linux-kernel - Re: [PATCH v2 2/2] sched: add READ_ONCE and use existing helper function to access ->avg

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Date: Thu, 4 Jan 2024 15:28:13 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
Cc: mingo@...nel.org, peterz@...radead.org, dietmar.eggemann@....com, 
	linux-kernel@...r.kernel.org, srikar@...ux.vnet.ibm.com, yu.c.chen@...el.com, 
	tim.c.chen@...ux.intel.com
Subject: Re: [PATCH v2 2/2] sched: add READ_ONCE and use existing helper
 function to access ->avg_irq

On Mon, 1 Jan 2024 at 16:47, Shrikanth Hegde <sshegde@...ux.vnet.ibm.com> wrote:
>
> Use existing helper function cpu_util_irq instead of referencing it
> directly.
>
> It was noted that avg_irq could be updated by different CPU than the one
> which is trying to access it. avg_irq is updated with WRITE_ONCE. Use
> READ_ONCE to access it in order to avoid any compiler optimizations.
>
> Signed-off-by: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
>  kernel/sched/fair.c  | 4 +---
>  kernel/sched/sched.h | 2 +-
>  2 files changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 1aeca3f943a8..02631060ca7e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9221,10 +9221,8 @@ static inline bool others_have_blocked(struct rq *rq)
>         if (thermal_load_avg(rq))
>                 return true;
>
> -#ifdef CONFIG_HAVE_SCHED_AVG_IRQ
> -       if (READ_ONCE(rq->avg_irq.util_avg))
> +       if (cpu_util_irq(rq))
>                 return true;
> -#endif
>
>         return false;
>  }
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index e58a54bda77d..edc20c5cc7ce 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3125,7 +3125,7 @@ static inline bool uclamp_rq_is_idle(struct rq *rq)
>  #ifdef CONFIG_HAVE_SCHED_AVG_IRQ
>  static inline unsigned long cpu_util_irq(struct rq *rq)
>  {
> -       return rq->avg_irq.util_avg;
> +       return READ_ONCE(rq->avg_irq.util_avg);
>  }
>
>  static inline
> --
> 2.39.3
>