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

[Pingfan Liu <kernelfans@gmail.com>]

On Tue, Oct 13, 2020 at 11:10 AM jun qian <qianjun.kernel@gmail.com> wrote:
>
> Pingfan Liu <kernelfans@gmail.com> 于2020年10月12日周一 下午9:54写道：
> >
> > __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.
> >
> > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Juri Lelli <juri.lelli@redhat.com>
> > Cc: Vincent Guittot <vincent.guittot@linaro.org>
> > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Ben Segall <bsegall@google.com>
> > Cc: Mel Gorman <mgorman@suse.de>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Andy Lutomirski <luto@kernel.org>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: "Paul E. McKenney" <paulmck@kernel.org>
> > Cc: Frederic Weisbecker <frederic@kernel.org>
> > Cc: Allen Pais <allen.lkml@gmail.com>
> > Cc: Romain Perier <romain.perier@gmail.com>
> > To: linux-kernel@vger.kernel.org
> > ---
> >  include/linux/hardirq.h |  4 ++--
> >  include/linux/vtime.h   | 12 ++++++------
> >  kernel/sched/cputime.c  |  4 ++--
> >  kernel/softirq.c        |  6 +++---
> >  4 files changed, 13 insertions(+), 13 deletions(-)
> >
> > diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
> > index 754f67a..56e7bb5 100644
> > --- a/include/linux/hardirq.h
> > +++ b/include/linux/hardirq.h
> > @@ -32,7 +32,7 @@ static __always_inline void rcu_irq_enter_check_tick(void)
> >   */
> >  #define __irq_enter()                                  \
> >         do {                                            \
> > -               account_irq_enter_time(current);        \
> > +               account_irq_enter_time(current, true);  \
> >                 preempt_count_add(HARDIRQ_OFFSET);      \
> >                 lockdep_hardirq_enter();                \
> >         } while (0)
> > @@ -63,7 +63,7 @@ void irq_enter_rcu(void);
> >  #define __irq_exit()                                   \
> >         do {                                            \
> >                 lockdep_hardirq_exit();                 \
> > -               account_irq_exit_time(current);         \
> > +               account_irq_exit_time(current, true);   \
> >                 preempt_count_sub(HARDIRQ_OFFSET);      \
> >         } while (0)
> >
> > diff --git a/include/linux/vtime.h b/include/linux/vtime.h
> > index 2cdeca0..294188ae1 100644
> > --- a/include/linux/vtime.h
> > +++ b/include/linux/vtime.h
> > @@ -98,21 +98,21 @@ static inline void vtime_flush(struct task_struct *tsk) { }
> >
> >
> >  #ifdef CONFIG_IRQ_TIME_ACCOUNTING
> > -extern void irqtime_account_irq(struct task_struct *tsk);
> > +extern void irqtime_account_irq(struct task_struct *tsk, bool hardirq);
> >  #else
> > -static inline void irqtime_account_irq(struct task_struct *tsk) { }
> > +static inline void irqtime_account_irq(struct task_struct *tsk, bool hardirq) { }
> >  #endif
> >
> > -static inline void account_irq_enter_time(struct task_struct *tsk)
> > +static inline void account_irq_enter_time(struct task_struct *tsk, bool hardirq)
> >  {
> >         vtime_account_irq_enter(tsk);
> > -       irqtime_account_irq(tsk);
> > +       irqtime_account_irq(tsk, hardirq);
> >  }
> >
> > -static inline void account_irq_exit_time(struct task_struct *tsk)
> > +static inline void account_irq_exit_time(struct task_struct *tsk, bool hardirq)
> >  {
> >         vtime_account_irq_exit(tsk);
> > -       irqtime_account_irq(tsk);
> > +       irqtime_account_irq(tsk, hardirq);
> >  }
> >
> >  #endif /* _LINUX_KERNEL_VTIME_H */
> > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> > index 5a55d23..166f1d7 100644
> > --- a/kernel/sched/cputime.c
> > +++ b/kernel/sched/cputime.c
> > @@ -47,7 +47,7 @@ static void irqtime_account_delta(struct irqtime *irqtime, u64 delta,
> >   * Called before incrementing preempt_count on {soft,}irq_enter
> >   * and before decrementing preempt_count on {soft,}irq_exit.
> >   */
> > -void irqtime_account_irq(struct task_struct *curr)
> > +void irqtime_account_irq(struct task_struct *curr, bool hardirq)
> >  {
> >         struct irqtime *irqtime = this_cpu_ptr(&cpu_irqtime);
> >         s64 delta;
> > @@ -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);
> >  }
>
> In my opinion, we don't need to use the hardirq flag, the code: if
> (hardirq_count())
> already tell us that where the delt time is from.

Considering the scenario in which hardirq happens immediately after
__do_softirq()->local_irq_enable(). The following code shows that
hardirq_count() can not help.
#define __irq_enter() \
do { \
account_irq_enter_time(current); \
preempt_count_add(HARDIRQ_OFFSET); \
lockdep_hardirq_enter(); \
} while (0)

Anything I missed?

Thanks,
Pingfan