netdev - Re: 3.7.8/amd64 full interrupt hangs due to iwlwifi under big nfs copies out

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Message-ID: <1361377243.8629.34.camel@jlt4.sipsolutions.net>
Date:	Wed, 20 Feb 2013 17:20:43 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Marc MERLIN <marc@...lins.org>, David Miller <davem@...emloft.net>,
	Larry.Finger@...inger.net, bhutchings@...arflare.com,
	linux-wireless@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: 3.7.8/amd64 full interrupt hangs due to iwlwifi under big nfs
 copies out

On Wed, 2013-02-20 at 07:11 -0800, Eric Dumazet wrote:
> On Wed, 2013-02-20 at 10:15 +0100, Johannes Berg wrote:
> 
> > OTOH, this affects the protocol, and when you really can't allocate any
> > order-1 pages you pointed out yourself that many other things also won't
> > work, so I'm not really sure it makes a big difference if we change the
> > driver?
> 
> It will make a huge difference, even on non pressure mode, as TCP
> receive window will grow twice faster.

Hmm, why does that depend on the allocation size?

> Unless wifi speed reaches 10Gbps, following patch should do the trick
> 
> 
> diff --git a/drivers/net/wireless/iwlwifi/dvm/rx.c b/drivers/net/wireless/iwlwifi/dvm/rx.c
> index a4eed20..77a3ee3 100644
> --- a/drivers/net/wireless/iwlwifi/dvm/rx.c
> +++ b/drivers/net/wireless/iwlwifi/dvm/rx.c
> @@ -750,7 +750,12 @@ static void iwlagn_pass_packet_to_mac80211(struct iwl_priv *priv,
>  	/* Dont use dev_alloc_skb(), we'll have enough headroom once
>  	 * ieee80211_hdr pulled.
>  	 */
> -	skb = alloc_skb(128, GFP_ATOMIC);
> +	fraglen = 128;
> +	/* if we use order-1 pages, copy to get better TCP performance */
> +	if (rxb->truesize > PAGE_SIZE)
> +		fraglen = max_t(unsigned, fraglen, len);
> +
> +	skb = alloc_skb(fraglen, GFP_ATOMIC);

Hmm, I don't quite understand -- that's not doing any copy?

FWIW if you do the copy you should not "steal" the pages, then they'd be
recycled in the RX ring right away.

johannes

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