linux-kernel - Re: [PATCH v2] wireguard: queueing: simplify wg_cpumask_next

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Message-ID: <aGLUKg8uNvNtimW0@yury>
Date: Mon, 30 Jun 2025 14:15:06 -0400
From: Yury Norov <yury.norov@...il.com>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Tejun Heo <tj@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	wireguard@...ts.zx2c4.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] wireguard: queueing: simplify wg_cpumask_next_online()

> > > From fbdce972342437fb12703cae0c3a4f8f9e218a1b Mon Sep 17 00:00:00 2001
> > > From: Yury Norov (NVIDIA) <yury.norov@...il.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@...il.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
> > >
> > 
> > Seems reasonable to me... Maybe submit this to Tejun and CC me?
> 
> Sure, no problem.

Hmm... So, actually WORK_CPU_UNBOUND is NR_CPUS, which is not the same
as nr_cpu_ids. For example, on my Ubuntu machine, the CONFIG_NR_CPUS
is 8192, and nr_cpu_ids is 8.

So, for the wg_cpumask_next_online() to work properly, we need to
return the WORK_CPU_UNBOUND in case of nothing is found.

I think I need to send a v3...

Thanks,
Yury