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Date:   Wed, 16 Sep 2020 10:38:54 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Saeed Mahameed <saeed@...nel.org>
Cc:     Yunsheng Lin <linyunsheng@...wei.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        "tanhuazhong@...wei.com" <tanhuazhong@...wei.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "yisen.zhuang@...wei.com" <yisen.zhuang@...wei.com>,
        "salil.mehta@...wei.com" <salil.mehta@...wei.com>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kuba@...nel.org" <kuba@...nel.org>
Subject: Re: [PATCH net-next 6/6] net: hns3: use napi_consume_skb() when
 cleaning tx desc

On Wed, Sep 16, 2020 at 10:33 AM Saeed Mahameed <saeed@...nel.org> wrote:
>
> On Tue, 2020-09-15 at 15:04 +0800, Yunsheng Lin wrote:
> > On 2020/9/15 13:09, Saeed Mahameed wrote:
> > > On Mon, 2020-09-14 at 20:06 +0800, Huazhong Tan wrote:
> > > > From: Yunsheng Lin <linyunsheng@...wei.com>
> > > >
> > > > Use napi_consume_skb() to batch consuming skb when cleaning
> > > > tx desc in NAPI polling.
> > > >
> > > > Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> > > > Signed-off-by: Huazhong Tan <tanhuazhong@...wei.com>
> > > > ---
> > > >  drivers/net/ethernet/hisilicon/hns3/hns3_enet.c    | 27
> > > > +++++++++++-
> > > > ----------
> > > >  drivers/net/ethernet/hisilicon/hns3/hns3_enet.h    |  2 +-
> > > >  drivers/net/ethernet/hisilicon/hns3/hns3_ethtool.c |  4 ++--
> > > >  3 files changed, 17 insertions(+), 16 deletions(-)
> > > >
> > > > diff --git a/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
> > > > b/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
> > > > index 4a49a76..feeaf75 100644
> > > > --- a/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
> > > > +++ b/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c
> > > > @@ -2333,10 +2333,10 @@ static int hns3_alloc_buffer(struct
> > > > hns3_enet_ring *ring,
> > > >  }
> > > >
> > > >  static void hns3_free_buffer(struct hns3_enet_ring *ring,
> > > > -                      struct hns3_desc_cb *cb)
> > > > +                      struct hns3_desc_cb *cb, int budget)
> > > >  {
> > > >   if (cb->type == DESC_TYPE_SKB)
> > > > -         dev_kfree_skb_any((struct sk_buff *)cb->priv);
> > > > +         napi_consume_skb(cb->priv, budget);
> > >
> > > This code can be reached from hns3_lb_clear_tx_ring() below which
> > > is
> > > your loopback test and called with non-zero budget, I am not sure
> > > you
> > > are allowed to call napi_consume_skb() with non-zero budget outside
> > > napi context, perhaps the cb->type for loopback test is different
> > > in lb
> > > test case ? Idk.. , please double check other code paths.
> >
> > Yes, loopback test may call napi_consume_skb() with non-zero budget
> > outside
> > napi context. Thanks for pointing out this case.
> >
> > How about add the below WARN_ONCE() in napi_consume_skb() to catch
> > this
> > kind of error?
> >
> > WARN_ONCE(!in_serving_softirq(), "napi_consume_skb() is called with
> > non-zero budget outside napi context");
> >
>
> Cc: Eric
>
> I don't know, need to check performance impact.
> And in_serving_softirq() doesn't necessarily mean in napi
> but looking at _kfree_skb_defer(), i think it shouldn't care if napi or
> not as long as it runs in soft irq it will push the skb to that
> particular cpu napi_alloc_cache, which should be fine.
>
> Maybe instead of the WARN_ONCE just remove the budget condition and
> replace it with
>
> if (!in_serving_softirq())
>       dev_consume_skb_any(skb);
>

I think we need to keep costs small.

So lets add a CONFIG_DEBUG_NET option so that developers can add
various DEBUG_NET() clauses.

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