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

[Alexander Duyck <alexander.duyck@gmail.com>]

On Thu, Feb 3, 2022 at 9:34 AM Eric Dumazet <edumazet@google.com> wrote:
>
> On Thu, Feb 3, 2022 at 9:26 AM Alexander H Duyck
> <alexander.duyck@gmail.com> wrote:
> >
> > On Wed, 2022-02-02 at 17:51 -0800, Eric Dumazet wrote:
> > > From: Eric Dumazet <edumazet@google.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@google.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 ?

This is all code in the drivers. Most drivers have them as the logic
is used to avoid having to return NETIDEV_TX_BUSY. Basically the
assumption is there is a 1:1 correlation between descriptors and
individual frags. So most drivers would need to increase the size of
their Tx descriptor rings if they were optimized for a lower value.

The other thing is that most of the tuning for things like interrupt
moderation assume a certain fill level on the queues and those would
likely need to be updated to account for this change.

> 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.

A config option would probably be preferred. The big issue as I see it
is that changing MAX_SKB_FRAGS is going to have ripples throughout the
ecosystem as the shared info size will be increasing and the queueing
behavior for most drivers will be modified as a result.