netdev - Re: [PATCH] netem: Segment GSO packets on enqueue.

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Message-ID: <20160427112748.GA23299@hmsreliant.think-freely.org>
Date:	Wed, 27 Apr 2016 07:27:48 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, Jamal Hadi Salim <jhs@...atatu.com>,
	"David S. Miller" <davem@...emloft.net>,
	netem@...ts.linux-foundation.org
Subject: Re: [PATCH] netem: Segment GSO packets on enqueue.

On Tue, Apr 26, 2016 at 01:19:15PM -0700, Eric Dumazet wrote:
> On Tue, 2016-04-26 at 15:00 -0400, Neil Horman wrote:
> > I can understand that, but that raises two questions in my mind:
> > 
> > 1)  Doesn't that make all the statistical manipulation for netem wrong?  That is
> > to say, if netem drops 5% of packets, and it happens to drop a GSO packet, its
> > actually dropping several, instead of the single one required.
> 
> 
> Please take a look at tbf_segment(), where you can find a proper way to
> handle this.
> 
> Note that for the case q->corrupt is 0, we definitely do not want to
> segment TSO packets.
> 
> > 2) How are you getting netem to work with GSO at all?  The warning is triggered
> > for me on every GSO packet, which I think would impact throughput :)
> 
> I mostly use netem to add delays and drops.
> I never had this bug, since q->corrupt = 0
> 

I see what you're saying now, I should only be segmenting the packet if the
qdisc needs to compute the checksum on each packet.  Other packets that aren't
selected to be mangled can pass through un-segmented (assuming they meet any
other queue constraints).

Ok, thanks.  Self-nak.  I'll respin/test and post a new version

Best
Neil

> 
> 
>