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

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Message-ID: <CANn89iLAPvPn2C9J+sSHsQ5EeF0hq47iMiiAfwoQkdZurQiyzA@mail.gmail.com>
Date:   Tue, 18 Sep 2018 06:45:15 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     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, kernel-team <Kernel-team@...com>
Subject: Re: pegged softirq and NAPI race (?)

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://www.spinics.net/lists/netdev/msg522328.html

You have not traced ixgbe to understand why driver hits
"clean_complete=false" all the time ?