linux-kernel - Re: [RFC PATCH v2 2/6] timekeeping: Fix cross-timestamp interpolation corner case decision

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Message-ID: <874jjv1cr4.ffs@tglx>
Date:   Fri, 15 Sep 2023 21:02:07 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Hilber <peter.hilber@...nsynergy.com>,
        linux-kernel@...r.kernel.org
Cc:     John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>
Subject: Re: [RFC PATCH v2 2/6] timekeeping: Fix cross-timestamp
 interpolation corner case decision

On Fri, Sep 15 2023 at 19:30, Peter Hilber wrote:
> On 15.09.23 18:10, Thomas Gleixner wrote:
>> So the explanation in the changelog makes some sense, but this code
>> without any further explanation just makes my brain explode.
>> 
>> This whole thing screams for a change to cycle_between() so it becomes:
>> 
>>      timestamp_in_interval(start, end, ts)
>> 
>> and make start inclusive and not exclusive, no?
>
> I tried like this in v1 (having 'end' inclusive as well), but didn't like
> the effect at the second usage site.
>
>> 
>> That's actually correct for both usage sites because for interpolation
>> the logic is the same. history_begin->cycles is a valid timestamp, no?
>
> AFAIU, with the timestamp_in_interval() change, history_begin->cycles would
> become a valid timestamp. To me it looks like
> adjust_historical_crosststamp() should then work unmodified for now. But
> one would have to be careful with the additional corner case in the future.
>
> So, document the current one-line change, or switch to
> timestamp_in_interval()?

I really prefer the consistent function which treats the start as
inclusive as that makes the most sense and is self explanatory.

Thanks,

        tglx