netdev - Re: [Patch net-next v3 3/3] net_sched: implement ->change_tx_queue_len() for pfifo

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Message-ID: <CAM_iQpWLNhm-k_2Ncv2D=K1FCnUJtcqfemu6oKt6DXQk7rQy7g@mail.gmail.com>
Date:   Thu, 25 Jan 2018 20:01:42 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        John Fastabend <john.fastabend@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [Patch net-next v3 3/3] net_sched: implement ->change_tx_queue_len()
 for pfifo_fast

On Thu, Jan 25, 2018 at 7:57 PM, Jason Wang <jasowang@...hat.com> wrote:
>
>
> On 2018年01月26日 10:26, Cong Wang wrote:
>>
>> pfifo_fast used to drop based on qdisc_dev(qdisc)->tx_queue_len,
>> so we have to resize skb array when we change tx_queue_len.
>>
>> Other qdiscs which read tx_queue_len are fine because they
>> all save it to sch->limit or somewhere else in qdisc during init.
>> They don't have to implement this, it is nicer if they do so
>> that users don't have to re-configure qdisc after changing
>> tx_queue_len.
>>
>> Cc: John Fastabend <john.fastabend@...il.com>
>> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
>> ---
>>   net/sched/sch_generic.c | 18 ++++++++++++++++++
>>   1 file changed, 18 insertions(+)
>>
>> diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
>> index 08f9fa27e06e..190570f21b20 100644
>> --- a/net/sched/sch_generic.c
>> +++ b/net/sched/sch_generic.c
>> @@ -763,6 +763,23 @@ static void pfifo_fast_destroy(struct Qdisc *sch)
>>         }
>>   }
>>   +static int pfifo_fast_change_tx_queue_len(struct Qdisc *sch,
>> +                                         unsigned int new_len)
>> +{
>> +       struct pfifo_fast_priv *priv = qdisc_priv(sch);
>> +       struct skb_array *bands[PFIFO_FAST_BANDS];
>> +       int prio;
>> +
>> +       for (prio = 0; prio < PFIFO_FAST_BANDS; prio++) {
>> +               struct skb_array *q = band2list(priv, prio);
>> +
>> +               bands[prio] = q;
>> +       }
>> +
>> +       return skb_array_resize_multiple(bands, PFIFO_FAST_BANDS, new_len,
>> +                                        GFP_KERNEL);
>> +}
>> +
>>   struct Qdisc_ops pfifo_fast_ops __read_mostly = {
>>         .id             =       "pfifo_fast",
>>         .priv_size      =       sizeof(struct pfifo_fast_priv),
>> @@ -773,6 +790,7 @@ struct Qdisc_ops pfifo_fast_ops __read_mostly = {
>>         .destroy        =       pfifo_fast_destroy,
>>         .reset          =       pfifo_fast_reset,
>>         .dump           =       pfifo_fast_dump,
>> +       .change_tx_queue_len =  pfifo_fast_change_tx_queue_len,
>>         .owner          =       THIS_MODULE,
>>         .static_flags   =       TCQ_F_NOLOCK | TCQ_F_CPUSTATS,
>>   };
>
>
> Is __skb_array_empty() in pfifo_fast_dequeue() still safe after this change?

Yes, we sync with dequeue path before calling ->change_tx_queue_len().
I already mentioned this in patch 2/3.