linux-kernel - Re: [RFC][PATCH] time: Fix ktime_get_raw() issues caused by incorrect base accumulation

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Date:   Sat, 26 Aug 2017 11:20:41 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     John Stultz <john.stultz@...aro.org>,
        "lkml" <linux-kernel@...r.kernel.org>
Cc:     "John Stultz" <john.stultz@...aro.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...nel.org>,
        "Miroslav Lichvar" <mlichvar@...hat.com>,
        "Richard Cochran" <richardcochran@...il.com>,
        "Prarit Bhargava" <prarit@...hat.com>,
        "Stephen Boyd" <stephen.boyd@...aro.org>,
        "Kevin Brodsky" <kevin.brodsky@....com>,
        "Will Deacon" <will.deacon@....com>,
        "Daniel Mentz" <danielmentz@...gle.com>
Subject: Re: [RFC][PATCH] time: Fix ktime_get_raw() issues caused by incorrect base
 accumulation

Quoting John Stultz (2017-08-25 23:57:04)
> In commit fc6eead7c1e2 ("time: Clean up CLOCK_MONOTONIC_RAW time
> handling"), I mistakenly added the following:
> 
>  /* Update the monotonic raw base */
>  seconds = tk->raw_sec;
>  nsec = (u32)(tk->tkr_raw.xtime_nsec >> tk->tkr_raw.shift);
>  tk->tkr_raw.base = ns_to_ktime(seconds * NSEC_PER_SEC + nsec);
> 
> Which adds the raw_sec value and the shifted down raw xtime_nsec
> to the base value.
> 
> This is problematic as when calling ktime_get_raw(), we add the
> tk->tkr_raw.xtime_nsec and current offset, shift it down and add
> it to the raw base.
> 
> This results in the shifted down tk->tkr_raw.xtime_nsec being
> added twice.
> 
> My mistake, was that I was matching the monotonic base logic
> above:
> 
>  seconds = (u64)(tk->xtime_sec + tk->wall_to_monotonic.tv_sec);
>  nsec = (u32) tk->wall_to_monotonic.tv_nsec;
>  tk->tkr_mono.base = ns_to_ktime(seconds * NSEC_PER_SEC + nsec);
> 
> Which adds the wall_to_monotonic.tv_nsec value, but not the
> tk->tkr_mono.xtime_nsec value to the base.
> 
> The result of this is that ktime_get_raw() users (which are all
> internal users) see the raw time move faster then it should
> (the rate at which can vary with the current size of
> tkr_raw.xtime_nsec), which has resulted in at least problems
> with graphics rendering performance.
> 
> To fix this, we simplify the tkr_raw.base accumulation to only
> accumulate the raw_sec portion, and do not include the
> tkr_raw.xtime_nsec portion, which will be added at read time.

This fixes our testcase of using ktime_get_raw() deltas. Thanks,
Tested-by: Chris Wilson <chris@...is-wilson.co.uk>
-Chris