Messages in this thread | | | Subject | Re: [PATCH net-next] vhost_net: conditionally enable tx polling | From | Jason Wang <> | Date | Wed, 1 Nov 2017 20:51:36 +0800 |
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On 2017年11月01日 00:36, Michael S. Tsirkin wrote: > On Tue, Oct 31, 2017 at 06:27:20PM +0800, Jason Wang wrote: >> We always poll tx for socket, this is sub optimal since: >> >> - we only want to be notified when sndbuf is available >> - this will slightly increase the waitqueue traversing time and more >> important, vhost could not benefit from commit >> commit 9e641bdcfa4e >> ("net-tun: restructure tun_do_read for better sleep/wakeup efficiency") >> even if we've stopped rx polling during handle_rx() since tx poll >> were still left in the waitqueue. >> >> Pktgen from a remote host to VM over mlx4 shows 5.5% improvements on >> rx PPS. (from 1.27Mpps to 1.34Mpps) >> >> Cc: Wei Xu <wexu@redhat.com> >> Cc: Matthew Rosato <mjrosato@linux.vnet.ibm.com> >> Signed-off-by: Jason Wang <jasowang@redhat.com> >> --- > Now that vhost_poll_stop happens on data path > a lot, I'd say > if (poll->wqh) > there should be unlikely().
It has been there since 8241a1e466cd ("vhost_net: stop polling socket during rx processing"). So it will be used for rx path too which unlikely() does not work as well as the case in tx.
> > >> drivers/vhost/net.c | 11 ++++++++--- >> 1 file changed, 8 insertions(+), 3 deletions(-) >> >> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c >> index 68677d9..286c3e4 100644 >> --- a/drivers/vhost/net.c >> +++ b/drivers/vhost/net.c >> @@ -471,6 +471,7 @@ static void handle_tx(struct vhost_net *net) >> goto out; >> >> vhost_disable_notify(&net->dev, vq); >> + vhost_net_disable_vq(net, vq); >> >> hdr_size = nvq->vhost_hlen; >> zcopy = nvq->ubufs; >> @@ -556,6 +557,8 @@ static void handle_tx(struct vhost_net *net) >> % UIO_MAXIOV; >> } >> vhost_discard_vq_desc(vq, 1); >> + if (err == -EAGAIN) >> + vhost_net_enable_vq(net, vq); >> break; >> } >> if (err != len) > I would probably just enable it unconditionally here. Why not? >
I thought we only care about the case of tun_sock_write_space() and for the errors other than -EAGAIN, they have nothing to do with polling.
>> @@ -1145,9 +1148,11 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd) >> r = vhost_vq_init_access(vq); >> if (r) >> goto err_used; >> - r = vhost_net_enable_vq(n, vq); >> - if (r) >> - goto err_used; >> + if (index == VHOST_NET_VQ_RX) { >> + r = vhost_net_enable_vq(n, vq); >> + if (r) >> + goto err_used; >> + } >> >> oldubufs = nvq->ubufs; >> nvq->ubufs = ubufs; > This last chunk seems questionable. If queue has stuff in it > when we connect the backend, we'll miss a wakeup. > I suspect this can happen during migration.
Unless qemu pass a tap which s already had pending tx packets.
I can remove this chuck, but if guest does not transmit any packet, rx can't benefit from this.
Thanks
> > >> -- >> 2.7.4
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