linux-kernel - Re: [PATCH 1/7] sched_ext: Use rhashtable_lookup() instead of rhashtable_lookup

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Message-ID: <aNJJydxk8ItUpBJd@gpd4>
Date: Tue, 23 Sep 2025 09:18:33 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	linux-kernel@...r.kernel.org, sched-ext@...ts.linux.dev
Subject: Re: [PATCH 1/7] sched_ext: Use rhashtable_lookup() instead of
 rhashtable_lookup_fast()

Hi Tejun,

On Sun, Sep 21, 2025 at 03:32:40PM -1000, Tejun Heo wrote:
> The find_user_dsq() function is called from contexts that are already
> under RCU read lock protection. Switch from rhashtable_lookup_fast() to
> rhashtable_lookup() to avoid redundant RCU locking.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> ---
>  kernel/sched/ext.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index f5873f8ed669..df433f6fab4b 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -207,7 +207,7 @@ static struct scx_dispatch_q *find_global_dsq(struct task_struct *p)
>  
>  static struct scx_dispatch_q *find_user_dsq(struct scx_sched *sch, u64 dsq_id)
>  {

Maybe we should add a WARN_ON_ONCE(!rcu_read_lock_held()) to make sure the
assumption is correct and catch potential non RCU read locked usage in the
future?

> -	return rhashtable_lookup_fast(&sch->dsq_hash, &dsq_id, dsq_hash_params);
> +	return rhashtable_lookup(&sch->dsq_hash, &dsq_id, dsq_hash_params);
>  }
>  
>  /*

Thanks,
-Andrea