each other.
> 
> Additionally, in the previous patch, we corrected an issue
> where we unintentionally in effect created a 50% saftey margin,
> which these 12.5% margins where then added to.
> 
> So to siplify the logic here, this patch removes the various

Typo.

> 12.5% margins, and consolidates adding the margin in one place:
> clocks_calc_max_nsecs().
> 
> Addtionally, Linus prefers a 50% safety margin, as it allows
> bad clock values to be more easily caught. This should really
> have no net effect, due to the corrected issue earlier which
> caused greater then 50% margins to be used w/o issue.
> 
> Cc: Dave Jones <davej@...emonkey.org.uk>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Richard Cochran <richardcochran@...il.com>
> Cc: Prarit Bhargava <prarit@...hat.com>
> Cc: Stephen Boyd <sboyd@...eaurora.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Acked-by: Stephen Boyd <sboyd@...eaurora.org> (for sched_clock.c bit)
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
>  kernel/time/clocksource.c | 26 ++++++++++++--------------
>  kernel/time/sched_clock.c |  4 ++--
>  2 files changed, 14 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> index 11323f4..e5d00e6 100644
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -469,6 +469,9 @@ static u32 clocksource_max_adjustment(struct clocksource *cs)
>   * @shift:	cycle to nanosecond divisor (power of two)
>   * @maxadj:	maximum adjustment value to mult (~11%)
>   * @mask:	bitmask for two's complement subtraction of non 64 bit counters
> + *
> + * NOTE: This function includes a saftey margin of 50%, so that bad clock values

Typo.

There's also the same typo in the title of the patch.
 
Thanks,

	Ingo
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