linux-kernel - Re: [PATCH] sched/cputime: Fix using smp_processor

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Message-ID: <20170707081120.ht2gxuzpytoq2by4@hirez.programming.kicks-ass.net>
Date:   Fri, 7 Jul 2017 10:11:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] sched/cputime: Fix using smp_processor_id() in
 preemptible

On Thu, Jul 06, 2017 at 08:18:41PM -0700, Wanpeng Li wrote:
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -683,7 +683,7 @@ static u64 vtime_delta(struct vtime *vtime)
>  {
>  	unsigned long long clock;
>  
> -	clock = sched_clock_cpu(smp_processor_id());
> +	clock = sched_clock();
>  	if (clock < vtime->starttime)
>  		return 0;
>  

This is broken. sched_clock_cpu() and sched_clock() do not have the
exact same value :-/

So this ends up doing:

	vtime->starttime = sched_clock_cpu(cpu);

	/* ... */

	clock = sched_clock();

	delta = clock - vtime->starttime;


And you compute a delta using two different clocks.