linux-kernel - Re: [PATCH 1/4] sched,time: count actually elapsed irq & softirq time

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Date:	Tue, 05 Jul 2016 09:08:16 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	mingo@...nel.org, pbonzini@...hat.com, fweisbec@...hat.com,
	wanpeng.li@...mail.com, efault@....de, tglx@...utronix.de,
	rkrcmar@...hat.com
Subject: Re: [PATCH 1/4] sched,time: count actually elapsed irq & softirq
 time

On Tue, 2016-07-05 at 14:40 +0200, Frederic Weisbecker wrote:
> On Thu, Jun 30, 2016 at 03:35:47PM -0400, riel@...hat.com wrote:
> > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> > index 3d60e5d76fdb..018bae2ada36 100644
> > --- a/kernel/sched/cputime.c
> > +++ b/kernel/sched/cputime.c
> > @@ -79,40 +79,50 @@ void irqtime_account_irq(struct task_struct
> > *curr)
> >  }
> >  EXPORT_SYMBOL_GPL(irqtime_account_irq);
> >  
> > -static int irqtime_account_hi_update(void)
> > +static cputime_t irqtime_account_hi_update(cputime_t maxtime)
> >  {
> >  	u64 *cpustat = kcpustat_this_cpu->cpustat;
> >  	unsigned long flags;
> > -	u64 latest_ns;
> > -	int ret = 0;
> > +	cputime_t irq_cputime;
> >  
> >  	local_irq_save(flags);
> > -	latest_ns = this_cpu_read(cpu_hardirq_time);
> > -	if (nsecs_to_cputime64(latest_ns) > cpustat[CPUTIME_IRQ])
> > -		ret = 1;
> > +	irq_cputime =
> > nsecs_to_cputime(this_cpu_read(cpu_hardirq_time)) -
> > +		      cpustat[CPUTIME_IRQ];
> 
> We might want to keep nsecs_to_cputime64(). If cputime_t == jiffies_t
> == unsigned long,
> we may have a problem after 49 days of interrupts. Arguably that's a
> lot of IRQs
> but lets be paranoid.

The macro nsecs_to_cputime64 is only defined in
cputime_jiffies.h though, not in cputime_nsecs.h

Want me to add a #define to the second file?

> > +	irq_cputime = min(irq_cputime, maxtime);
> > +	cpustat[CPUTIME_IRQ] += irq_cputime;
> >  	local_irq_restore(flags);
> > -	return ret;
> > +	return irq_cputime;
> >  }
> >  
> > -static int irqtime_account_si_update(void)
> > +static cputime_t irqtime_account_si_update(cputime_t maxtime)
> >  {
> >  	u64 *cpustat = kcpustat_this_cpu->cpustat;
> >  	unsigned long flags;
> > -	u64 latest_ns;
> > -	int ret = 0;
> > +	cputime_t softirq_cputime;
> >  
> >  	local_irq_save(flags);
> > -	latest_ns = this_cpu_read(cpu_softirq_time);
> > -	if (nsecs_to_cputime64(latest_ns) >
> > cpustat[CPUTIME_SOFTIRQ])
> > -		ret = 1;
> > +	softirq_cputime =
> > nsecs_to_cputime(this_cpu_read(cpu_softirq_time)) -
> 
> Ditto.
> 
> > +			  cpustat[CPUTIME_SOFTIRQ];
> > +	softirq_cputime = min(softirq_cputime, maxtime);
> > +	cpustat[CPUTIME_SOFTIRQ] += softirq_cputime;
> >  	local_irq_restore(flags);
> > -	return ret;
> > +	return softirq_cputime;
> >  }
> >  
> >  #else /* CONFIG_IRQ_TIME_ACCOUNTING */
> >  
> >  #define sched_clock_irqtime	(0)
> >  
> > +static cputime_t irqtime_account_hi_update(cputime_t dummy)
> > +{
> > +	return 0;
> > +}
> > +
> > +static cputime_t irqtime_account_si_update(cputime_t dummy)
> > +{
> > +	return 0;
> > +}
> > +
> >  #endif /* !CONFIG_IRQ_TIME_ACCOUNTING */
> >  
> >  static inline void task_group_account_field(struct task_struct *p,
> > int index,
> > @@ -257,32 +267,45 @@ void account_idle_time(cputime_t cputime)
> >  		cpustat[CPUTIME_IDLE] += (__force u64) cputime;
> >  }
> >  
> > -static __always_inline unsigned long
> > steal_account_process_tick(unsigned long max_jiffies)
> > +static __always_inline cputime_t
> > steal_account_process_time(cputime_t maxtime)
> >  {
> >  #ifdef CONFIG_PARAVIRT
> >  	if (static_key_false(&paravirt_steal_enabled)) {
> > +		cputime_t steal_cputime;
> >  		u64 steal;
> > -		unsigned long steal_jiffies;
> >  
> >  		steal = paravirt_steal_clock(smp_processor_id());
> >  		steal -= this_rq()->prev_steal_time;
> > +		this_rq()->prev_steal_time += steal;
> 
> We are accounting steal_cputime but you make it remember steal_nsecs.
> This is
> leaking quite some steal time in the way.
> 
> Imagine that cputime_t == jiffies_t and HZ=100.
> paravirt_steal_clock() returns 199 nsecs. prev_steal_time gets added
> those 199.
> nsecs_to_cputime() return 1 jiffy (we are one nsec off the next
> jiffy). So
> account_steal_time() is accounting 1 jiffy and the 99 remaining nsecs
> are leaked.
> If some more steal time is to be accounted on the next tick, the 99
> previous nsecs
> are forgotten.
> 
> A non-leaking sequence would rather be:
> 
> 	steal = paravirt_steal_clock(smp_processor_id());
> 	steal -= this_rq()->prev_steal_time;
> 	steal_cputime = min(nsecs_to_cputime(steal), maxtime);
> 	account_steal_time(steal_cputime);
> 	this_rq()->prev_steal_time += cputime_to_nsecs(steal_cputime);
> 
> Thanks!

Good catch. I will fix this!

Thanks for reviewing.

-- 

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