Re: [PATCH 1/7] sched_ext: Use rhashtable_lookup() instead of rhashtable_lookup_fast()

[Andrea Righi <arighi@nvidia.com>]

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@kernel.org>

It looks like the ttwu_queue() path isn't RCU read lock protected.
With this applied:

[    6.647598] =============================
[    6.647603] WARNING: suspicious RCU usage
[    6.647605] 6.17.0-rc7-virtme #1 Not tainted
[    6.647608] -----------------------------
[    6.647608] ./include/linux/rhashtable.h:602 suspicious rcu_dereference_check() usage!
[    6.647610]
[    6.647610] other info that might help us debug this:
[    6.647610]
[    6.647612]
[    6.647612] rcu_scheduler_active = 2, debug_locks = 1
[    6.647613] 1 lock held by swapper/10/0:
[    6.647614]  #0: ffff8b14bbb3cc98 (&rq->__lock){-.-.}-{2:2}, at: raw_spin_rq_lock_nested+0x20/0x90
[    6.647630]
[    6.647630] stack backtrace:
[    6.647633] CPU: 10 UID: 0 PID: 0 Comm: swapper/10 Not tainted 6.17.0-rc7-virtme #1 PREEMPT(full)
[    6.647643] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[    6.647646] Sched_ext: beerland_1.0.2_g27d63fc3_x86_64_unknown_linux_gnu (enabled+all)
[    6.647648] Call Trace:
[    6.647652]  <IRQ>
[    6.647655]  dump_stack_lvl+0x78/0xe0
[    6.647665]  lockdep_rcu_suspicious+0x14a/0x1b0
[    6.647672]  __rhashtable_lookup.constprop.0+0x1d5/0x250
[    6.647680]  find_dsq_for_dispatch+0xbc/0x190
[    6.647684]  do_enqueue_task+0x25b/0x550
[    6.647689]  enqueue_task_scx+0x21d/0x360
[    6.647692]  ? trace_lock_acquire+0x22/0xb0
[    6.647695]  enqueue_task+0x2e/0xd0
[    6.647698]  ttwu_do_activate+0xa2/0x290
[    6.647703]  sched_ttwu_pending+0xfd/0x250
[    6.647706]  __flush_smp_call_function_queue+0x1cd/0x610
[    6.647714]  __sysvec_call_function_single+0x34/0x150
[    6.647720]  sysvec_call_function_single+0x6e/0x80
[    6.647726]  </IRQ>
[    6.647726]  <TASK>
[    6.647727]  asm_sysvec_call_function_single+0x1a/0x20

Should we revert this?

Thanks,
-Andrea

> ---
>  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)
>  {
> -	return rhashtable_lookup_fast(&sch->dsq_hash, &dsq_id, dsq_hash_params);
> +	return rhashtable_lookup(&sch->dsq_hash, &dsq_id, dsq_hash_params);
>  }
>  
>  /*
> -- 
> 2.51.0
>