Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Apr 24, 2014 at 08:45:58PM +0200, Denys Vlasenko wrote:
> Before this change, if last IO-blocked task wakes up
> on a different CPU, the original CPU may stay idle for much longer,
> and the entire time it stays idle is accounted as iowait time.
> 
> This change adds struct tick_sched::iowait_exittime member.
> On entry to idle, it is set to KTIME_MAX.
> Last IO-blocked task, if migrated, sets it to current time.
> Note that this can happen only once per each idle period:
> new iowaiting tasks can't magically appear on idle CPU's rq.
> 
> If iowait_exittime is set, then (iowait_exittime - idle_entrytime)
> gets accounted as iowait, and the remaining (now - iowait_exittime)
> as "true" idle.
> 
> Run-tested: /proc/stat counters no longer go backwards.
> 
> Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
> Cc: Fernando Luis Vazquez Cao <fernando_b1@lab.ntt.co.jp>
> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Arjan van de Ven <arjan@linux.intel.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> ---
>  include/linux/tick.h     |  2 ++
>  kernel/sched/core.c      | 14 +++++++++++
>  kernel/time/tick-sched.c | 64 ++++++++++++++++++++++++++++++++++++++++--------
>  3 files changed, 70 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 4de1f9e..1bf653e 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -67,6 +67,7 @@ struct tick_sched {
>  	ktime_t				idle_exittime;
>  	ktime_t				idle_sleeptime;
>  	ktime_t				iowait_sleeptime;
> +	ktime_t				iowait_exittime;
>  	seqcount_t			idle_sleeptime_seq;
>  	ktime_t				sleep_length;
>  	unsigned long			last_jiffies;
> @@ -140,6 +141,7 @@ extern void tick_nohz_irq_exit(void);
>  extern ktime_t tick_nohz_get_sleep_length(void);
>  extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
>  extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
> +extern void tick_nohz_iowait_to_idle(int cpu);
>  
>  # else /* !CONFIG_NO_HZ_COMMON */
>  static inline int tick_nohz_tick_stopped(void)
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 268a45e..ffea757 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4218,7 +4218,14 @@ void __sched io_schedule(void)
>  	current->in_iowait = 1;
>  	schedule();
>  	current->in_iowait = 0;
> +#ifdef CONFIG_NO_HZ_COMMON
> +	if (atomic_dec_and_test(&rq->nr_iowait)) {
> +		if (raw_smp_processor_id() != cpu_of(rq))
> +			tick_nohz_iowait_to_idle(cpu_of(rq));
> +	}
> +#else
>  	atomic_dec(&rq->nr_iowait);
> +#endif
>  	delayacct_blkio_end();
>  }
>  EXPORT_SYMBOL(io_schedule);
> @@ -4234,7 +4241,14 @@ long __sched io_schedule_timeout(long timeout)
>  	current->in_iowait = 1;
>  	ret = schedule_timeout(timeout);
>  	current->in_iowait = 0;
> +#ifdef CONFIG_NO_HZ_COMMON
> +	if (atomic_dec_and_test(&rq->nr_iowait)) {
> +		if (raw_smp_processor_id() != cpu_of(rq))
> +			tick_nohz_iowait_to_idle(cpu_of(rq));
> +	}
> +#else
>  	atomic_dec(&rq->nr_iowait);
> +#endif
>  	delayacct_blkio_end();
>  	return ret;
>  }
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 47ed7cf..d78c942 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -408,15 +408,27 @@ static void tick_nohz_update_jiffies(ktime_t now)
>  
>  static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
>  {
> -	ktime_t delta;
> +	ktime_t delta, entry, end;
>  
>  	/* Updates the per cpu time idle statistics counters */
>  	write_seqcount_begin(&ts->idle_sleeptime_seq);
> -	delta = ktime_sub(now, ts->idle_entrytime);
> -	if (ts->idle_active == 2)
> +	entry = ts->idle_entrytime;
> +	delta = ktime_sub(now, entry);
> +	if (ts->idle_active == 2) {
> +		end = ts->iowait_exittime;
> +		if (end.tv64 != KTIME_MAX) {
> +			/*
> +			 * Last iowaiting task on our rq was woken up on other CPU
> +			 * sometime in the past, it updated ts->iowait_exittime.
> +			 */
> +			delta = ktime_sub(now, end);
> +			ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
> +			delta = ktime_sub(end, entry);
> +		}
>  		ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
> -	else
> +	} else {
>  		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
> +	}
>  	ts->idle_active = 0;
>  	write_seqcount_end(&ts->idle_sleeptime_seq);
>  
> @@ -430,6 +442,7 @@ static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
>  	write_seqcount_begin(&ts->idle_sleeptime_seq);
>  	ts->idle_entrytime = now;
>  	ts->idle_active = nr_iowait_cpu(smp_processor_id()) ? 2 : 1;
> +	ts->iowait_exittime.tv64 = KTIME_MAX;
>  	write_seqcount_end(&ts->idle_sleeptime_seq);
>  
>  	sched_clock_idle_sleep_event();
> @@ -437,6 +450,16 @@ static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
>  	return now;
>  }
>  
> +void tick_nohz_iowait_to_idle(int cpu)
> +{
> +	struct tick_sched *ts = tick_get_tick_sched(cpu);
> +	ktime_t now = ktime_get();
> +
> +	write_seqcount_begin(&ts->idle_sleeptime_seq);
> +	ts->iowait_exittime = now;
> +	write_seqcount_end(&ts->idle_sleeptime_seq);

So now you have two concurrent updaters using the seqcount, which is
very dangerous as the counters aren't updated atomically.

seqcount is only suitable when there is a single sequential updater.
Once you deal with concurrent updaters you need seqlock.

And once you add seqlock in the hot scheduler path, you're hitting
a big scalability issue.