netdev - Re: TI CPSW Ethernet Tx performance regression

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Date:	Mon, 3 Feb 2014 11:24:49 -0800
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Mugunthan V N <mugunthanvnm@...com>
Cc:	netdev <netdev@...r.kernel.org>,
	Ben Hutchings <ben@...adent.org.uk>
Subject: Re: TI CPSW Ethernet Tx performance regression

2014-01-15 Mugunthan V N <mugunthanvnm@...com>:
> Hi
>
> On Thursday 16 January 2014 02:51 AM, Florian Fainelli wrote:
>> 2014/1/15 Ben Hutchings <bhutchings@...arflare.com>:
>>> On Wed, 2014-01-15 at 18:18 +0530, Mugunthan V N wrote:
>>>> Hi
>>>>
>>>> I am seeing a performance regression with CPSW driver on AM335x EVM. AM335x EVM
>>>> CPSW has 3.2 kernel support [1] and Mainline support from 3.7. When I am
>>>> comparing the performance between 3.2 and 3.13-rc4. TCP receive performance of
>>>> CPSW between 3.2 and 3.13-rc4 is same (~180Mbps) but TCP Transmit performance
>>>> is poor comparing to 3.2 kernel. In 3.2 kernel is it *256Mbps* and in 3.13-rc4
>>>> it is *70Mbps*
>>>>
>>>> Iperf version is *iperf version 2.0.5 (08 Jul 2010) pthreads* on both PC and EVM
>>>>
>>>> On UDP transmit also performance is down comparing to 3.2 kernel. In 3.2 it is
>>>> 196Mbps for 200Mbps band width and in 3.13-rc4 it is 92Mbps
>>>>
>>>> Can someone point me out where can I look for improving Tx performance. I also
>>>> checked whether there is Tx descriptor over flow and there is none. I have
>>>> tries 3.11 and some older kernel, all are giving ~75Mbps Transmit performance
>>>> only.
>>>>
>>>> [1] - http://arago-project.org/git/projects/?p=linux-am33x.git;a=summary
>>> If you don't get any specific suggestions, you could try bisecting to
>>> find out which specific commit(s) changed the performance.
>> Not necessarily related to that issue, but there are a few
>> weird/unusual things done in the CPSW interrupt handler:
>>
>> static irqreturn_t cpsw_interrupt(int irq, void *dev_id)
>> {
>>         struct cpsw_priv *priv = dev_id;
>>
>>         cpsw_intr_disable(priv);
>>         if (priv->irq_enabled == true) {
>>                 cpsw_disable_irq(priv);
>>                 priv->irq_enabled = false;
>>         }
>>
>>         if (netif_running(priv->ndev)) {
>>                 napi_schedule(&priv->napi);
>>                 return IRQ_HANDLED;
>>         }
>>
>> Checking for netif_running() should not be required, you should not
>> get any TX/RX interrupts if your interface is not running.
>
> The driver also supports Dual EMAC with one physical device. More
> description can be found in [1] under the topic *9.2.1.5.2 Dual Mac
> Mode*. If the first interface is down and the second interface is up,
> without checking the interface we will not know which napi to schedule.
>
>>
>>
>>         priv = cpsw_get_slave_priv(priv, 1);
>>         if (!priv)
>>                 return IRQ_NONE;
>>
>> Should not this be moved up as the very first conditional check to do?
>> is not there a risk to leave the interrupts disabled and not
>> re-enabled due to the first 5 lines at the top?
>
> This has to be kept here to check if the interrupt is triggered by the
> second Ethernet port interface when the first interface is down.

Ok,the priv pointer when we enter the interrupt handler could point to
e.g: slave 0, so we need to get it re-assigned to the second slave
using cpsw_get_slave_priv(). How do you ensure that "priv" at the
beginning of the interrupt handler does not already point to slave 1?
In that case, is not there a chance to starve slave 0, or at least
cause an excessive latency by exiting the interrupt handler for slave
1, and then re-entering it for slave 0?

-- 
Florian
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