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

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Date:   Tue, 25 Sep 2018 07:02:28 -0700
From:   Michael Chan <michael.chan@...adcom.com>
To:     Song Liu <songliubraving@...com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Ariel Elior <ariel.elior@...ium.com>,
        Eric Dumazet <eric.dumazet@...il.com>, tariqt@...lanox.com,
        Saeed Mahameed <saeedm@...lanox.com>,
        jeffrey.t.kirsher@...el.com,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        j.vosburgh@...il.com, 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 2:18 PM Song Liu <songliubraving@...com> wrote:
>
>
>
> > On Sep 24, 2018, at 2:05 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> >
> >>
> >> 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.
> >
> > Hmm, actually a revert might not be enough, since code at lines 2030-2031
> > would fire and we might not call napi_complete_done() anyway.
> >
> > Unfortunately this driver logic is quite complex.
> >
> > Could you test on other NIC eventually ?
> >
>
> It actually runs OK on ixgbe.
>
> @Michael, could you please help us with this?
>
I've taken a quick look using today's net tree plus Eric's
poll_one_napi() patch.  The problem I'm seeing is that netpoll calls
bnxt_poll() with budget 0.  And since work_done >= budget of 0, we
return without calling napi_complete_done() and without arming the
interrupt.  netpoll doesn't always call us back until we call
napi_complete_done(), right?  So I think if there are in-flight TX
completions, we'll miss those.