Subject: Re: [PATCH net-next v3 1/7] virtio_net: correct netdev_tx_reset_queue() invocation point

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 5 Dec 2024 10:14:19 -0500

On Thu, Dec 05, 2024 at 10:16:35PM +0900, Koichiro Den wrote:
> On Thu, Dec 05, 2024 at 09:43:38PM +0900, Koichiro Den wrote:
> > On Thu, Dec 05, 2024 at 05:33:36AM -0500, Michael S. Tsirkin wrote:
> > > On Wed, Dec 04, 2024 at 02:07:18PM +0900, Koichiro Den wrote:
> > > > When virtnet_close is followed by virtnet_open, some TX completions can
> > > > possibly remain unconsumed, until they are finally processed during the
> > > > first NAPI poll after the netdev_tx_reset_queue(), resulting in a crash
> > > > [1].
> > > 
> > > 
> > > So it's a bugfix. Why net-next not net?
> > 
> > I was mistaken (I just read netdev-FAQ again). I'll resend to net, with
> > adjustments reflecting your feedback.
> > 
> > > 
> > > > Commit b96ed2c97c79 ("virtio_net: move netdev_tx_reset_queue() call
> > > > before RX napi enable") was not sufficient to eliminate all BQL crash
> > > > cases for virtio-net.
> > > > 
> > > > This issue can be reproduced with the latest net-next master by running:
> > > > `while :; do ip l set DEV down; ip l set DEV up; done` under heavy network
> > > > TX load from inside the machine.
> > > > 
> > > > netdev_tx_reset_queue() can actually be dropped from virtnet_open path;
> > > > the device is not stopped in any case. For BQL core part, it's just like
> > > > traffic nearly ceases to exist for some period. For stall detector added
> > > > to BQL, even if virtnet_close could somehow lead to some TX completions
> > > > delayed for long, followed by virtnet_open, we can just take it as stall
> > > > as mentioned in commit 6025b9135f7a ("net: dqs: add NIC stall detector
> > > > based on BQL"). Note also that users can still reset stall_max via sysfs.
> > > > 
> > > > So, drop netdev_tx_reset_queue() from virtnet_enable_queue_pair(). This
> > > > eliminates the BQL crashes. Note that netdev_tx_reset_queue() is now
> > > > explicitly required in freeze/restore path, so this patch adds it to
> > > > free_unused_bufs().
> > > 
> > > I don't much like that free_unused_bufs now has this side effect.
> > > I think would be better to just add a loop in virtnet_restore.
> > > Or if you want to keep it there, pls rename the function
> > > to hint it does more.
> > 
> > It makes sense. I would go for the former. Thanks.
> 
> Hmm, as Jacob pointed out in v1
> (https://lore.kernel.org/all/[email-masked]/),
> it looks better to follow the rule of thumb. Taking both suggestions
> from Jacob and you, adding a loop to remove_vq_common(), just after
> free_unused_bufs(), seems more fitting now, like this:
> 
>      static void remove_vq_common(struct virtnet_info *vi)
>      {
>     +       int i;
>     +
>             virtio_reset_device(vi->vdev);
>     
>             /* Free unused buffers in both send and recv, if any. */
>             free_unused_bufs(vi);
>     
>     +       /* Tx unused buffers flushed, so reset BQL counter */
>     +       for (i = 0; i < vi->max_queue_pairs; i++)
>     +               netdev_tx_reset_queue(netdev_get_tx_queue(vi->dev, i));
>     +
>             free_receive_bufs(vi);
> 
> What do you think?
> 
> Thanks,
> 
> -Koichiro Den

why in remove_vq_common? It's only used on freeze/restore.

> > 
> > > 
> > > 
> > > > 
> > > > [1]:
> > > > ------------[ cut here ]------------
> > > > kernel BUG at lib/dynamic_queue_limits.c:99!
> > > > Oops: invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
> > > > CPU: 7 UID: 0 PID: 1598 Comm: ip Tainted: G    N 6.12.0net-next_main+ #2
> > > > Tainted: [N]=TEST
> > > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), \
> > > > BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
> > > > RIP: 0010:dql_completed+0x26b/0x290
> > > > Code: b7 c2 49 89 e9 44 89 da 89 c6 4c 89 d7 e8 ed 17 47 00 58 65 ff 0d
> > > > 4d 27 90 7e 0f 85 fd fe ff ff e8 ea 53 8d ff e9 f3 fe ff ff <0f> 0b 01
> > > > d2 44 89 d1 29 d1 ba 00 00 00 00 0f 48 ca e9 28 ff ff ff
> > > > RSP: 0018:ffffc900002b0d08 EFLAGS: 00010297
> > > > RAX: 0000000000000000 RBX: ffff888102398c80 RCX: 0000000080190009
> > > > RDX: 0000000000000000 RSI: 000000000000006a RDI: 0000000000000000
> > > > RBP: ffff888102398c00 R08: 0000000000000000 R09: 0000000000000000
> > > > R10: 00000000000000ca R11: 0000000000015681 R12: 0000000000000001
> > > > R13: ffffc900002b0d68 R14: ffff88811115e000 R15: ffff8881107aca40
> > > > FS:  00007f41ded69500(0000) GS:ffff888667dc0000(0000)
> > > > knlGS:0000000000000000
> > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: 0000556ccc2dc1a0 CR3: 0000000104fd8003 CR4: 0000000000772ef0
> > > > PKRU: 55555554
> > > > Call Trace:
> > > >  <IRQ>
> > > >  ? die+0x32/0x80
> > > >  ? do_trap+0xd9/0x100
> > > >  ? dql_completed+0x26b/0x290
> > > >  ? dql_completed+0x26b/0x290
> > > >  ? do_error_trap+0x6d/0xb0
> > > >  ? dql_completed+0x26b/0x290
> > > >  ? exc_invalid_op+0x4c/0x60
> > > >  ? dql_completed+0x26b/0x290
> > > >  ? asm_exc_invalid_op+0x16/0x20
> > > >  ? dql_completed+0x26b/0x290
> > > >  __free_old_xmit+0xff/0x170 [virtio_net]
> > > >  free_old_xmit+0x54/0xc0 [virtio_net]
> > > >  virtnet_poll+0xf4/0xe30 [virtio_net]
> > > >  ? __update_load_avg_cfs_rq+0x264/0x2d0
> > > >  ? update_curr+0x35/0x260
> > > >  ? reweight_entity+0x1be/0x260
> > > >  __napi_poll.constprop.0+0x28/0x1c0
> > > >  net_rx_action+0x329/0x420
> > > >  ? enqueue_hrtimer+0x35/0x90
> > > >  ? trace_hardirqs_on+0x1d/0x80
> > > >  ? kvm_sched_clock_read+0xd/0x20
> > > >  ? sched_clock+0xc/0x30
> > > >  ? kvm_sched_clock_read+0xd/0x20
> > > >  ? sched_clock+0xc/0x30
> > > >  ? sched_clock_cpu+0xd/0x1a0
> > > >  handle_softirqs+0x138/0x3e0
> > > >  do_softirq.part.0+0x89/0xc0
> > > >  </IRQ>
> > > >  <TASK>
> > > >  __local_bh_enable_ip+0xa7/0xb0
> > > >  virtnet_open+0xc8/0x310 [virtio_net]
> > > >  __dev_open+0xfa/0x1b0
> > > >  __dev_change_flags+0x1de/0x250
> > > >  dev_change_flags+0x22/0x60
> > > >  do_setlink.isra.0+0x2df/0x10b0
> > > >  ? rtnetlink_rcv_msg+0x34f/0x3f0
> > > >  ? netlink_rcv_skb+0x54/0x100
> > > >  ? netlink_unicast+0x23e/0x390
> > > >  ? netlink_sendmsg+0x21e/0x490
> > > >  ? ____sys_sendmsg+0x31b/0x350
> > > >  ? avc_has_perm_noaudit+0x67/0xf0
> > > >  ? cred_has_capability.isra.0+0x75/0x110
> > > >  ? __nla_validate_parse+0x5f/0xee0
> > > >  ? __pfx___probestub_irq_enable+0x3/0x10
> > > >  ? __create_object+0x5e/0x90
> > > >  ? security_capable+0x3b/0x70
> > > >  rtnl_newlink+0x784/0xaf0
> > > >  ? avc_has_perm_noaudit+0x67/0xf0
> > > >  ? cred_has_capability.isra.0+0x75/0x110
> > > >  ? stack_depot_save_flags+0x24/0x6d0
> > > >  ? __pfx_rtnl_newlink+0x10/0x10
> > > >  rtnetlink_rcv_msg+0x34f/0x3f0
> > > >  ? do_syscall_64+0x6c/0x180
> > > >  ? entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > > >  ? __pfx_rtnetlink_rcv_msg+0x10/0x10
> > > >  netlink_rcv_skb+0x54/0x100
> > > >  netlink_unicast+0x23e/0x390
> > > >  netlink_sendmsg+0x21e/0x490
> > > >  ____sys_sendmsg+0x31b/0x350
> > > >  ? copy_msghdr_from_user+0x6d/0xa0
> > > >  ___sys_sendmsg+0x86/0xd0
> > > >  ? __pte_offset_map+0x17/0x160
> > > >  ? preempt_count_add+0x69/0xa0
> > > >  ? __call_rcu_common.constprop.0+0x147/0x610
> > > >  ? preempt_count_add+0x69/0xa0
> > > >  ? preempt_count_add+0x69/0xa0
> > > >  ? _raw_spin_trylock+0x13/0x60
> > > >  ? trace_hardirqs_on+0x1d/0x80
> > > >  __sys_sendmsg+0x66/0xc0
> > > >  do_syscall_64+0x6c/0x180
> > > >  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> > > > RIP: 0033:0x7f41defe5b34
> > > > Code: 15 e1 12 0f 00 f7 d8 64 89 02 b8 ff ff ff ff eb bf 0f 1f 44 00 00
> > > > f3 0f 1e fa 80 3d 35 95 0f 00 00 74 13 b8 2e 00 00 00 0f 05 <48> 3d 00
> > > > f0 ff ff 77 4c c3 0f 1f 00 55 48 89 e5 48 83 ec 20 89 55
> > > > RSP: 002b:00007ffe5336ecc8 EFLAGS: 00000202 ORIG_RAX: 000000000000002e
> > > > RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f41defe5b34
> > > > RDX: 0000000000000000 RSI: 00007ffe5336ed30 RDI: 0000000000000003
> > > > RBP: 00007ffe5336eda0 R08: 0000000000000010 R09: 0000000000000001
> > > > R10: 00007ffe5336f6f9 R11: 0000000000000202 R12: 0000000000000003
> > > > R13: 0000000067452259 R14: 0000556ccc28b040 R15: 0000000000000000
> > > >  </TASK>
> > > > [...]
> > > > ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---
> > > > 
> > > > Fixes: c8bd1f7f3e61 ("virtio_net: add support for Byte Queue Limits")
> > > > Cc: <[email-masked]> # v6.11+
> > > > Signed-off-by: Koichiro Den <[email-masked]>
> > > > ---
> > > >  drivers/net/virtio_net.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > index 64c87bb48a41..48ce8b3881b6 100644
> > > > --- a/drivers/net/virtio_net.c
> > > > +++ b/drivers/net/virtio_net.c
> > > > @@ -3054,7 +3054,6 @@ static int virtnet_enable_queue_pair(struct virtnet_info *vi, int qp_index)
> > > >  	if (err < 0)
> > > >  		goto err_xdp_reg_mem_model;
> > > >  
> > > > -	netdev_tx_reset_queue(netdev_get_tx_queue(vi->dev, qp_index));
> > > >  	virtnet_napi_enable(vi->rq[qp_index].vq, &vi->rq[qp_index].napi);
> > > >  	virtnet_napi_tx_enable(vi, vi->sq[qp_index].vq, &vi->sq[qp_index].napi);
> > > >  
> > > > @@ -6243,6 +6242,7 @@ static void free_unused_bufs(struct virtnet_info *vi)
> > > >  		struct virtqueue *vq = vi->sq[i].vq;
> > > >  		while ((buf = virtqueue_detach_unused_buf(vq)) != NULL)
> > > >  			virtnet_sq_free_unused_buf(vq, buf);
> > > > +		netdev_tx_reset_queue(netdev_get_tx_queue(vi->dev, i));
> > > >  		cond_resched();
> > > >  	}
> > > >  
> > > > -- 
> > > > 2.43.0
> > >