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Date: Fri, 14 Dec 2018 15:16:38 +0100
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Ilias Apalodimas <ilias.apalodimas@...aro.org>
Cc: "<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
Jaswinder Singh <jaswinder.singh@...aro.org>,
Masami Hiramatsu <masami.hiramatsu@...aro.org>,
Masahisa Kojima <masahisa.kojima@...aro.org>
Subject: Re: [net-next, PATCH 2/2] net: socionext: remove mmio reads on Tx
On Fri, 14 Dec 2018 at 12:18, Ilias Apalodimas
<ilias.apalodimas@...aro.org> wrote:
>
> On Fri, Dec 14, 2018 at 11:59:00AM +0100, Ard Biesheuvel wrote:
> > On Fri, 14 Dec 2018 at 09:59, Ilias Apalodimas
> > <ilias.apalodimas@...aro.org> wrote:
> > >
> > > Currently the driver issues 2 mmio reads to figure out the number of
> > > transmitted packets and clean them. We can get rid of the expensive
> > > reads since BIT 31 of the Tx descriptor can be used for that.
> > > We can also remove the budget counting of Tx completions since all of
> > > the descriptors are not deliberately processed.
> > >
> >
> > This last sentence reflects what we discussed off-line, right? That
> > reaping used Tx descriptors shouldn't be counted against the budget,
> > but should only occur if the budget is > 0 after processing the Rx
> > queue?
>
> yes
>
> > > Performance numbers using pktgen are:
> > > size pre-patch(pps) post-patch(pps)
> > > 64 362483 427916
> > > 128 358315 411686
> > > 256 352725 389683
> > > 512 215675 216464
> > > 1024 113812 114442
> > >
> > > Signed-off-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
> > > ---
> > > drivers/net/ethernet/socionext/netsec.c | 97 ++++++++++++++-----------
> > > 1 file changed, 54 insertions(+), 43 deletions(-)
> > >
> > > diff --git a/drivers/net/ethernet/socionext/netsec.c b/drivers/net/ethernet/socionext/netsec.c
> > > index 584a6b3f6542..05a0948ad929 100644
> > > --- a/drivers/net/ethernet/socionext/netsec.c
> > > +++ b/drivers/net/ethernet/socionext/netsec.c
> > > @@ -257,7 +257,6 @@ struct netsec_desc_ring {
> > > dma_addr_t desc_dma;
> > > struct netsec_desc *desc;
> > > void *vaddr;
> > > - u16 pkt_cnt;
> > > u16 head, tail;
> > > };
> > >
> > > @@ -598,33 +597,26 @@ static void netsec_set_rx_de(struct netsec_priv *priv,
> > > dring->desc[idx].len = desc->len;
> > > }
> > >
> > > -static int netsec_clean_tx_dring(struct netsec_priv *priv, int budget)
> > > +static bool netsec_clean_tx_dring(struct netsec_priv *priv)
> > > {
> > > struct netsec_desc_ring *dring = &priv->desc_ring[NETSEC_RING_TX];
> > > unsigned int pkts, bytes;
> > > -
> > > - dring->pkt_cnt += netsec_read(priv, NETSEC_REG_NRM_TX_DONE_PKTCNT);
> > > -
> > > - if (dring->pkt_cnt < budget)
> > > - budget = dring->pkt_cnt;
> > > + struct netsec_de *entry;
> > > + int tail = dring->tail;
> > > + int cnt = 0;
> > >
> > > pkts = 0;
> > > bytes = 0;
> > > + entry = dring->vaddr + DESC_SZ * tail;
> > >
> > > - while (pkts < budget) {
> > > + while (!(entry->attr & (1U << NETSEC_TX_SHIFT_OWN_FIELD)) &&
> > > + cnt < DESC_NUM) {
> > > struct netsec_desc *desc;
> > > - struct netsec_de *entry;
> > > - int tail, eop;
> > > -
> > > - tail = dring->tail;
> > > -
> > > - /* move tail ahead */
> > > - dring->tail = (tail + 1) % DESC_NUM;
> > > + int eop;
> > >
> > > desc = &dring->desc[tail];
> > > - entry = dring->vaddr + DESC_SZ * tail;
> > > -
> > > eop = (entry->attr >> NETSEC_TX_LAST) & 1;
> > > + dma_rmb();
> > >
> > > dma_unmap_single(priv->dev, desc->dma_addr, desc->len,
> > > DMA_TO_DEVICE);
> > > @@ -633,38 +625,51 @@ static int netsec_clean_tx_dring(struct netsec_priv *priv, int budget)
> > > bytes += desc->skb->len;
> > > dev_kfree_skb(desc->skb);
> > > }
> > > + /* clean up so netsec_uninit_pkt_dring() won't free the skb
> > > + * again
> > > + */
> > > *desc = (struct netsec_desc){};
> > > +
> > > + /* entry->attr is not going to be accessed by the NIC until
> > > + * netsec_set_tx_de() is called. No need for a dma_wmb() here
> > > + */
> > > + entry->attr = 1U << NETSEC_TX_SHIFT_OWN_FIELD;
> > > + /* move tail ahead */
> > > + dring->tail = (tail + 1) % DESC_NUM;
> > > +
> > > + tail = dring->tail;
> > > + entry = dring->vaddr + DESC_SZ * tail;
> > > + cnt++;
> > > }
> > > - dring->pkt_cnt -= budget;
> > >
> > > - priv->ndev->stats.tx_packets += budget;
> > > + if (!cnt)
> > > + return false;
> > > +
> > > + /* reading the register clears the irq */
> > > + netsec_read(priv, NETSEC_REG_NRM_TX_DONE_PKTCNT);
> > > +
> > > + priv->ndev->stats.tx_packets += cnt;
> > > priv->ndev->stats.tx_bytes += bytes;
> > >
> > > - netdev_completed_queue(priv->ndev, budget, bytes);
> > > + netdev_completed_queue(priv->ndev, cnt, bytes);
> > >
> > > - return budget;
> > > + return true;
> > > }
> > >
> > > -static int netsec_process_tx(struct netsec_priv *priv, int budget)
> > > +static void netsec_process_tx(struct netsec_priv *priv)
> > > {
> > > struct net_device *ndev = priv->ndev;
> > > - int new, done = 0;
> > > + bool cleaned;
> > >
> > > - do {
> > > - new = netsec_clean_tx_dring(priv, budget);
> > > - done += new;
> > > - budget -= new;
> > > - } while (new);
> > > + cleaned = netsec_clean_tx_dring(priv);
> > >
> > > - if (done && netif_queue_stopped(ndev)) {
> > > + if (cleaned && netif_queue_stopped(ndev)) {
> > > /* Make sure we update the value, anyone stopping the queue
> > > * after this will read the proper consumer idx
> > > */
> > > smp_wmb();
> > > netif_wake_queue(ndev);
> > > }
> > > -
> > > - return done;
> > > }
> > >
> > > static void *netsec_alloc_rx_data(struct netsec_priv *priv,
> > > @@ -813,24 +818,17 @@ static int netsec_process_rx(struct netsec_priv *priv, int budget)
> > > static int netsec_napi_poll(struct napi_struct *napi, int budget)
> > > {
> > > struct netsec_priv *priv;
> > > - int tx, rx, done, todo;
> > > + int rx, done, todo;
> > >
> > > priv = container_of(napi, struct netsec_priv, napi);
> > >
> > > + netsec_process_tx(priv);
> > > +
> >
> > So shouldn't we do this *after* processing Rx, and only if there is budget left?
>
> I am not really sure, this would drown Tx processing if you had a bunch of
> received packets that exhausted the budget.
> Intel 1gbit drivers are doing something similar. They reclaim Tx prior to
> processing Rx. The only thing that could be checked here i guess is keep the
> napi poll running if *all* Tx descriptors were processed at some point instead
> of re-enabling irqs.
>
The reason I suggest it is because you quoted it from the documentation :-)
But if reality deviates from that, then sure, let's follow the
examples of others.
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