linux-kernel - Re: [PATCH] sched/cputime: correct account of irqtime

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Message-ID: <20201014130223.GF2628@hirez.programming.kicks-ass.net>
Date:   Wed, 14 Oct 2020 15:02:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Allen Pais <allen.lkml@...il.com>,
        Romain Perier <romain.perier@...il.com>
Subject: Re: [PATCH] sched/cputime: correct account of irqtime

On Mon, Oct 12, 2020 at 09:50:44PM +0800, Pingfan Liu wrote:
> __do_softirq() may be interrupted by hardware interrupts. In this case,
> irqtime_account_irq() will account the time slice as CPUTIME_SOFTIRQ by
> mistake.
> 
> By passing irqtime_account_irq() an extra param about either hardirq or
> softirq, irqtime_account_irq() can handle the above case.

I'm not sure I see the scenario in which it goes wrong.

irqtime_account_irq() is designed such that we're called with the old
preempt_count on enter and the new preempt_count on exit. This way we'll
accumuate the delta to the previous context.

> @@ -68,7 +68,7 @@ void irqtime_account_irq(struct task_struct *curr)
>  	 */
>  	if (hardirq_count())
>  		irqtime_account_delta(irqtime, delta, CPUTIME_IRQ);
> -	else if (in_serving_softirq() && curr != this_cpu_ksoftirqd())
> +	else if (in_serving_softirq() && curr != this_cpu_ksoftirqd() && !hardirq)
>  		irqtime_account_delta(irqtime, delta, CPUTIME_SOFTIRQ);
>  }