Subject: Re: [PATCH net-next] virtio_net: Add TX stop and wake counters

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 30 Jan 2024 10:53:29 -0500

On Tue, Jan 30, 2024 at 03:50:29PM +0000, Daniel Jurgens wrote:
> > From: Michael S. Tsirkin <[email-masked]>
> > Sent: Tuesday, January 30, 2024 9:42 AM
> > On Tue, Jan 30, 2024 at 03:40:21PM +0000, Daniel Jurgens wrote:
> > > > From: Michael S. Tsirkin <[email-masked]>
> > > > Sent: Tuesday, January 30, 2024 8:58 AM
> > > >
> > > > On Tue, Jan 30, 2024 at 08:25:21AM -0600, Daniel Jurgens wrote:
> > > > > Add a tx queue stop and wake counters, they are useful for debugging.
> > > > >
> > > > > 	$ ethtool -S ens5f2 | grep 'tx_stop\|tx_wake'
> > > > > 	...
> > > > > 	tx_queue_1_tx_stop: 16726
> > > > > 	tx_queue_1_tx_wake: 16726
> > > > > 	...
> > > > > 	tx_queue_8_tx_stop: 1500110
> > > > > 	tx_queue_8_tx_wake: 1500110
> > > > >
> > > > > Signed-off-by: Daniel Jurgens <[email-masked]>
> > > > > Reviewed-by: Parav Pandit <[email-masked]>
> > > >
> > > > Hmm isn't one always same as the other, except when queue is stopped?
> > > > And when it is stopped you can see that in the status?
> > > > So how is having two useful?
> > >
> > > At idle the counters will be the same, unless a tx_timeout occurs. But
> > under load they can be monitored to see which queues are stopped and get
> > an idea of how long they are stopped.
> > 
> > how does it give you the idea of how long they are stopped?
> 
> By serially monitoring the counter you can see stops that persist long intervals that are less than the tx_timeout time.

Why don't you monitor queue status directly?

> > 
> > > Other net drivers (not all), also have the wake counter.
> > 
> > Examples?
> 
> [danielj@sw-mtx-051 upstream]$ ethtool -i ens2f1np1
> driver: mlx5_core                                  
> version: 6.7.0+                                    
> ...
> [danielj@sw-mtx-051 upstream]$ ethtool -S ens2f1np1 | grep wake
>      tx_queue_wake: 0
>      tx0_wake: 0

Do they have a stop counter too?

> > 
> > > In my opinion it makes the stop counter more useful, at little cost.
> > >
> > > >
> > > >
> > > > > ---
> > > > >  drivers/net/virtio_net.c | 26 ++++++++++++++++++++++++--
> > > > >  1 file changed, 24 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > > index 3cb8aa193884..7e3c31ceaf7e 100644
> > > > > --- a/drivers/net/virtio_net.c
> > > > > +++ b/drivers/net/virtio_net.c
> > > > > @@ -88,6 +88,8 @@ struct virtnet_sq_stats {
> > > > >  	u64_stats_t xdp_tx_drops;
> > > > >  	u64_stats_t kicks;
> > > > >  	u64_stats_t tx_timeouts;
> > > > > +	u64_stats_t tx_stop;
> > > > > +	u64_stats_t tx_wake;
> > > > >  };
> > > > >
> > > > >  struct virtnet_rq_stats {
> > > > > @@ -112,6 +114,8 @@ static const struct virtnet_stat_desc
> > > > virtnet_sq_stats_desc[] = {
> > > > >  	{ "xdp_tx_drops",	VIRTNET_SQ_STAT(xdp_tx_drops) },
> > > > >  	{ "kicks",		VIRTNET_SQ_STAT(kicks) },
> > > > >  	{ "tx_timeouts",	VIRTNET_SQ_STAT(tx_timeouts) },
> > > > > +	{ "tx_stop",		VIRTNET_SQ_STAT(tx_stop) },
> > > > > +	{ "tx_wake",		VIRTNET_SQ_STAT(tx_wake) },
> > > > >  };
> > > > >
> > > > >  static const struct virtnet_stat_desc virtnet_rq_stats_desc[] = {
> > > > > @@
> > > > > -843,6 +847,9 @@ static void check_sq_full_and_disable(struct
> > > > > virtnet_info
> > > > *vi,
> > > > >  	 */
> > > > >  	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {
> > > > >  		netif_stop_subqueue(dev, qnum);
> > > > > +		u64_stats_update_begin(&sq->stats.syncp);
> > > > > +		u64_stats_inc(&sq->stats.tx_stop);
> > > > > +		u64_stats_update_end(&sq->stats.syncp);
> > > > >  		if (use_napi) {
> > > > >  			if (unlikely(!virtqueue_enable_cb_delayed(sq->vq)))
> > > > >  				virtqueue_napi_schedule(&sq->napi, sq- vq);
> > @@ -851,6 +858,9
> > > > >@@  static void check_sq_full_and_disable(struct virtnet_info *vi,
> > > > >  			free_old_xmit_skbs(sq, false);
> > > > >  			if (sq->vq->num_free >= 2+MAX_SKB_FRAGS) {
> > > > >  				netif_start_subqueue(dev, qnum);
> > > > > +				u64_stats_update_begin(&sq->stats.syncp);
> > > > > +				u64_stats_inc(&sq->stats.tx_wake);
> > > > > +				u64_stats_update_end(&sq->stats.syncp);
> > > > >  				virtqueue_disable_cb(sq->vq);
> > > > >  			}
> > > > >  		}
> > > > > @@ -2163,8 +2173,14 @@ static void virtnet_poll_cleantx(struct
> > > > receive_queue *rq)
> > > > >  			free_old_xmit_skbs(sq, true);
> > > > >  		} while (unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
> > > > >
> > > > > -		if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> > > > > +		if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS) {
> > > > > +			if (netif_tx_queue_stopped(txq)) {
> > > > > +				u64_stats_update_begin(&sq->stats.syncp);
> > > > > +				u64_stats_inc(&sq->stats.tx_wake);
> > > > > +				u64_stats_update_end(&sq->stats.syncp);
> > > > > +			}
> > > > >  			netif_tx_wake_queue(txq);
> > > > > +		}
> > > > >
> > > > >  		__netif_tx_unlock(txq);
> > > > >  	}
> > > > > @@ -2310,8 +2326,14 @@ static int virtnet_poll_tx(struct
> > > > > napi_struct
> > > > *napi, int budget)
> > > > >  	virtqueue_disable_cb(sq->vq);
> > > > >  	free_old_xmit_skbs(sq, true);
> > > > >
> > > > > -	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS)
> > > > > +	if (sq->vq->num_free >= 2 + MAX_SKB_FRAGS) {
> > > > > +		if (netif_tx_queue_stopped(txq)) {
> > > > > +			u64_stats_update_begin(&sq->stats.syncp);
> > > > > +			u64_stats_inc(&sq->stats.tx_wake);
> > > > > +			u64_stats_update_end(&sq->stats.syncp);
> > > > > +		}
> > > > >  		netif_tx_wake_queue(txq);
> > > > > +	}
> > > > >
> > > > >  	opaque = virtqueue_enable_cb_prepare(sq->vq);
> > > > >
> > > > > --
> > > > > 2.42.0