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SubjectRe: [PATCH] net-sysfs: export gso_max_size attribute
On Wed, Nov 22, 2017 at 04:30:41PM -0800, Solio Sarabia wrote:
> The netdevice gso_max_size is exposed to allow users fine-control on
> systems with multiple NICs with different GSO buffer sizes, and where
> the virtual devices like bridge and veth, need to be aware of the GSO
> size of the underlying devices.
>
> In a virtualized environment, setting the right GSO sizes for physical
> and virtual devices makes all TSO work to be on physical NIC, improving
> throughput and reducing CPU util. If virtual devices send buffers
> greater than what NIC supports, it forces host to do TSO for buffers
> exceeding the limit, increasing CPU utilization in host.
>
> Suggested-by: Shiny Sebastian <shiny.sebastian@intel.com>
> Signed-off-by: Solio Sarabia <solio.sarabia@intel.com>
> ---
> In one test scenario with Hyper-V host, Ubuntu 16.04 VM, with Docker
> inside VM, and NTttcp sending 40 Gbps from one container, setting the
> right gso_max_size values for all network devices in the chain, reduces
> CPU overhead about 3x (for the sender), since all TSO work is done by
> physical NIC.
>
> net/core/net-sysfs.c | 30 ++++++++++++++++++++++++++++++
> 1 file changed, 30 insertions(+)
>
> diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> index 799b752..7314bc8 100644
> --- a/net/core/net-sysfs.c
> +++ b/net/core/net-sysfs.c
> @@ -376,6 +376,35 @@ static ssize_t gro_flush_timeout_store(struct device *dev,
> }
> NETDEVICE_SHOW_RW(gro_flush_timeout, fmt_ulong);
>
> +static int change_gso_max_size(struct net_device *dev, unsigned long new_size)
> +{
> + unsigned int orig_size = dev->gso_max_size;
> +
> + if (new_size != (unsigned int)new_size)
> + return -ERANGE;
> +
> + if (new_size == orig_size)
> + return 0;
> +
> + if (new_size <= 0 || new_size > GSO_MAX_SIZE)
> + return -ERANGE;
> +
> + dev->gso_max_size = new_size;
> + return 0;
> +}
Hindsight, we need to re-evaluate the valid range. As it is now, in a
virtualized environment, users could set the gso to a value greater than
what NICs expose, which would inflict the original issue: overhead in
the host os due to a configuration value in the vm.

> +
> +static ssize_t gso_max_size_store(struct device *dev,
> + struct device_attribute *attr,
> + const char *buf, size_t len)
> +{
> + if (!capable(CAP_NET_ADMIN))
> + return -EPERM;
> +
> + return netdev_store(dev, attr, buf, len, change_gso_max_size);
> +}
> +
> +NETDEVICE_SHOW_RW(gso_max_size, fmt_dec);
> +
> static ssize_t ifalias_store(struct device *dev, struct device_attribute *attr,
> const char *buf, size_t len)
> {
> @@ -543,6 +572,7 @@ static struct attribute *net_class_attrs[] __ro_after_init = {
> &dev_attr_flags.attr,
> &dev_attr_tx_queue_len.attr,
> &dev_attr_gro_flush_timeout.attr,
> + &dev_attr_gso_max_size.attr,
> &dev_attr_phys_port_id.attr,
> &dev_attr_phys_port_name.attr,
> &dev_attr_phys_switch_id.attr,
> --
> 2.7.4
>

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