Re: [PATCH 3/3] nohz: Fix iowait overcounting if iowait task migrates

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 23, 2014 at 09:00:36PM +0200, Denys Vlasenko wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9cae286..08dd220 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4255,6 +4255,9 @@ EXPORT_SYMBOL_GPL(yield_to);
>   */
>  void __sched io_schedule(void)
>  {
> +#ifdef CONFIG_NO_HZ_COMMON
> +	int cpu_on_entry = smp_processor_id();
> +#endif
>  	struct rq *rq = raw_rq();
>  
>  	delayacct_blkio_start();
> @@ -4263,13 +4266,23 @@ 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 (smp_processor_id() != cpu_on_entry)
> +			tick_nohz_iowait_to_idle(cpu_on_entry);
> +	}
> +#else
>  	atomic_dec(&rq->nr_iowait);
> +#endif
>  	delayacct_blkio_end();
>  }
>  EXPORT_SYMBOL(io_schedule);
>  
>  long __sched io_schedule_timeout(long timeout)
>  {
> +#ifdef CONFIG_NO_HZ_COMMON
> +	int cpu_on_entry = smp_processor_id();
> +#endif
>  	struct rq *rq = raw_rq();
>  	long ret;
>  
> @@ -4279,7 +4292,14 @@ 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 (smp_processor_id() != cpu_on_entry)
> +			tick_nohz_iowait_to_idle(cpu_on_entry);
> +	}
> +#else
>  	atomic_dec(&rq->nr_iowait);
> +#endif
>  	delayacct_blkio_end();
>  	return ret;
>  }

Why do you insist on writing the same (buggy, see later) code twice?
Really, get lazy already and write it once but use it twice!

Its buggy because the smp_processor_id() is used in preemptible context,
its further buggy because the raw_rq() does it again and could get a rq
on a different cpu.

What you want is something like:

static inline void io_wait_start(struct rq *rq)
{
	atomic_inc(&rq->nr_iowait);
	current->in_iowait = 1;
}

static inline void io_wait_end(struct rq *rq)
{
	current->in_iowait = 0;
#ifdef CONFIG_NO_HZ_COMMON
	if (atomic_dec_and_test(&rq->nr_iowait) && 
		cpu_of(rq) != raw_smp_processor_id()) {
		tick_nohz_iowait_end(cpu_of(rq));
	}
#else
	atomic_dec(&rq->nr_iowait);
#endif
}

Anyway, I suspect that's still broken and you really need that lock
around the state like I did earlier, because the above isn't serialized
between remote wakeup and the cpu waking from nohz.