netdev - RE: [PATCH net-next v2 06/10] net: stmmac: Do not disable interrupts when cleaning TX

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Date:   Mon, 1 Jul 2019 10:15:17 +0000
From:   Jose Abreu <Jose.Abreu@...opsys.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jose Abreu <Jose.Abreu@...opsys.com>
CC:     linux-kernel <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        "David S . Miller" <davem@...emloft.net>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>
Subject: RE: [PATCH net-next v2 06/10] net: stmmac: Do not disable interrupts
 when cleaning TX

From: Willem de Bruijn <willemdebruijn.kernel@...il.com>

> By the
> 
>         if ((status & handle_rx) && (chan < priv->plat->rx_queues_to_use)) {
>                 stmmac_disable_dma_irq(priv, priv->ioaddr, chan);
>                 napi_schedule_irqoff(&ch->rx_napi);
>         }
> 
> branch directly above? If so, is it possible to have fewer rx than tx
> queues and miss this?

Yes, it is possible.

> this logic seems more complex than needed?
> 
>         if (status)
>                 status |= handle_rx | handle_tx;
> 
>         if ((status & handle_rx) && (chan < priv->plat->rx_queues_to_use)) {
> 
>         }
> 
>         if ((status & handle_tx) && (chan < priv->plat->tx_queues_to_use)) {
> 
>         }
> 
> status & handle_rx implies status & handle_tx and vice versa.

This is removed in patch 09/10.

> > -       if (work_done < budget && napi_complete_done(napi, work_done))
> > -               stmmac_enable_dma_irq(priv, priv->ioaddr, chan);
> > +       if (work_done < budget)
> > +               napi_complete_done(napi, work_done);
> 
> It does seem odd that stmmac_napi_poll_rx and stmmac_napi_poll_tx both
> call stmmac_enable_dma_irq(..) independent of the other. Shouldn't the
> IRQ remain masked while either is active or scheduled? That is almost
> what this patch does, though not exactly.

After patch 09/10 the interrupts will only be disabled by RX NAPI and 
re-enabled by it again. I can do some tests on whether disabling 
interrupts independently gives more performance but I wouldn't expect so 
because the real bottleneck when I do iperf3 tests is the RX path ...