netdev - Re: [PATCH net-next] net: save some cycles when doing skb_attempt_defer

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Date: Thu, 11 Apr 2024 09:11:53 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: pablo@...filter.org, kuba@...nel.org, pabeni@...hat.com, 
	davem@...emloft.net, horms@...nel.org, aleksander.lobakin@...el.com, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next] net: save some cycles when doing skb_attempt_defer_free()

On Thu, Apr 11, 2024 at 8:33 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> On Thu, Apr 11, 2024 at 1:27 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Thu, Apr 11, 2024 at 5:25 AM Jason Xing <kerneljasonxing@...il.com> wrote:
> > >
> > > From: Jason Xing <kernelxing@...cent.com>
> > >
> > > Normally, we don't face these two exceptions very often meanwhile
> > > we have some chance to meet the condition where the current cpu id
> > > is the same as skb->alloc_cpu.
> > >
> > > One simple test that can help us see the frequency of this statement
> > > 'cpu == raw_smp_processor_id()':
> > > 1. running iperf -s and iperf -c [ip] -P [MAX CPU]
> > > 2. using BPF to capture skb_attempt_defer_free()
> > >
> > > I can see around 4% chance that happens to satisfy the statement.
> > > So moving this statement at the beginning can save some cycles in
> > > most cases.
> > >
> > > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> > > ---
> > >  net/core/skbuff.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > index ab970ded8a7b..b4f252dc91fb 100644
> > > --- a/net/core/skbuff.c
> > > +++ b/net/core/skbuff.c
> > > @@ -7002,9 +7002,9 @@ void skb_attempt_defer_free(struct sk_buff *skb)
> > >         unsigned int defer_max;
> > >         bool kick;
> > >
> > > -       if (WARN_ON_ONCE(cpu >= nr_cpu_ids) ||
> > > +       if (cpu == raw_smp_processor_id() ||
> > >             !cpu_online(cpu) ||
> > > -           cpu == raw_smp_processor_id()) {
> > > +           WARN_ON_ONCE(cpu >= nr_cpu_ids)) {
> > >  nodefer:       kfree_skb_napi_cache(skb);
> > >                 return;
> > >         }
> >
> > Wrong patch.
> >
> > cpu_online(X) is undefined and might crash if X is out of bounds on CONFIG_SMP=y
>
> Even if skb->alloc_cpu is larger than nr_cpu_ids, I don't know why the
> integer test statement could cause crashing the kernel. It's just a
> simple comparison. And if the statement is true,
> raw_smp_processor_id() can guarantee the validation, right?

Please read again the code you wrote, or run it with skb->alloc_cpu
being set to 45000 on a full DEBUG kernel.

You are focusing on skb->alloc_cpu == raw_smp_processor_id(), I am
focusing on what happens
when this condition is not true.