linux-kernel - Re: [tip:sched/core] sched/cputime: Refactor the cputime

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Date:   Tue, 4 Jul 2017 11:17:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org, fransklaver@...il.com, hpa@...or.com,
        fweisbec@...il.com, torvalds@...ux-foundation.org, riel@...hat.com,
        tglx@...utronix.de, wanpeng.li@...mail.com,
        linux-kernel@...r.kernel.org, garsilva@...eddedor.com,
        sgruszka@...hat.com
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched/cputime: Refactor the cputime_adjust()
 code

On Fri, Jun 30, 2017 at 06:10:35AM -0700, tip-bot for Gustavo A. R. Silva wrote:
> Commit-ID:  72298e5c92c50edd8cb7cfda4519483ce65fa166
> Gitweb:     http://git.kernel.org/tip/72298e5c92c50edd8cb7cfda4519483ce65fa166
> Author:     Gustavo A. R. Silva <garsilva@...eddedor.com>
> AuthorDate: Thu, 29 Jun 2017 13:41:28 -0500
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Fri, 30 Jun 2017 09:37:59 +0200
> 
> sched/cputime: Refactor the cputime_adjust() code
> 
> Address a Coverity false positive, which is caused by overly
> convoluted code:
> 
> Value assigned to variable 'utime' at line 619:utime = rtime;
> is overwritten at line 642:utime = rtime - stime; before it
> can be used. This makes such variable assignment useless.
> 
> Remove this variable assignment and refactor the code related.
> 
> Addresses-Coverity-ID: 1371643
> Signed-off-by: Gustavo A. R. Silva <garsilva@...eddedor.com>
> Cc: Frans Klaver <fransklaver@...il.com>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Stanislaw Gruszka <sgruszka@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Wanpeng Li <wanpeng.li@...mail.com>
> Link: http://lkml.kernel.org/r/20170629184128.GA5271@embeddedgus
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  kernel/sched/cputime.c | 16 +++++-----------
>  1 file changed, 5 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index aea3135..67c70e2 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -615,19 +615,13 @@ static void cputime_adjust(struct task_cputime *curr,
>  	 * userspace. Once a task gets some ticks, the monotonicy code at
>  	 * 'update' will ensure things converge to the observed ratio.
>  	 */
> -	if (stime == 0) {
> -		utime = rtime;
> -		goto update;
> +	if (stime != 0) {
> +		if (utime == 0)
> +			stime = rtime;
> +		else
> +			stime = scale_stime(stime, rtime, stime + utime);
>  	}
>  
> -	if (utime == 0) {
> -		stime = rtime;
> -		goto update;
> -	}
> -
> -	stime = scale_stime(stime, rtime, stime + utime);
> -
> -update:
>  	/*
>  	 * Make sure stime doesn't go backwards; this preserves monotonicity
>  	 * for utime because rtime is monotonic.


Argh, no... That code was perfectly fine. The new code otoh is
convoluted crap.

It had the form:

	if (exception1)
	  deal with exception1

	if (execption2)
	  deal with exception2

	do normal stuff

Which is as simple and straight forward as it gets.

The new code otoh reads like:

	if (!exception1) {
		if (exception2)
		  deal with exception 2
		else
		  do normal stuff
	}

which is absolute shit.

So NAK on this.