netdev - Re: [PATCH net-next] r8152: divide the tx and rx bottom functions

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Date:   Fri, 16 Aug 2019 10:19:33 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Hayes Wang <hayeswang@...ltek.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc:     nic_swsd <nic_swsd@...ltek.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next] r8152: divide the tx and rx bottom functions



On 8/16/19 10:10 AM, Hayes Wang wrote:
> Eric Dumazet [mailto:eric.dumazet@...il.com]
>> Sent: Friday, August 16, 2019 2:40 PM
> [...]
>> tasklet and NAPI are scheduled on the same core (the current
>> cpu calling napi_schedule() or tasklet_schedule())
>>
>> I would rather not add this dubious tasklet, and instead try to understand
>> what is wrong in this driver ;)
>>
>> The various napi_schedule() calls are suspect IMO.
> 
> The original method as following.
> 
> static int r8152_poll(struct napi_struct *napi, int budget)
> {
> 	struct r8152 *tp = container_of(napi, struct r8152, napi);
> 	int work_done;
> 
> 	work_done = rx_bottom(tp, budget); <-- RX
> 	bottom_half(tp); <-- Tx (tx_bottom)
> 	[...]
> 
> The rx_bottom and tx_bottom would only be called in r8152_poll.
> That is, tx_bottom wouldn't be run unless rx_bottom is finished.
> And, rx_bottom would be called if tx_bottom is running.
> 
> If the traffic is busy. rx_bottom or tx_bottom may take a lot
> of time to deal with the packets. And the one would increase
> the latency time for the other one.
> 
> Therefore, when I separate the tx_bottom and rx_bottom to
> different tasklet and napi, the callback functions of tx and
> rx may schedule the tasklet and napi to different cpu. Then,
> the rx_bottom and tx_bottom may be run at the same time.


Your patch makes absolutely no guarantee of doing what you
want, I am afraid.

> 
> Take our arm platform for example. There are five cpus to
> handle the interrupt of USB host controller. When the rx is
> completed, cpu #1 may handle the interrupt and napi would
> be scheduled. When the tx is finished, cpu #2 may handle
> the interrupt and the tasklet is scheduled. Then, napi is
> run on cpu #1, and tasklet is run on cpu #2.
> 
>> Also rtl8152_start_xmit() uses skb_queue_tail(&tp->tx_queue, skb);
>>
>> But I see nothing really kicking the transmit if tx_free is empty ?
> 
> Tx callback function "write_bulk_callback" would deal with it.
> The callback function would check if there are packets waiting
> to be sent.

Which callback ?

After an idle period (no activity, no prior packets being tx-completed ...),
a packet is sent by the upper stack, enters the ndo_start_xmit() of a network driver.

This driver ndo_start_xmit() simply adds an skb to a local list, and returns.

Where/how is scheduled this callback ?

Some kind of timer ?
An (unrelated) incoming packet ?

> 
> 
> Best Regards,
> Hayes
> 
>