netdev - Re: [RFC PATCH] Regression in linux 2.6.32 virtio

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Message-ID: <20091217143635.GA388@gondor.apana.org.au>
Date:	Thu, 17 Dec 2009 22:36:35 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Jarek Poplawski <jarkao2@...il.com>, mst@...hat.com,
	netdev@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>,
	Sridhar Samudrala <sri@...ibm.com>
Subject: Re: [RFC PATCH] Regression in linux 2.6.32 virtio_net seen with
	vhost-net

On Thu, Dec 17, 2009 at 08:05:02PM +0530, Krishna Kumar2 wrote:
>
> I think the bug is in this check:
> 
> +     if (vi->capacity >= 2 + MAX_SKB_FRAGS) {
> +           /* Suppress further xmit interrupts. */
> +           vi->svq->vq_ops->disable_cb(vi->svq);
> +           napi_complete(xmit_napi);
> +
> +           /* Don't wake it if link is down. */
> +           if (likely(netif_carrier_ok(vi->vdev)))
> +                 netif_wake_queue(vi->dev);
> +     }
> 
> We wake up too fast, just enough space for one more skb to be sent
> before the queue is stopped again. And hence no more messages about
> queue full, but lot of requeues. The qdisc code is doing the correct
> thing, but we need to increase the limit here.
> 
> Can we try with some big number, 64, 128?

Good point.  Sridhar, could you please test doubling or quadrupling
the threshold? 128 is probably a bit too much though as the ring
only has 256 entries.

Cheers,
-- 
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Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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