linux-kernel - Re: [PATCH net-next 6/6] net: hns3: use napi_consume

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Date:   Wed, 16 Sep 2020 01:33:35 -0700
From:   Saeed Mahameed <saeed@...nel.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        "tanhuazhong@...wei.com" <tanhuazhong@...wei.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     "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 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);