netdev - Re: [RFC PATCH] net: don't keep lonely packets forever in the gro hash

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Date:   Tue, 20 Nov 2018 16:42:48 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [RFC PATCH] net: don't keep lonely packets forever in the gro
 hash

Hi,

On Tue, 2018-11-20 at 05:49 -0800, Eric Dumazet wrote:
> 
> On 11/20/2018 02:17 AM, Paolo Abeni wrote:
> > Eric noted that with UDP GRO and napi timeout, we could keep a single
> > UDP packet inside the GRO hash forever, if the related NAPI instance
> > calls napi_gro_complete() at an higher frequency than the napi timeout.
> > Willem noted that even TCP packets could be trapped there, till the
> > next retransmission.
> > This patch tries to address the issue, flushing the oldest packets before
> > scheduling the NAPI timeout. The rationale is that such a timeout should be
> > well below a jiffy and we are not flushing packets eligible for sane GRO.
> > 
> > Reported-by: Eric Dumazet <eric.dumazet@...il.com>
> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > ---
> > Sending as RFC, as I fear I'm missing some relevant pieces.
> > Also I'm unsure if this should considered a fixes for "udp: implement
> > GRO for plain UDP sockets." or for "net: gro: add a per device gro flush timer"

Thank you for your feedback!

> Truth be told, relying on jiffies change is a bit fragile for HZ=100 or HZ=250 kernels.

Yes, we have higher bound there.

> See recent TCP commit that got rid of tcp_tso_should_defer() dependency on HZ/jiffies
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/commit/?id=a682850a114aef947da5d603f7fd2cfe7eabbd72

I'm unsure I follow correctly. Are you suggesting to use ns precision
for skb aging in GRO? If so, could that be a separate change? (looks
more invasive)

Thanks,

Paolo