linux-kernel - Re: [PATCH 4/5] irqtime: Move irqtime entry accounting after irq offset incrementation

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Date:   Tue, 1 Dec 2020 10:20:11 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Tony Luck <tony.luck@...el.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Heiko Carstens <hca@...ux.ibm.com>
Subject: Re: [PATCH 4/5] irqtime: Move irqtime entry accounting after irq
 offset incrementation

On Tue, Dec 01, 2020 at 01:12:25AM +0100, Frederic Weisbecker wrote:
> +static s64 irqtime_get_delta(struct irqtime *irqtime)
>  {
> +	int cpu = smp_processor_id();
>  	s64 delta;
>  
>  	delta = sched_clock_cpu(cpu) - irqtime->irq_start_time;
>  	irqtime->irq_start_time += delta;
>  
> +	return delta;
> +}
> +
> +/* Called after incrementing preempt_count on {soft,}irq_enter */
> +void irqtime_account_enter(struct task_struct *curr)
> +{
> +	struct irqtime *irqtime = this_cpu_ptr(&cpu_irqtime);
> +	u64 delta;
> +
> +	if (!sched_clock_irqtime)
> +		return;
> +
> +	delta = irqtime_get_delta(irqtime);
> +	/*
> +	 * We do not account for softirq time from ksoftirqd here.
> +	 * We want to continue accounting softirq time to ksoftirqd thread
> +	 * in that case, so as not to confuse scheduler with a special task
> +	 * that do not consume any time, but still wants to run.
> +	 */
> +	if ((irq_count() == (SOFTIRQ_OFFSET | HARDIRQ_OFFSET)) &&
> +	    curr != this_cpu_ksoftirqd())
> +		irqtime_account_delta(irqtime, delta, CPUTIME_SOFTIRQ);
> +}
> +
> +/* Called before decrementing preempt_count on {soft,}irq_exit */
> +void irqtime_account_exit(struct task_struct *curr)
> +{
> +	struct irqtime *irqtime = this_cpu_ptr(&cpu_irqtime);
> +	u64 delta;
> +
> +	if (!sched_clock_irqtime)
> +		return;
> +
> +	delta = irqtime_get_delta(irqtime);
>  	/*
>  	 * We do not account for softirq time from ksoftirqd here.
>  	 * We want to continue accounting softirq time to ksoftirqd thread


Urgh...


Why not something like:

void irqtime_account_irq(struct task_struct *curr, unsigned int offset)
{
	struct irqtime *irqtime = this_cpu_ptr(&cpu_irqtime);
	unsigned int pc = preempt_count() - offset;
	s64 delta;
	int cpu;

	if (!sched_clock_irqtime)
		return;

	cpu = smp_processor_id();
	delta = sched_clock_cpu(cpu) - irqtime->irq_start_time;
	irqtime->irq_start_time += delta;

	/*
	 * We do not account for softirq time from ksoftirqd here.
	 * We want to continue accounting softirq time to ksoftirqd thread
	 * in that case, so as not to confuse scheduler with a special task
	 * that do not consume any time, but still wants to run.
	 */
	if (pc & HARDIRQ_MASK)
		irqtime_account_delta(irqtime, delta, CPUTIME_IRQ);
	else if ((pc & SOFTIRQ_OFFSET) && curr != this_cpu_ksoftirqd())
		irqtime_account_delta(irqtime, delta, CPUTIME_SOFTIRQ);
}