Re: [dpdk-dev] [PATCH] vhost: add pmd xstats

["Yang, Zhiyong" <zhiyong.yang@intel.com>]

> -----Original Message-----
> From: Panu Matilainen [mailto:pmatilai@redhat.com]
> Sent: Wednesday, August 24, 2016 8:37 PM
> To: Thomas Monjalon <thomas.monjalon@6wind.com>; Yuanhan Liu
> <yuanhan.liu@linux.intel.com>
> Cc: dev@dpdk.org; Yang, Zhiyong <zhiyong.yang@intel.com>
> Subject: Re: [dpdk-dev] [PATCH] vhost: add pmd xstats
> 
> On 08/24/2016 11:44 AM, Thomas Monjalon wrote:
> > 2016-08-24 13:46, Yuanhan Liu:
> >> On Tue, Aug 23, 2016 at 12:45:54PM +0300, Panu Matilainen wrote:
> >>>>>> Since collecting data of vhost_update_packet_xstats will have
> >>>>>> some effect on RX/TX performance, so, Setting compiling switch
> >>>>>> CONFIG_RTE_LIBRTE_PMD_VHOST_UPDATE_XSTATS=n by default
> in the
> >>>>> file
> >>>>>> config/common_base, if needing xstats data, you can enable it(y).
> >>>>>
> >>>>> NAK, such things need to be switchable at run-time.
> >>>>>
> >>>>> 	- Panu -
> >>>>
> >>>> Considering the following reasons using the compiler switch, not
> >>>> command-line at run-time.
> >>>>
> >>>> 1.Similar xstats update functions are always collecting stats data
> >>>> in the background when rx/tx are running, such as the physical NIC
> >>>> or virtio, which have no switch. Compiler switch for vhost pmd
> >>>> xstats is added as a option when performance is viewed as critical
> factor.
> >>>>
> >>>> 2. No data structure and API in any layer support the xstats update
> >>>> switch at run-time. Common data structure (struct rte_eth_dev_data)
> >>>> has no device-specific data member, if implementing enable/disable
> >>>> of vhost_update _packet_xstats at run-time, must define a
> >>>> flag(device-specific) in it, because the definition of struct
> >>>> vhost_queue in the driver code (eth_vhost_rx/eth_vhost_tx
> processing)is not visible from device perspective.
> >>>>
> >>>> 3. I tested RX/TX with v1 patch (y) as reference based on Intel(R)
> >>>> Xeon(R) CPU E5-2699 v3 @ 2.30GHz, for 64byts packets in burst mode,
> >>>> 32 packets in one RX/TX processing. Overhead of
> >>>> vhost_update_packet_xstats is less than 3% for the rx/tx
> >>>> processing. It looks that vhost_update_packet_xstats has a limited
> effect on performance drop.
> >>>
> >>> Well, either the performance overhead is acceptable and it should
> >>> always be on (like with physical NICs I think). Or it is not. In
> >>> which case it needs to be turnable on and off, at run-time.
> >>> Rebuilding is not an option in the world of distros.
> >>
> >> I think the less than 3% overhead is acceptable here, that I agree
> >> with Panu we should always keep it on. If someone compains it later
> >> that even 3% is too big for them, let's consider to make it be
> >> switchable at run-time. Either we could introduce a generic eth API
> >> for that, Or just introduce a vhost one if that doesn't make too much
> >> sense to other eth drivers.
> >
> > +1
> > It may have sense to introduce a generic run-time option for stats.
> >
> 
> Yup, sounds good.
> 
It sounds better , if DPDK can add generic API and structure to the switch
of xstats update. So, any device can use it at run time if necessary.

Can we define one bit data member (xstats_update) in the data structure
struct rte_eth_dev_data?
such as:
	uint8_t promiscuous : 1, /**< RX promiscuous mode ON(1) / OFF(0). */
	scattered_rx : 1,  /**< RX of scattered packets is ON(1) / OFF(0) */
	all_multicast : 1, /**< RX all multicast mode ON(1) / OFF(0). */
	dev_started : 1,   /**< Device state: STARTED(1) / STOPPED(0). */
	lro         : 1,   /**< RX LRO is ON(1) / OFF(0) */
	xstats_update: 1;   /**< xstats update is ON(1) / OFF(0) */

Define 3 functions:

void rte_eth_xstats_update_enable(uint8_t port_id);
void rte_eth_xstats_update_disable(uint8_t port_id);
int rte_eth_xstats_update_get(uint8_t port_id);

Or define two:

/* uint8_t xstats_update ; 1 on, 0 off*/
void rte_eth_xstats_update_enable(uint8_t port_id, uint8_t xstats_update);
int rte_eth_xstats_update_get(uint8_t port_id);

In the struct eth_dev_ops, adding two functions to pass xstats_update to
driver, because the rxq/txq can't access xstats_update directly.
So, add a xstats flag per queue data structure. 
for example 
struct vhost_queue {
	......
	Uint64_t  xstats_flag;
	......
};
typedef uint16_t (*eth_rx_burst_t)(void *rxq,
				   struct rte_mbuf **rx_pkts,
				   uint16_t nb_pkts);
typedef uint16_t (*eth_tx_burst_t)(void *txq,
				   struct rte_mbuf **tx_pkts,
				   uint16_t nb_pkts);

struct eth_dev_ops {
......
	eth_xstats_update_enable_t  xstats_update_enable; /**< xstats ON. */
	eth_xstats_update_disable_t xstats_update_disable;/**< xstats OFF. */
......
};

	--zhiyong--