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

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Date:   Wed, 24 Feb 2021 16:07:23 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
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, 25 Feb 2021 00:59:25 +0100 Eric Dumazet wrote:
> On Thu, Feb 25, 2021 at 12:52 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > 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.

Are you okay with adding a SCHED_THREADED bit for threaded NAPI to be
set in addition to SCHED? At least that way the bit is associated with
it's user. IIUC since the extra clear_bit() in busy poll was okay so
should be a new set_bit()?