Message-ID: <685565722327f_3ffda42943d@willemb.c.googlers.com.notmuch>
Date: Fri, 20 Jun 2025 09:43:14 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jason Xing <kerneljasonxing@...il.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 bjorn@...nel.org, 
 magnus.karlsson@...el.com, 
 maciej.fijalkowski@...el.com, 
 jonathan.lemon@...il.com, 
 sdf@...ichev.me, 
 ast@...nel.org, 
 daniel@...earbox.net, 
 hawk@...nel.org, 
 john.fastabend@...il.com, 
 joe@...a.to, 
 bpf@...r.kernel.org, 
 netdev@...r.kernel.org, 
 Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v3] net: xsk: introduce XDP_MAX_TX_BUDGET
 set/getsockopt

Jason Xing wrote:
> On Thu, Jun 19, 2025 at 9:53 PM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > Jason Xing wrote:
> > > From: Jason Xing <kernelxing@...cent.com>
> > >
> > > The patch does the following things:
> > > - Add XDP_MAX_TX_BUDGET socket option.
> > > - Unify TX_BATCH_SIZE and MAX_PER_SOCKET_BUDGET into single one
> > >   tx_budget_spent.
> > > - tx_budget_spent is set to 32 by default in the initialization phase.
> > >   It's a per-socket granular control.
> > >
> > > The idea behind this comes out of real workloads in production. We use a
> > > user-level stack with xsk support to accelerate sending packets and
> > > minimize triggering syscall. When the packets are aggregated, it's not
> > > hard to hit the upper bound (namely, 32). The moment user-space stack
> > > fetches the -EAGAIN error number passed from sendto(), it will loop to try
> > > again until all the expected descs from tx ring are sent out to the driver.
> > > Enlarging the XDP_MAX_TX_BUDGET value contributes to less frequencies of
> > > sendto(). Besides, applications leveraging this setsockopt can adjust
> > > its proper value in time after noticing the upper bound issue happening.
> > >
> > > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> > > ---
> > > V3
> > > Link: https://lore.kernel.org/all/20250618065553.96822-1-kerneljasonxing@gmail.com/
> > > 1. use a per-socket control (suggested by Stanislav)
> > > 2. unify both definitions into one
> > > 3. support setsockopt and getsockopt
> > > 4. add more description in commit message
> >
> > +1 on an XSK setsockopt only
> 
> May I ask why only setsockopt? In tradition, dev_tx_weight can be read
> and written through running sysctl. I think they are the same?

This is not dev_tx_weight, which is per device.

This is a per-socket choice. The reason for adding it that you gave,
a specific application that is known to be able to batch more than 32,
can tune this configurable in the application.

I see no immediately need to set this at a per netns or global level.
If so, the extra cacheline space in those structs is not warranted.

> >
> > >
> > > V2
> > > Link: https://lore.kernel.org/all/20250617002236.30557-1-kerneljasonxing@gmail.com/
> > > 1. use a per-netns sysctl knob
> > > 2. use sysctl_xsk_max_tx_budget to unify both definitions.
> > > ---
> > >  include/net/xdp_sock.h            |  3 ++-
> > >  include/uapi/linux/if_xdp.h       |  1 +
> > >  net/xdp/xsk.c                     | 36 +++++++++++++++++++++++++------
> > >  tools/include/uapi/linux/if_xdp.h |  1 +
> > >  4 files changed, 34 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> > > index e8bd6ddb7b12..8eecafad92c0 100644
> > > --- a/include/net/xdp_sock.h
> > > +++ b/include/net/xdp_sock.h
> > > @@ -65,11 +65,12 @@ struct xdp_sock {
> > >       struct xsk_queue *tx ____cacheline_aligned_in_smp;
> > >       struct list_head tx_list;
> > >       /* record the number of tx descriptors sent by this xsk and
> > > -      * when it exceeds MAX_PER_SOCKET_BUDGET, an opportunity needs
> > > +      * when it exceeds max_tx_budget, an opportunity needs
> > >        * to be given to other xsks for sending tx descriptors, thereby
> > >        * preventing other XSKs from being starved.
> > >        */
> > >       u32 tx_budget_spent;
> > > +     u32 max_tx_budget;
> >
> > This probably does not need to be a u32?
> 
> From what I've known, it's not possible to set a very large value like
> 1000 which probably brings side effects.
> 
> But it seems we'd better not limit the use of this max_tx_budget? We
> don't know what the best fit for various use cases is. If the type
> needs to be downsized to a smaller one like u16, another related
> consideration is that dev_tx_weight deserves the same treatment?

If the current constant is 32, is U16_MAX really a limiting factor.
See also the next point.

> Or let me adjust to 'int' then?
> > > @@ -1437,6 +1436,18 @@ static int xsk_setsockopt(struct socket *sock, int level, int optname,
> > >               mutex_unlock(&xs->mutex);
> > >               return err;
> > >       }
> > > +     case XDP_MAX_TX_BUDGET:
> > > +     {
> > > +             unsigned int budget;
> > > +
> > > +             if (optlen < sizeof(budget))
> > > +                     return -EINVAL;
> > > +             if (copy_from_sockptr(&budget, optval, sizeof(budget)))
> > > +                     return -EFAULT;
> > > +
> > > +             WRITE_ONCE(xs->max_tx_budget, budget);
> >
> > Sanitize input: bounds check
> 
> Thanks for catching this.
> 
> I will change it like this:
>     WRITE_ONCE(xs->max_tx_budget, min_t(int, budget, INT_MAX));?

INT_MAX is not a valid upper bound. The current constant is 32.
I would expect an upper bound to perhaps be a few orders larger.

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