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

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Message-ID: <aNLrL4QKoaljw01m@gpd4>
Date: Tue, 23 Sep 2025 20:47:11 +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()

On Tue, Sep 23, 2025 at 05:59:47AM -1000, Tejun Heo wrote:
> On Tue, Sep 23, 2025 at 09:18:33AM +0200, Andrea Righi wrote:
> > 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?
> 
> rhashtable_lookup() already has that as it uses rcu_dereference_check()
> internally.

Oh that's right, I see it now. Then looks good.

Acked-by: Andrea Righi <arighi@...dia.com>

-Andrea