netdev - Re: pegged softirq and NAPI race (?)

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Message-ID: <a946c4d2-5e29-b1d4-3af6-8657caeebc11@fb.com>
Date:   Tue, 18 Sep 2018 17:51:11 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Eric Dumazet <edumazet@...gle.com>,
        Song Liu <songliubraving@...com>
CC:     netdev <netdev@...r.kernel.org>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        "michael.chan@...adcom.com" <michael.chan@...adcom.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: pegged softirq and NAPI race (?)

On 9/18/18 6:45 AM, Eric Dumazet wrote:
> On Tue, Sep 18, 2018 at 1:41 AM Song Liu <songliubraving@...com> wrote:
>>
>> We are debugging this issue that netconsole message triggers pegged softirq
>> (ksoftirqd taking 100% CPU for many seconds). We found this issue in
>> production with both bnxt and ixgbe, on a 4.11 based kernel. This is easily
>> reproducible with ixgbe on 4.11, and latest net/net-next (see [1] for more
>> detail).
>>
>> After debugging for some time, we found that this issue is likely related
>> to 39e6c8208d7b ("net: solve a NAPI race"). After reverting this commit,
>> the steps described in [1] cannot reproduce the issue on ixgbe. Reverting
>> this commit also reduces the chances we hit the issue with bnxt (it still
>> happens with a lower rate).
>>
>> I tried to fix this issue with relaxed variant (or older version) of
>> napi_schedule_prep() in netpoll, just like the one on napi_watchdog().
>> However, my tests do not always go as expected.
>>
>> Please share your comments/suggestions on which direction shall we try
>> to fix this.
>>
>> Thanks in advance!
>> Song
>>
>>
>> [1] https://urldefense.proofpoint.com/v2/url?u=https-3A__www.spinics.net_lists_netdev_msg522328.html&d=DwIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=i6WobKxbeG3slzHSIOxTVtYIJw7qjCE6S0spDTKL-J4&m=iSaOapj1kxjhGYLgQr0Qd8mQCzVdobmgT1L4JwFvzxs&s=lCEhrz6wQJUUaJOkxFmtOszAgkf3Jh4reX_i1GbI5RI&e=
>
> You have not traced ixgbe to understand why driver hits
> "clean_complete=false" all the time ?

Eric,

I'm looking at commit 39e6c8208d7b and wondering that it's doing
clear_bit(NAPI_STATE_MISSED,..);
for busy_poll_stop(), but not for netpoll.
Can that be an issue?

and then something like below is needed:
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 57557a6a950c..a848be6b503c 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -172,6 +172,7 @@ static void poll_one_napi(struct napi_struct *napi)
         trace_napi_poll(napi, work, 0);

         clear_bit(NAPI_STATE_NPSVC, &napi->state);
+       clear_bit(NAPI_STATE_MISSED, &napi->state);
  }