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

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2020/9/21 15:19, Eric Dumazet wrote:
> On Mon, Sep 21, 2020 at 4:08 AM Yunsheng Lin <linyunsheng@huawei.com> 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@huawei.com>
>> ---
>> 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().

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.

> .
>