Re: Re: [PATCH v2 net-next] xsk: Avoid starving xsk at the end of the list

From: Magnus Karlsson
Date: Mon Oct 16 2023 - 05:14:10 EST

On Mon, 16 Oct 2023 at 10:54, 黄杰 <huangjie.albert@xxxxxxxxxxxxx> wrote:
>
> Magnus Karlsson <magnus.karlsson@xxxxxxxxx> 于2023年10月16日周一 14:41写道：
> >
> > On Mon, 16 Oct 2023 at 05:17, Albert Huang
> > <huangjie.albert@xxxxxxxxxxxxx> wrote:
> > >
> > > In the previous implementation, when multiple xsk sockets were
> > > associated with a single xsk_buff_pool, a situation could arise
> > > where the xsk_tx_list maintained data at the front for one xsk
> > > socket while starving the xsk sockets at the back of the list.
> > > This could result in issues such as the inability to transmit packets,
> > > increased latency, and jitter. To address this problem, we introduced
> > > a new variable called tx_budget_cache, which limits each xsk to transmit
> > > a maximum of MAX_XSK_TX_BUDGET tx descriptors. This allocation ensures
> > > equitable opportunities for subsequent xsk sockets to send tx descriptors.
> > > The value of MAX_XSK_TX_BUDGET is temporarily set to 16.
> >
> > Hi Albert. Yes you are correct that there is nothing hindering this to
> > happen in the code at the moment, so let us fix it.
> >
> thanks.
>
> > > Signed-off-by: Albert Huang <huangjie.albert@xxxxxxxxxxxxx>
> > > ---
> > > include/net/xdp_sock.h | 6 ++++++
> > > net/xdp/xsk.c | 18 ++++++++++++++++++
> > > 2 files changed, 24 insertions(+)
> > >
> > > diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
> > > index 69b472604b86..f617ff54e38c 100644
> > > --- a/include/net/xdp_sock.h
> > > +++ b/include/net/xdp_sock.h
> > > @@ -44,6 +44,7 @@ struct xsk_map {
> > > struct xdp_sock __rcu *xsk_map[];
> > > };
> > >
> > > +#define MAX_XSK_TX_BUDGET 16
> >
> > I think something like MAX_PER_SOCKET_BUDGET would be clearer.
> >
>
> OK, this will be considered in the next patch.
>
> > > struct xdp_sock {
> > > /* struct sock must be the first member of struct xdp_sock */
> > > struct sock sk;
> > > @@ -63,6 +64,11 @@ struct xdp_sock {
> > >
> > > struct xsk_queue *tx ____cacheline_aligned_in_smp;
> > > struct list_head tx_list;
> > > + /* Record the actual number of times xsk has transmitted a tx
> > > + * descriptor, with a maximum limit not exceeding MAX_XSK_TX_BUDGET
> > > + */
> > > + u32 tx_budget_cache;
> > > +
> > > /* Protects generic receive. */
> > > spinlock_t rx_lock;
> > >
> > > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > > index f5e96e0d6e01..087f2675333c 100644
> > > --- a/net/xdp/xsk.c
> > > +++ b/net/xdp/xsk.c
> > > @@ -413,16 +413,25 @@ EXPORT_SYMBOL(xsk_tx_release);
> > >
> > > bool xsk_tx_peek_desc(struct xsk_buff_pool *pool, struct xdp_desc *desc)
> > > {
> > > + u32 xsk_full_count = 0;
> >
> > Enough with a bool;
> >
> > > struct xdp_sock *xs;
> > >
> > > rcu_read_lock();
> > > +again:
> > > list_for_each_entry_rcu(xs, &pool->xsk_tx_list, tx_list) {
> > > + if (xs->tx_budget_cache >= MAX_XSK_TX_BUDGET) {
> > > + xsk_full_count++;
> > > + continue;
> > > + }
> >
> > The problem here is that the fixed MAX_XSK_TX_BUDGET is only useful
> > for the <= 2 socket case. If I have 3 sockets sharing a
> > netdev/queue_id, the two first sockets can still starve the third one
> > since the total budget per send is 32.
>
> Why is there a limit of 32? I'm not quite clear on the implications of these,
> Did I miss something?
> BR
> Albert

There is a define TX_BATCH_SIZE 32 that controls the max number of
packets a sendto() call can send before it exits. It is used in
__xsk_generic_xmit().

> >You need to go through the list
> > of sockets in the beginning to compute the MAX_XSK_TX_BUDGET to
> > compute this dynamically before each call. Or cache this value
> > somehow, in the pool for example. Actually, the refcount in the
> > buf_pool will tell you how many sockets are sharing the same buf_pool.
> > Try using that to form MAX_XSK_TX_BUDGET on the fly.
> >
> > Another simpler way of accomplishing this would be to just reorder the
> > list every time. Put the first socket last in the list every time. The
> > drawback of this is that you need to hold the xsk_tx_list_lock while
> > doing this so might be slower. The per socket batch size would also be
> > 32 and you would not receive "fairness" over a single call to
> > sendto(). Would that be a problem for you?
> >
>
> Yes, I did consider this approach, but I abandoned it because it would lose
> the performance advantages of lock-free operations(RCU read)
> thanks
> Albert

OK, then let us not consider it and try to make your current approach work.

>
> > > +
> > > if (!xskq_cons_peek_desc(xs->tx, desc, pool)) {
> > > if (xskq_has_descs(xs->tx))
> > > xskq_cons_release(xs->tx);
> > > continue;
> > > }
> > >
> > > + xs->tx_budget_cache++;
> > > +
> > > /* This is the backpressure mechanism for the Tx path.
> > > * Reserve space in the completion queue and only proceed
> > > * if there is space in it. This avoids having to implement
> > > @@ -436,6 +445,14 @@ bool xsk_tx_peek_desc(struct xsk_buff_pool *pool, struct xdp_desc *desc)
> > > return true;
> > > }
> > >
> > > + if (unlikely(xsk_full_count > 0)) {
> > > + list_for_each_entry_rcu(xs, &pool->xsk_tx_list, tx_list) {
> > > + xs->tx_budget_cache = 0;
> > > + }
> > > + xsk_full_count = 0;
> > > + goto again;
> > > + }
>
> this section of code only enters when it's unable to acquire any TX
> descriptors and
> xsk_full_count > 0.
>
> > > +
> > > out:
> > > rcu_read_unlock();
> > > return false;
> > > @@ -1230,6 +1247,7 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
> > > xs->zc = xs->umem->zc;
> > > xs->sg = !!(xs->umem->flags & XDP_UMEM_SG_FLAG);
> > > xs->queue_id = qid;
> > > + xs->tx_budget_cache = 0;
> > > xp_add_xsk(xs->pool, xs);
> > >
> > > out_unlock:
> > > --
> > > 2.20.1
> > >
> > >