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

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Message-ID: <1361379597.8629.39.camel@jlt4.sipsolutions.net>
Date:	Wed, 20 Feb 2013 17:59:57 +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 08:55 -0800, Eric Dumazet wrote:

> > > 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?
> 
> I guess you missed all the patches about skb->truesize on netdev

Yeah, I don't follow netdev much any more...

> > > -	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.
> 
> Code should just works, please read the following lines in the same
> function....
> 
>         /* If frame is small enough to fit in skb->head, pull it completely.
>          * If not, only pull ieee80211_hdr so that splice() or TCP coalesce
>          * are more efficient.
>          */

Oh, right, though I guess the comment is now wrong since practically
every packet will be copied either here or in mac80211 (A-MSDUs are
split up there)

johannes

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