linux-kernel - Re: [PATCH 2/4] sched/core: change rq->nr_iowait type to atomic_long

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Date: Wed, 24 Apr 2024 11:56:35 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Jens Axboe <axboe@...nel.dk>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH 2/4] sched/core: change rq->nr_iowait type to
 atomic_long_t on 64-bit

On Tue, Apr 16, 2024 at 06:11:19AM -0600, Jens Axboe wrote:
> In preparation for storing two separate iowait states in there, bump the
> size from a 32-bit to a 64-bit size, for 64-bit kernels.
> 
> Note that on 32-bit, the number of tasks are limited to 0x8000, which
> fits just fine in even half of the existiing 32-bit atomic_t. For 64-bit,
> no such limit exists, hence play it safe and make it a 64-bit atomic.

We still have the tid limit, no? Anyway...

> 
> Signed-off-by: Jens Axboe <axboe@...nel.dk>
> ---
>  kernel/sched/core.c  | 14 +++++++++++++-
>  kernel/sched/sched.h |  4 ++++
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 977bb08a33d2..6a6c985220b1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3723,17 +3723,29 @@ static inline cpumask_t *alloc_user_cpus_ptr(int node)
>  
>  static void task_iowait_inc(struct task_struct *p)
>  {
> +#ifdef CONFIG_64BIT
> +	atomic_long_inc(&task_rq(p)->nr_iowait);
> +#else
>  	atomic_inc(&task_rq(p)->nr_iowait);
> +#endif
>  }
>  
>  static void task_iowait_dec(struct task_struct *p)
>  {
> +#ifdef CONFIG_64BIT
> +	atomic_long_dec(&task_rq(p)->nr_iowait);
> +#else
>  	atomic_dec(&task_rq(p)->nr_iowait);
> +#endif
>  }
>  
>  int rq_iowait(struct rq *rq)
>  {
> +#ifdef CONFIG_64BIT
> +	return atomic_long_read(&rq->nr_iowait);
> +#else
>  	return atomic_read(&rq->nr_iowait);
> +#endif
>  }
>  
>  static void
> @@ -10065,7 +10077,7 @@ void __init sched_init(void)
>  #endif
>  #endif /* CONFIG_SMP */
>  		hrtick_rq_init(rq);
> -		atomic_set(&rq->nr_iowait, 0);
> +		atomic_long_set(&rq->nr_iowait, 0);
>  

This one site lacks the ifdeffery, which seems superfluous anyway, since
long is already 32bit / 64bit like you want. Hmm?

>  #ifdef CONFIG_SCHED_CORE
>  		rq->core = rq;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 387f67ddf18a..c2802d066615 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1049,7 +1049,11 @@ struct rq {
>  	u64			clock_idle_copy;
>  #endif
>  
> +#ifdef CONFIG_64BIT
> +	atomic_long_t		nr_iowait;
> +#else
>  	atomic_t		nr_iowait;
> +#endif
>  
>  #ifdef CONFIG_SCHED_DEBUG
>  	u64 last_seen_need_resched_ns;
> -- 
> 2.43.0
>