Re: [PATCH net-next] net: use in_softirq() to indicate the NAPI context in napi_consume_skb()

[Eric Dumazet]

On Mon, Sep 21, 2020 at 10:10 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>
> On 2020/9/21 15:19, Eric Dumazet wrote:
> > On Mon, Sep 21, 2020 at 4:08 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> >>
> >> When napi_consume_skb() is called in the tx desc cleaning process,
> >> it is usually in the softirq context(BH disabled, or are processing
> >> softirqs), but it may also be in the task context, such as in the
> >> netpoll or loopback selftest process.
> >>
> >> Currently napi_consume_skb() uses non-zero budget to indicate the
> >> NAPI context, the driver writer may provide the wrong budget when
> >> tx desc cleaning function is reused for both NAPI and non-NAPI
> >> context, see [1].
> >>
> >> So this patch uses in_softirq() to indicate the NAPI context, which
> >> doesn't necessarily mean in NAPI context, but it shouldn't care if
> >> NAPI context or not as long as it runs in softirq context or with BH
> >> disabled, then _kfree_skb_defer() will push the skb to the particular
> >> cpu' napi_alloc_cache atomically.
> >>
> >> [1] https://lkml.org/lkml/2020/9/15/38
> >>
> >> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
> >> ---
> >> note that budget parameter is not removed in this patch because it
> >> involves many driver changes, we can remove it in separate patch if
> >> this patch is accepted.
> >> ---
> >>  net/core/skbuff.c | 6 ++++--
> >>  1 file changed, 4 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> >> index e077447..03d0d28 100644
> >> --- a/net/core/skbuff.c
> >> +++ b/net/core/skbuff.c
> >> @@ -895,8 +895,10 @@ void __kfree_skb_defer(struct sk_buff *skb)
> >>
> >>  void napi_consume_skb(struct sk_buff *skb, int budget)
> >>  {
> >> -       /* Zero budget indicate non-NAPI context called us, like netpoll */
> >> -       if (unlikely(!budget)) {
> >> +       /* called by non-softirq context, which usually means non-NAPI
> >> +        * context, like netpoll.
> >> +        */
> >> +       if (unlikely(!in_softirq())) {
> >>                 dev_consume_skb_any(skb);
> >>                 return;
> >>         }
> >> --
> >
> >
> > I do not think we should add this kind of fuzzy logic, just because
> > _one_ driver author made a mistake.
> >
> > Add a disable_bh() in the driver slow path, and accept the _existing_
> > semantic, the one that was understood by dozens.
>
> As my understanding, this patch did not change _existing_ semantic,
> it still only call _kfree_skb_defer() in softirq context. This patch
> just remove the requirement that a softirq context hint need to be
> provided to decide whether calling _kfree_skb_defer().

I do not want to remove the requirement.

>
> Yes, we can add DEBUG_NET() clauses to catch this kind of error as
> you suggested.
>
> But why we need such a debug clauses, when we can decide if delaying
> skb freeing is possible in napi_consume_skb(), why not just use
> in_softirq() to make this API more easy to use? Just as __dev_kfree_skb_any()
> API use "in_irq() || irqs_disabled()" checking to handle the irq context
> and non-irq context.


I just do not like your patch.

Copying another piece of fuzzy logic, inherited from legacy code is
not an excuse.

Add a local_bh_disable() in the driver slow path to meet _existing_
requirement, so that we can keep the hot path fast.