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SubjectRe: [PATCH net] tuntap: try not batch packets for devmap
On Wed, Feb 14, 2018 at 11:36:05AM +0800, Jason Wang wrote:
> Commit 762c330d670e ("tuntap: add missing xdp flush") tries to fix the
> devmap stall caused by missed xdp flush by counting the pending xdp
> redirected packets and flush when it exceeds NAPI_POLL_WEIGHT or
> MSG_MORE is clear. This may lead BUG() since xdp_do_flush() was
> called under process context with preemption enabled. Simply disable
> preemption may silent the warning but be not enough since process may
> move between different CPUS during a batch which cause xdp_do_flush()
> misses some CPU where the process run previously. For -net, fix this
> by simply calling xdp_do_flush() immediately after xdp_do_redirect(),
> a side effect is that this removes any possibility of batching which
> could be addressed in the future.
>
> Reported-by: Christoffer Dall <christoffer.dall@linaro.org>
> Fixes: 762c330d670e ("tuntap: add missing xdp flush")

Much of that patch is reverted here. How about a revert
followed by a one-liner calling xdp_do_redirect?
Will make backporting easier for people.

> Signed-off-by: Jason Wang <jasowang@redhat.com>

change itself is fine, feel free to include my

Acked-by: Michael S. Tsirkin <mst@redhat.com>


> ---
> drivers/net/tun.c | 15 +--------------
> 1 file changed, 1 insertion(+), 14 deletions(-)
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 17e496b..6a4cd97 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -181,7 +181,6 @@ struct tun_file {
> struct tun_struct *detached;
> struct ptr_ring tx_ring;
> struct xdp_rxq_info xdp_rxq;
> - int xdp_pending_pkts;
> };
>
> struct tun_flow_entry {
> @@ -1666,10 +1665,10 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
> case XDP_REDIRECT:
> get_page(alloc_frag->page);
> alloc_frag->offset += buflen;
> - ++tfile->xdp_pending_pkts;
> err = xdp_do_redirect(tun->dev, &xdp, xdp_prog);
> if (err)
> goto err_redirect;
> + xdp_do_flush_map();
> rcu_read_unlock();
> return NULL;
> case XDP_TX:
> @@ -1988,11 +1987,6 @@ static ssize_t tun_chr_write_iter(struct kiocb *iocb, struct iov_iter *from)
> result = tun_get_user(tun, tfile, NULL, from,
> file->f_flags & O_NONBLOCK, false);
>
> - if (tfile->xdp_pending_pkts) {
> - tfile->xdp_pending_pkts = 0;
> - xdp_do_flush_map();
> - }
> -
> tun_put(tun);
> return result;
> }
> @@ -2330,12 +2324,6 @@ static int tun_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
> m->msg_flags & MSG_DONTWAIT,
> m->msg_flags & MSG_MORE);
>
> - if (tfile->xdp_pending_pkts >= NAPI_POLL_WEIGHT ||
> - !(m->msg_flags & MSG_MORE)) {
> - tfile->xdp_pending_pkts = 0;
> - xdp_do_flush_map();
> - }
> -
> tun_put(tun);
> return ret;
> }
> @@ -3167,7 +3155,6 @@ static int tun_chr_open(struct inode *inode, struct file * file)
> sock_set_flag(&tfile->sk, SOCK_ZEROCOPY);
>
> memset(&tfile->tx_ring, 0, sizeof(tfile->tx_ring));
> - tfile->xdp_pending_pkts = 0;
>
> return 0;
> }
> --
> 2.7.4

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