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Tsirkin" To: Simon Schippers Cc: willemdebruijn.kernel@gmail.com, jasowang@redhat.com, eperezma@redhat.com, stephen@networkplumber.org, leiyang@redhat.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux.dev, kvm@vger.kernel.org, Tim Gebauer Subject: Re: [PATCH net-next v5 4/8] TUN & TAP: Wake netdev queue after consuming an entry Message-ID: <20250928182445-mutt-send-email-mst@kernel.org> References: <20250922221553.47802-1-simon.schippers@tu-dortmund.de> <20250922221553.47802-5-simon.schippers@tu-dortmund.de> <20250923123101-mutt-send-email-mst@kernel.org> <4dde6d41-2a26-47b8-aef1-4967f7fc94ab@tu-dortmund.de> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4dde6d41-2a26-47b8-aef1-4967f7fc94ab@tu-dortmund.de> On Sun, Sep 28, 2025 at 11:27:25PM +0200, Simon Schippers wrote: > On 23.09.25 18:36, Michael S. Tsirkin wrote: > > On Tue, Sep 23, 2025 at 12:15:49AM +0200, Simon Schippers wrote: > >> The new wrappers tun_ring_consume/tap_ring_consume deal with consuming an > >> entry of the ptr_ring and then waking the netdev queue when entries got > >> invalidated to be used again by the producer. > >> To avoid waking the netdev queue when the ptr_ring is full, it is checked > >> if the netdev queue is stopped before invalidating entries. Like that the > >> netdev queue can be safely woken after invalidating entries. > >> > >> The READ_ONCE in __ptr_ring_peek, paired with the smp_wmb() in > >> __ptr_ring_produce within tun_net_xmit guarantees that the information > >> about the netdev queue being stopped is visible after __ptr_ring_peek is > >> called. > >> > >> The netdev queue is also woken after resizing the ptr_ring. > >> > >> Co-developed-by: Tim Gebauer > >> Signed-off-by: Tim Gebauer > >> Signed-off-by: Simon Schippers > >> --- > >> drivers/net/tap.c | 44 +++++++++++++++++++++++++++++++++++++++++++- > >> drivers/net/tun.c | 47 +++++++++++++++++++++++++++++++++++++++++++++-- > >> 2 files changed, 88 insertions(+), 3 deletions(-) > >> > >> diff --git a/drivers/net/tap.c b/drivers/net/tap.c > >> index 1197f245e873..f8292721a9d6 100644 > >> --- a/drivers/net/tap.c > >> +++ b/drivers/net/tap.c > >> @@ -753,6 +753,46 @@ static ssize_t tap_put_user(struct tap_queue *q, > >> return ret ? ret : total; > >> } > >> > >> +static struct sk_buff *tap_ring_consume(struct tap_queue *q) > >> +{ > >> + struct netdev_queue *txq; > >> + struct net_device *dev; > >> + bool will_invalidate; > >> + bool stopped; > >> + void *ptr; > >> + > >> + spin_lock(&q->ring.consumer_lock); > >> + ptr = __ptr_ring_peek(&q->ring); > >> + if (!ptr) { > >> + spin_unlock(&q->ring.consumer_lock); > >> + return ptr; > >> + } > >> + > >> + /* Check if the queue stopped before zeroing out, so no ptr get > >> + * produced in the meantime, because this could result in waking > >> + * even though the ptr_ring is full. > > > > So what? Maybe it would be a bit suboptimal? But with your design, I do > > not get what prevents this: > > > > > > stopped? -> No > > ring is stopped > > discard > > > > and queue stays stopped forever > > > > I think I found a solution to this problem, see below: > > > > >> The order of the operations > >> + * is ensured by barrier(). > >> + */ > >> + will_invalidate = __ptr_ring_will_invalidate(&q->ring); > >> + if (unlikely(will_invalidate)) { > >> + rcu_read_lock(); > >> + dev = rcu_dereference(q->tap)->dev; > >> + txq = netdev_get_tx_queue(dev, q->queue_index); > >> + stopped = netif_tx_queue_stopped(txq); > >> + } > >> + barrier(); > >> + __ptr_ring_discard_one(&q->ring, will_invalidate); > >> + > >> + if (unlikely(will_invalidate)) { > > Here I just check for > > if (will_invalidate || __ptr_ring_empty(&q->ring)) { > > instead because, if the ptr_ring is empty and the netdev queue stopped, > the race must have occurred. Then it is safe to wake the netdev queue, > because it is known that space in the ptr_ring was freed when the race > occurred. Also, it is guaranteed that tap_ring_consume is called at least > once after the race, because a new entry is generated by the producer at > the race. > In my adjusted implementation, it tests fine with pktgen without any lost > packets. what if it is not empty and ring is stopped? > > Generally now I think that the whole implementation can be fine without > using spinlocks at all. I am currently adjusting the implementation > regarding SMP memory barrier pairings, and I have a question: > In the v4 you mentioned "the stop -> wake bounce involves enough barriers > already". Does it, for instance, mean that netif_tx_wake_queue already > ensures memory ordering, and I do not have to use an smp_wmb() in front of > netif_tx_wake_queue() and smp_rmb() in front of the ptr_ring operations > in tun_net_xmit? > I dug through net/core/netdevice.h and dev.c but could not really > answer this question by myself... > Thanks :) Only if it wakes up something, I think. Read: SLEEP AND WAKE-UP FUNCTIONS in Documentation/memory-barriers.txt IIUC this is the same. > > >> + if (stopped) > >> + netif_tx_wake_queue(txq); > >> + rcu_read_unlock(); > >> + } > > > > > > After an entry is consumed, you can detect this by checking > > > > r->consumer_head >= r->consumer_tail > > > > > > so it seems you could keep calling regular ptr_ring_consume > > and check afterwards? > > > > > > > > > >> + spin_unlock(&q->ring.consumer_lock); > >> + > >> + return ptr; > >> +} > >> + > >> static ssize_t tap_do_read(struct tap_queue *q, > >> struct iov_iter *to, > >> int noblock, struct sk_buff *skb) > >> @@ -774,7 +814,7 @@ static ssize_t tap_do_read(struct tap_queue *q, > >> TASK_INTERRUPTIBLE); > >> > >> /* Read frames from the queue */ > >> - skb = ptr_ring_consume(&q->ring); > >> + skb = tap_ring_consume(q); > >> if (skb) > >> break; > >> if (noblock) { > >> @@ -1207,6 +1247,8 @@ int tap_queue_resize(struct tap_dev *tap) > >> ret = ptr_ring_resize_multiple_bh(rings, n, > >> dev->tx_queue_len, GFP_KERNEL, > >> __skb_array_destroy_skb); > >> + if (netif_running(dev)) > >> + netif_tx_wake_all_queues(dev); > >> > >> kfree(rings); > >> return ret; > >> diff --git a/drivers/net/tun.c b/drivers/net/tun.c > >> index c6b22af9bae8..682df8157b55 100644 > >> --- a/drivers/net/tun.c > >> +++ b/drivers/net/tun.c > >> @@ -2114,13 +2114,53 @@ static ssize_t tun_put_user(struct tun_struct *tun, > >> return total; > >> } > >> > >> +static void *tun_ring_consume(struct tun_file *tfile) > >> +{ > >> + struct netdev_queue *txq; > >> + struct net_device *dev; > >> + bool will_invalidate; > >> + bool stopped; > >> + void *ptr; > >> + > >> + spin_lock(&tfile->tx_ring.consumer_lock); > >> + ptr = __ptr_ring_peek(&tfile->tx_ring); > >> + if (!ptr) { > >> + spin_unlock(&tfile->tx_ring.consumer_lock); > >> + return ptr; > >> + } > >> + > >> + /* Check if the queue stopped before zeroing out, so no ptr get > >> + * produced in the meantime, because this could result in waking > >> + * even though the ptr_ring is full. The order of the operations > >> + * is ensured by barrier(). > >> + */ > >> + will_invalidate = __ptr_ring_will_invalidate(&tfile->tx_ring); > >> + if (unlikely(will_invalidate)) { > >> + rcu_read_lock(); > >> + dev = rcu_dereference(tfile->tun)->dev; > >> + txq = netdev_get_tx_queue(dev, tfile->queue_index); > >> + stopped = netif_tx_queue_stopped(txq); > >> + } > >> + barrier(); > >> + __ptr_ring_discard_one(&tfile->tx_ring, will_invalidate); > >> + > >> + if (unlikely(will_invalidate)) { > >> + if (stopped) > >> + netif_tx_wake_queue(txq); > >> + rcu_read_unlock(); > >> + } > >> + spin_unlock(&tfile->tx_ring.consumer_lock); > >> + > >> + return ptr; > >> +} > >> + > >> static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err) > >> { > >> DECLARE_WAITQUEUE(wait, current); > >> void *ptr = NULL; > >> int error = 0; > >> > >> - ptr = ptr_ring_consume(&tfile->tx_ring); > >> + ptr = tun_ring_consume(tfile); > >> if (ptr) > >> goto out; > >> if (noblock) { > >> @@ -2132,7 +2172,7 @@ static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err) > >> > >> while (1) { > >> set_current_state(TASK_INTERRUPTIBLE); > >> - ptr = ptr_ring_consume(&tfile->tx_ring); > >> + ptr = tun_ring_consume(tfile); > >> if (ptr) > >> break; > >> if (signal_pending(current)) { > >> @@ -3621,6 +3661,9 @@ static int tun_queue_resize(struct tun_struct *tun) > >> dev->tx_queue_len, GFP_KERNEL, > >> tun_ptr_free); > >> > >> + if (netif_running(dev)) > >> + netif_tx_wake_all_queues(dev); > >> + > >> kfree(rings); > >> return ret; > >> } > >> -- > >> 2.43.0 > >