netdev - Re: [PATCH net-next 09/15] net: increase MAX_SKB

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Message-ID: <CANn89iKxGvbXQqoRZZ5j22-5YkpiCLS13EGoQ1OYe3EHjEss6A@mail.gmail.com>
Date:   Thu, 3 Feb 2022 09:34:43 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alexander H Duyck <alexander.duyck@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>, Coco Li <lixiaoyan@...gle.com>
Subject: Re: [PATCH net-next 09/15] net: increase MAX_SKB_FRAGS

On Thu, Feb 3, 2022 at 9:26 AM Alexander H Duyck
<alexander.duyck@...il.com> wrote:
>
> On Wed, 2022-02-02 at 17:51 -0800, Eric Dumazet wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > Currently, MAX_SKB_FRAGS value is 17.
> >
> > For standard tcp sendmsg() traffic, no big deal because tcp_sendmsg()
> > attempts order-3 allocations, stuffing 32768 bytes per frag.
> >
> > But with zero copy, we use order-0 pages.
> >
> > For BIG TCP to show its full potential, we increase MAX_SKB_FRAGS
> > to be able to fit 45 segments per skb.
> >
> > This is also needed for BIG TCP rx zerocopy, as zerocopy currently
> > does not support skbs with frag list.
> >
> > We have used this MAX_SKB_FRAGS value for years at Google before
> > we deployed 4K MTU, with no adverse effect.
> > Back then, goal was to be able to receive full size (64KB) GRO
> > packets without the frag_list overhead.
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>
> So a big issue I see with this patch is the potential queueing issues
> it may introduce on Tx queues. I suspect it will cause a number of
> performance regressions and deadlocks as it will change the Tx queueing
> behavior for many NICs.
>
> As I recall many of the Intel drivers are using MAX_SKB_FRAGS as one of
> the ingredients for DESC_NEEDED in order to determine if the Tx queue
> needs to stop. With this change the value for igb for instance is
> jumping from 21 to 49, and the wake threshold is twice that, 98. As
> such the minimum Tx descriptor threshold for the driver would need to
> be updated beyond 80 otherwise it is likely to deadlock the first time
> it has to pause.

Are these limits hard coded in Intel drivers and firmware, or do you
think this can be changed ?

I could make  MAX_SKB_FRAGS a config option, and default to 17, until
all drivers have been fixed.

Alternative is that I remove this patch from the series and we apply
it to Google production kernels,
as we did before.