linux-kernel - Re: [PATCH net-next] net: dsa: Add support for 64-bit statistics

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Date:   Thu, 3 Aug 2017 12:43:03 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        vivien.didelot@...oirfairelinux.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] net: dsa: Add support for 64-bit statistics

On 08/03/2017 11:11 AM, Andrew Lunn wrote:
> On Thu, Aug 03, 2017 at 10:30:56AM -0700, Florian Fainelli wrote:
>> On 08/02/2017 04:49 PM, David Miller wrote:
>>> From: Florian Fainelli <f.fainelli@...il.com>
>>> Date: Tue,  1 Aug 2017 15:00:36 -0700
>>>
>>>> DSA slave network devices maintain a pair of bytes and packets counters
>>>> for each directions, but these are not 64-bit capable. Re-use
>>>> pcpu_sw_netstats which contains exactly what we need for that purpose
>>>> and update the code path to report 64-bit capable statistics.
>>>>
>>>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>>>
>>> Applied, thanks.
>>>
>>> I would run ethtool -S and ifconfig under perf to see where it is
>>> spending so much time.
>>>
>>
>> This appears to be way worse than I thought, will keep digging, but for
>> now, I may have to send a revert. Andrew, Vivien can you see if you have
>> the same problems on your boards? Thanks!
>>
>> # killall iperf
>> # [ ID] Interval       Transfer     Bandwidth
>> [  3]  0.0-19.1 sec   500 MBytes   220 Mbits/sec
>> # while true; do ethtool -S gphy; ifconfig gphy; done
>> ^C^C
>>
>>
>> [   64.566226] INFO: rcu_sched self-detected stall on CPU
>> [   64.571487]  0-...: (25999 ticks this GP) idle=006/140000000000001/0
> 
> Hi Florian

Hi Andrew,

> 
> I don't get anything so bad, but i think that is because of hardware
> restrictions. I see the ethtool; ifconfig loop goes a lot slower when
> there is iperf traffic, but i don't get an RCU stall. However, the
> board i tested on only has a 100Mbps CPU interface, and it can handle
> all that traffic without pushing the CPU to 100%. What is the CPU load
> when you run your test? Even if you are going to 100% CPU load, we
> still don't want RCU stalls.

This is a quad core 1.5 Ghz board pushing 1Gbit/sec worth of traffic,
this is about 25% loaded. What is needed to reproduce this is basically:

iperf -c 192.168.1.1 -t 30 &
while true; do ifconfig gphy; ethtool -S gphy; done

when iperf terminates, the lock-up reliably occurs. I just converted
net/dsa/ to use per-cpu statistics and of course, I can no longer
reproduce this problem now...
-- 
Florian