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

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Message-ID: <aNLD8w2ZILVbERwG@slm.duckdns.org>
Date: Tue, 23 Sep 2025 05:59:47 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
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 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.

Thanks.

-- 
tejun