Re: [PATCH v2] wireguard: queueing: simplify wg_cpumask_next_online()

[Yury Norov <yury.norov@gmail.com>]

On Mon, Jun 30, 2025 at 07:38:02PM +0200, Jason A. Donenfeld wrote:
> On Mon, Jun 30, 2025 at 01:33:37PM -0400, Yury Norov wrote:
> > On Mon, Jun 30, 2025 at 07:24:33PM +0200, Jason A. Donenfeld wrote:
> > > On Thu, Jun 19, 2025 at 10:54:59AM -0400, Yury Norov wrote:
> > > > From: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> > > > 
> > > > wg_cpumask_choose_online() opencodes cpumask_nth(). Use it and make the
> > > > function significantly simpler. While there, fix opencoded cpu_online()
> > > > too.
> > > > 
> > > > Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> > > > ---
> > > > v1: https://lore.kernel.org/all/20250604233656.41896-1-yury.norov@gmail.com/
> > > > v2:
> > > >  - fix 'cpu' undeclared;
> > > >  - change subject (Jason);
> > > >  - keep the original function structure (Jason);
> > > > 
> > > >  drivers/net/wireguard/queueing.h | 13 ++++---------
> > > >  1 file changed, 4 insertions(+), 9 deletions(-)
> > > > 
> > > > diff --git a/drivers/net/wireguard/queueing.h b/drivers/net/wireguard/queueing.h
> > > > index 7eb76724b3ed..56314f98b6ba 100644
> > > > --- a/drivers/net/wireguard/queueing.h
> > > > +++ b/drivers/net/wireguard/queueing.h
> > > > @@ -104,16 +104,11 @@ static inline void wg_reset_packet(struct sk_buff *skb, bool encapsulating)
> > > >  
> > > >  static inline int wg_cpumask_choose_online(int *stored_cpu, unsigned int id)
> > > >  {
> > > > -	unsigned int cpu = *stored_cpu, cpu_index, i;
> > > > +	unsigned int cpu = *stored_cpu;
> > > > +
> > > > +	if (unlikely(cpu >= nr_cpu_ids || !cpu_online(cpu)))
> > > > +		cpu = *stored_cpu = cpumask_nth(id % num_online_cpus(), cpu_online_mask);
> > > 
> > > I was about to apply this but then it occurred to me: what happens if
> > > cpu_online_mask changes (shrinks) after num_online_cpus() is evaluated?
> > > cpumask_nth() will then return nr_cpu_ids?
> > 
> > It will return >= nd_cpu_ids. The original version based a for-loop
> > does the same, so I decided that the caller is safe against it.
> 
> Good point. I just checked... This goes into queue_work_on() which
> eventually hits:
> 
>         /* pwq which will be used unless @work is executing elsewhere */
>         if (req_cpu == WORK_CPU_UNBOUND) {
> 
> And it turns out WORK_CPU_UNBOUND is the same as nr_cpu_ids. So I guess
> that's a fine failure mode.

Actually, cpumask_nth_cpu may return >= nr_cpu_ids because of
small_cpumask_nbits optimization. So it's safer to relax the
condition. 

Can you consider applying the following patch for that?

Thanks,
Yury


From fbdce972342437fb12703cae0c3a4f8f9e218a1b Mon Sep 17 00:00:00 2001
From: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Date: Mon, 30 Jun 2025 13:47:49 -0400
Subject: [PATCH] workqueue: relax condition in __queue_work()

Some cpumask search functions may return a number greater than
nr_cpu_ids when nothing is found. Adjust __queue_work() to it.

Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
---
 kernel/workqueue.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 9f9148075828..abacfe157fe6 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2261,7 +2261,7 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
 	rcu_read_lock();
 retry:
 	/* pwq which will be used unless @work is executing elsewhere */
-	if (req_cpu == WORK_CPU_UNBOUND) {
+	if (req_cpu >= WORK_CPU_UNBOUND) {
 		if (wq->flags & WQ_UNBOUND)
 			cpu = wq_select_unbound_cpu(raw_smp_processor_id());
 		else
-- 
2.43.0