netdev - Re: [PATCH net] net: fix race between napi kthread mode and busy poll

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Date:   Thu, 25 Feb 2021 00:59:25 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Wei Wang <weiwan@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Alexander Duyck <alexanderduyck@...com>,
        Martin Zaharinov <micron10@...il.com>
Subject: Re: [PATCH net] net: fix race between napi kthread mode and busy poll

On Thu, Feb 25, 2021 at 12:52 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Wed, 24 Feb 2021 23:30:23 +0100 Eric Dumazet wrote:
> > On Wed, Feb 24, 2021 at 10:30 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > > Just to find out what the LoC is I sketched this out:
> > >
> > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > > index ddf4cfc12615..77f09ced9ee4 100644
> > > --- a/include/linux/netdevice.h
> > > +++ b/include/linux/netdevice.h
> > > @@ -348,6 +348,7 @@ struct napi_struct {
> > >         struct hlist_node       napi_hash_node;
> > >         unsigned int            napi_id;
> > >         struct task_struct      *thread;
> > > +       struct list_head        thread_poll_list;
> > >  };
> >
> > offlist, since it seems this conversation is upsetting you.
>
> Interesting, vger seems to be CCed but it isn't appearing on the ML.
> Perhaps just a vger delay :S
>
> Not really upsetting. I'm just trying to share what I learned devising
> more advanced pollers. The bits get really messy really quickly.
> Especially that the proposed fix adds a bit for a poor bystander (busy
> poll) while it's the threaded IRQ that is incorrectly not preserving
> its ownership.
>
> > Additional 16 bytes here, possibly in a shared cache line, [1]
> > I prefer using a bit in hot n->state, we have plenty of them available.
>
> Right, presumably the location of the new member could be optimized.
> I typed this proposal up in a couple of minutes.
>
> > We worked hours with Alexander, Wei, I am sorry you think we did a poor job.
> > I really thought we instead solved the issue at hand.
> >
> > May I suggest you defer your idea of redesigning the NAPI model for
> > net-next ?
>
> Seems like you decided on this solution off list and now the fact that
> there is a discussion on the list is upsetting you. May I suggest that
> discussions should be conducted on list to avoid such situations?

We were trying to not pollute the list (with about 40 different emails so far)

(Note this was not something I initiated, I only hit Reply all button)

OK, I will shut up, since you seem to take over this matter, and it is
1am here in France.