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

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Message-ID: <1461701955.5535.38.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Tue, 26 Apr 2016 13:19:15 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Neil Horman <nhorman@...driver.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, 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