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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 30 Jan 2024 10:41:32 -0500

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?

> Other net drivers (not all), also have the wake counter.

Examples?

> 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