netdev - Re: [PATCH net 3/3] udp: avoid bulk memory scheduling on memory pressure.

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Date:   Fri, 17 Jan 2020 19:37:17 +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 <willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH net 3/3] udp: avoid bulk memory scheduling on memory
 pressure.

Hi,

On Fri, 2020-01-17 at 09:51 -0800, Eric Dumazet wrote:
> On 1/17/20 9:27 AM, Paolo Abeni wrote:
> > Williem reported that after commit 0d4a6608f68c ("udp: do rmem bulk
> > free even if the rx sk queue is empty") the memory allocated by
> > an almost idle system with many UDP sockets can grow a lot.
> > 
> > This change addresses the issue enabling memory pressure tracking
> > for UDP and flushing the fwd allocated memory on dequeue if the
> > UDP protocol is under memory pressure.
> > 
> > Note that with this patch applied, the system allocates more
> > liberally memory for UDP sockets while the total memory usage is
> > below udp_mem[1], while the vanilla kernel would allow at most a
> > single page per socket when UDP memory usage goes above udp_mem[0]
> > - see __sk_mem_raise_allocated().
> > 
> > Reported-and-diagnosed-by: Willem de Bruijn <willemdebruijn.kernel@...il.com>
> > Fixes: commit 0d4a6608f68c ("udp: do rmem bulk free even if the rx sk queue is empty")

Thank you for the feedback.

> Not a proper Fixes: tag
> 
> Frankly I would rather revert this patch, unless you show how much things were improved.

unpatched version:

# ensure we will hit memory pressure
echo "5000 10000 20000" > /proc/sys/net/ipv4/udp_mem

# run the repro from Willem
./repro.py

# it get stuck after opening a bunch of sockets, because
# __udp_enqueue_schedule_skb() hits the memory limit
# and packets are dropped.

patched kernel:
echo "5000 10000 20000" > /proc/sys/net/ipv4/udp_mem
./repro.py
# completes successfully, output trimmed
sockets: used 10179
TCP: inuse 4 orphan 0 tw 0 alloc 7 mem 1
UDP: inuse 4 mem 19860
UDPLITE: inuse 0
RAW: inuse 0
FRAG: inuse 0 memory 0

To complete successfully with an unpatched kernel the reproducer
requires memory limits 1 or 2 order of magnitude greaters.

> Where in the UDP code the forward allocations will be released while udp_memory_pressure
> is hit ?

fwd memory is released by udp_rmem_release(), so every time some
process tries to read from any UDP socket. Surely less effective than
the TCP infrastructure, but that other option looks overkill for UDP?!?

Thanks,

Paolo