linux-kernel - Re: [GIT PULL clocksource] Clocksource watchdog commits for v5.15

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Date:   Thu, 12 Aug 2021 19:11:55 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     paulmck@...nel.org
Cc:     linux-kernel@...r.kernel.org, john.stultz@...aro.org,
        kernel-team@...com, ak@...ux.intel.com, rong.a.chen@...el.com,
        sboyd@...nel.org
Subject: Re: [GIT PULL clocksource] Clocksource watchdog commits for v5.15

On Thu, Aug 12 2021 at 09:37, Paul E. McKenney wrote:

> On Thu, Aug 12, 2021 at 03:46:42PM +0200, Thomas Gleixner wrote:
>> On Wed, Aug 11 2021 at 17:01, Paul E. McKenney wrote:
>> > This pull request contains a single change that prevents clocksource
>> > watchdog testing on systems with HZ < 100, thus preventing the integer
>> > underflow that can occur on leisurely HZed systems.  This has been
>> > posted to LKML:
>> >
>> > https://lore.kernel.org/lkml/20210721212755.GA2066078@paulmck-ThinkPad-P17-Gen-1/
>> 
>> So with HZ < 100 .mult overflows, but why not simply adjusting the
>> mult, shift value to be
>> 
>>       .mult	= TICK_NSEC,
>>       .shift	= 0,
>> 
>> which is effectively the same as
>> 
>>       .mult	= TICK_NSEC << 8,
>>       .shift	= 8,
>> 
>> Hmm?
>
> Another option would be for me to be less lazy and to move this code:
>
> 	/* Since jiffies uses a simple TICK_NSEC multiplier
> 	 * conversion, the .shift value could be zero. However
> 	 * this would make NTP adjustments impossible as they are
> 	 * in units of 1/2^.shift. Thus we use JIFFIES_SHIFT to
> 	 * shift both the nominator and denominator the same
> 	 * amount, and give ntp adjustments in units of 1/2^8
> 	 *
> 	 * The value 8 is somewhat carefully chosen, as anything
> 	 * larger can result in overflows. TICK_NSEC grows as HZ
> 	 * shrinks, so values greater than 8 overflow 32bits when
> 	 * HZ=100.
> 	 */
> 	#if HZ < 34
> 	#define JIFFIES_SHIFT	6
> 	#elif HZ < 67
> 	#define JIFFIES_SHIFT	7
> 	#else
> 	#define JIFFIES_SHIFT	8
> 	#endif
>
> from kernel/time/jiffies.c to include/linux/clocksource.h.

No need to expose this globaly.

  kernel/time/tick-internal.h or kernel/time/jiffies.h

Thanks,

        tglx