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

Rik van Riel
2016-07-05T13:09:38Z

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On Tue, 2016-07-05 at 14:40 +0200, Frederic Weisbecker wrote:
> On Thu, Jun 30, 2016 at 03:35:47PM -0400, riel@redhat.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.

--

All Rights Reversed.

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