From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B48F123D280 for ; Wed, 24 Sep 2025 06:55:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758696938; cv=none; b=CuRBMQOizQyWeT8dJA/GLWZTrw6Zhzn5CaFKF+0EVmkgi4ZuulEOtz9MEK0cYrUpCSzqgE0j8+txJO+++PU+YKUixCht1/HhbxkMWDTLPtU0s4QPdpozvxZU89jvC7exvdKw612qk3u5xJGXDGUudgkRzsVbNpOWyHu/lCAfSkQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758696938; c=relaxed/simple; bh=IYyT2OvtNRU8mKssTIxBwKkXN9e3ywY0bC2fvBkYVDY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=VT2I5NPMQzz15ROEp+vM7cwDRyx/OXjUEAQV7dlo4QHQdRAiNMdhjOqpLR5j9qdHI56yDjs0rB7nsDhw7GxdoqzA1d49mYUZfShd9evkbqAxAY5EhLH1Yh0PJ/7HqnnvbsfUzQ/GWS9jB/uWjJ3gqdnxAD46NCoBjP91Em5unU0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=PtlFQGMB; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="PtlFQGMB" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1758696935; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=AMFaozSmWaP2nSuZAYoKtFmuv42SSqXkQF+k+CWOpeA=; b=PtlFQGMB/rEbDa39+2i0TesKsEseGzfL5IK4st3W3TxlCswHs+imzGc6Wy3lm/3idCfJT9 20CBduoCwjQrXakAXU/QRQR1kiCbeds76F2cEfZWcurNXBzl9XuhiRuj5iTD4RcUf/7VWP 5q2N81sFvV8L3QAOHXv7ViX5L+UqfaU= Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-663-5SS1_EPJP6OugJXQQ5o7dw-1; Wed, 24 Sep 2025 02:55:33 -0400 X-MC-Unique: 5SS1_EPJP6OugJXQQ5o7dw-1 X-Mimecast-MFC-AGG-ID: 5SS1_EPJP6OugJXQQ5o7dw_1758696933 Received: by mail-wm1-f71.google.com with SMTP id 5b1f17b1804b1-45b986a7b8aso38030725e9.0 for ; Tue, 23 Sep 2025 23:55:33 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1758696932; x=1759301732; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=AMFaozSmWaP2nSuZAYoKtFmuv42SSqXkQF+k+CWOpeA=; b=FkdS9e2U/YEscpxLdeMtdrcGQfl5qn/SGStSE/mWRZk5YjtqJxdB9G/E65+BD5E3u8 fpsEvZu8Z9l//S6qywFk6eRUtdlgUFu2bk5o58IoZKAu8bthpZ/rxn6vX4+Vma1UaE3r O6d0PbcCV1UMRueAAi1kIFsKUQn7I6i62LKAaevZy0Tp11zqbc2umZ/DiZJtxBywxbeO S8N1o5gN4Ks1+5KdZuFJYObxE8jEEX2VALn94o9GHD0S+W0z56REDggmjCpIG+2SASZQ A++jLg2v8E3qgG2un5x3lvn1GI3OqTxBAf74QgTc7EFbR5gh2/6/WeB3DlA9ulc+VAQ9 mO/Q== X-Forwarded-Encrypted: i=1; AJvYcCWm6wRi7DusXyHZeJv/mRgvBd/GcpkVxyWHT0Z5MmpjhfUuPnE8DmzFVtbiZlf+OG4BRRIGVfU=@vger.kernel.org X-Gm-Message-State: AOJu0Yzr66+EqjaUvhSD73jMvBFAGteie4xAFUNyxa4fMCGv0EaokkOU JR/Z0D5+dtuFmEmVUU5USYdHt9jzlb9/xJRyCo5Nn6/0VmsuteGphvrw99K4DaK5MD11V0FVzXV tjcy5yui+zboWcolX8D6PlEtnHJpECW8OFQ1Vm2deh96cua9pkGwzVjllSQ== X-Gm-Gg: ASbGncu6SCF3LyMYkUwgugd08ZF72oLmDVcjJdVp7DvS4nw4IQ/K3UT/oU40NbqPPtG sGFvQADL68QDihsCULL8+Rdpkl+q9jBGYJvemdWUFT/xLDsNbtHRGAK8tQRpVGcfkqtwBRrwMOR PnIh0nDU+jagfK4WSf6LRA/SP/iJnZszf/unEZ/JqS2zjhFMNTD6PMjGW2OFGvWtmmMncc7zBB5 aXwFSnaGpKPebxnjl5c9/HiCHWADWc1TOWWk1g0Gu5J4JWgvFnCh4WOdCEpb3AFysb9Tai5FYZI NZ/c45roggLxTHjPCxIIDuLsrBy4953RN0o= X-Received: by 2002:a05:600c:198f:b0:45d:d9ab:b86d with SMTP id 5b1f17b1804b1-46e1dac6457mr53314995e9.31.1758696932402; Tue, 23 Sep 2025 23:55:32 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFzQnJawr202LYF/GiG0rNphKRkGOKkt//h6RbI08Bh3KFW0jxQ55p1coTiwy7Vic6NwDCxDw== X-Received: by 2002:a05:600c:198f:b0:45d:d9ab:b86d with SMTP id 5b1f17b1804b1-46e1dac6457mr53314625e9.31.1758696931825; Tue, 23 Sep 2025 23:55:31 -0700 (PDT) Received: from redhat.com ([2a06:c701:73ea:f900:52ee:df2b:4811:77e0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3f99178390csm13966234f8f.44.2025.09.23.23.55.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 23 Sep 2025 23:55:31 -0700 (PDT) Date: Wed, 24 Sep 2025 02:55:28 -0400 From: "Michael S. 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: <20250924024416-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> 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: On Wed, Sep 24, 2025 at 07:56:33AM +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 totally missed this (but I am not sure why it did not happen in my > testing with different ptr_ring sizes..). > > I guess you are right, there must be some type of locking. > It probably makes sense to lock the netdev txq->_xmit_lock whenever the > consumer invalidates old ptr_ring entries (so when r->consumer_head >= > r->consumer_tail). The producer holds this lock with dev->lltx=false. Then > the consumer is able to wake the queue safely. > > So I would now just change the implementation to: > tun_net_xmit: > ... > if ptr_ring_produce > // Could happen because of unproduce in vhost_net.. > netif_tx_stop_queue > ... > goto drop > > if ptr_ring_full > netif_tx_stop_queue > ... > > tun_ring_recv/tap_do_read (the implementation for the batched methods > would be done in the similar way): > ... > ptr_ring_consume > if r->consumer_head >= r->consumer_tail > __netif_tx_lock_bh > netif_tx_wake_queue > __netif_tx_unlock_bh > > This implementation does not need any new ptr_ring helpers and no fancy > ordering tricks. > Would this implementation be sufficient in your opinion? Maybe you mean == ? Pls don't poke at ptr ring internals though. What are we testing for here? I think the point is that a batch of entries was consumed? Maybe __ptr_ring_consumed_batch ? and a comment explaining this returns true when last successful call to consume freed up a batch of space in the ring for producer to make progress. consumer_head == consumer_tail also happens rather a lot, though thankfully not on every entry. So taking tx lock each time this happens, even if queue is not stopped, seems heavyweight. > >> 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)) { > >> + 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 > >