On Mon, Sep 21, 2020 at 10:10 AM Yunsheng Lin <linyunsh...@huawei.com> wrote: > > On 2020/9/21 15:19, Eric Dumazet wrote: > > On Mon, Sep 21, 2020 at 4:08 AM Yunsheng Lin <linyunsh...@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 <linyunsh...@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().
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.