netdev - Re: [PATCH net] ixgbe: check return value of napi_complete

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Date:   Fri, 21 Sep 2018 07:55:06 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Song Liu <songliubraving@...com>
CC:     Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        netdev <netdev@...r.kernel.org>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH net] ixgbe: check return value of napi_complete_done()

On 9/21/18 6:33 AM, Eric Dumazet wrote:
>
>
> On 09/21/2018 12:17 AM, Song Liu wrote:
>>
>>
>>> On Sep 20, 2018, at 4:49 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>>
>>>
>>>
>>> On 09/20/2018 04:43 PM, Song Liu wrote:
>>>>
>>>
>>>> I tried to totally skip ndo_poll_controller() here. It did avoid hitting
>>>> the issue. However, netpoll will drop (fail to send) more packets.
>>>>
>>>
>>> Why is it failing ?
>>>
>>> If you are under high memory pressure, then maybe if you absolutely want memory to send
>>> netpoll packets, you want to grab all NAPI contexts as a way to prevent other cpus
>>> from feeding incoming packets to the host and add more memory pressure ;)
>>>
>>
>> I did the test with Eric's latest patch (and disable ndo_poll_controller
>> in driver). The result didn't show significant increase in drop packets.
>> I guess packet drops in my earlier test was caused by some other changes
>> I mixed there.
>>
>> So I think this patch does fix the issue. Thanks Eric!
>
> Great, this is awesome.
>
> I will prepare a patch series for net tree.
>
> The core infrastructure is just better at being able to drain TX completions
> without risking stealing the NAPI context forever.

should we remove ndo_poll_controller then?
My understanding that the patch helps by not letting
drivers do napi_schedule() for all queues into this_cpu, right?
But most of the drivers do exactly that in their ndo_poll_controller
implementations. Means most of the drivers will experience
this nasty behavior.