linux-kernel - Re: [PATCH] Revert "sched/cputime: Fix mul_u64_u64_div

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Message-ID: <5j22peklzybliocy6lg3rx6a6qsvrij3kvnjs74djpqpufv4kx@ql2zrdtsor3i>
Date: Mon, 7 Oct 2024 09:50:19 +0200
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: Nicolas Pitre <npitre@...libre.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Zheng Zucheng <zhengzucheng@...wei.com>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "sched/cputime: Fix mul_u64_u64_div_u64()
 precision for cputime"

Hello,

On Fri, Oct 04, 2024 at 08:19:54PM -0400, Nicolas Pitre wrote:
> This reverts commit 77baa5bafcbe1b2a15ef9c37232c21279c95481c.
> 
> After commit b29a62d87cc0 ("mul_u64_u64_div_u64: make it precise always")
> it is no longer necessary to have this workaround.
> 
> Signed-off-by: Nicolas Pitre <npitre@...libre.com>
> ---
>  kernel/sched/cputime.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 0bed0fa1ac..a5e00293ae 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -582,12 +582,6 @@ void cputime_adjust(struct task_cputime *curr, struct prev_cputime *prev,
>  	}
>  
>  	stime = mul_u64_u64_div_u64(stime, rtime, stime + utime);
> -	/*
> -	 * Because mul_u64_u64_div_u64() can approximate on some
> -	 * achitectures; enforce the constraint that: a*b/(b+c) <= a.
> -	 */
> -	if (unlikely(stime > rtime))
> -		stime = rtime;

I didn't look in detail, but even with mul_u64_u64_div_u64() being exact
now, stime > rtime can still be hit if stime + utime overflows. Can this
happen? (Can stime + utime become 0?)

The example from the commit log of 77baa5bafcbe ("sched/cputime: Fix
mul_u64_u64_div_u64() precision for cputime") however won't occur any
more. IMHO that is good enough to justify this patch.

Acked-by: Uwe Kleine-König <u.kleine-koenig@...libre.com>

Thanks for following up your improvement to mul_u64_u64_div_u64() with
this change.

Best regards
Uwe

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