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

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Date:   Tue, 1 Dec 2020 12:23:38 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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 10:20:11AM +0100, Peter Zijlstra wrote:
> 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);
> }
> 
> 

Right that's better, I'll do that.

Thanks.