netdev - Re: [PATCH net 00/15] netpoll: avoid capture effects for NAPI drivers

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Message-ID: <CANn89iL46aONnTP6+Mb=tSq2-ywntAMwasLEuCs8VNBd5jqitg@mail.gmail.com>
Date:   Mon, 24 Sep 2018 13:56:04 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     songliubraving@...com
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, michael.chan@...adcom.com,
        Ariel Elior <ariel.elior@...ium.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        jakub.kicinski@...ronome.com, Jay Vosburgh <j.vosburgh@...il.com>,
        Veaceslav Falico <vfalico@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>
Subject: Re: [PATCH net 00/15] netpoll: avoid capture effects for NAPI drivers

On Mon, Sep 24, 2018 at 1:00 PM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Sep 24, 2018, at 12:41 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Mon, Sep 24, 2018 at 12:31 PM Song Liu <songliubraving@...com> wrote:
> >
> >> This triggers dev_watchdog() on a simple netperf TCP_RR on bnxt (I haven't
> >> tested other drivers yet).
> >>
> >> I guess this is because NAPI_STATE_SCHED is set when poll_one_napi() calls
> >> napi->poll(). And then cleared by napi->poll().
> >
> > Which part of napi->poll() could possibly clear NAPI_STATE_SCHED when
> > called by netpoll ?
> >
> > AFAIK, napi_complete_done() should exit early (before having a chance
> > to clear NAPI_STATE_SCHED)
> > because of :
> >
> > if (unlikely(n->state & (NAPIF_STATE_NPSVC | NAPIF_STATE_IN_BUSY_POLL)))
> >     return false;
> >
> > Thanks !
>
> You are right on this condition. But this does trigger dev_watchdog() for
> some reason.

Interesting, maybe a bnxt specific issue.

It seems their model is to process TX/RX notification in the same queue,
they throw away RX events if budget == 0

It means commit e7b9569102995ebc26821789628eef45bd9840d8 is wrong and
must be reverted.

Otherwise, we have a possibility of blocking a queue under netpoll pressure.