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Date: Tue, 08 Dec 2020 16:28:52 -0800 (PST) From: David Miller <davem@...emloft.net> To: eric.dumazet@...il.com Cc: kuba@...nel.org, netdev@...r.kernel.org, edumazet@...gle.com, soheil@...gle.com, ncardwell@...gle.com, ycheng@...gle.com, abuehaze@...zon.com Subject: Re: [PATCH net] tcp: select sane initial rcvq_space.space for big MSS From: Eric Dumazet <eric.dumazet@...il.com> Date: Tue, 8 Dec 2020 08:21:31 -0800 > From: Eric Dumazet <edumazet@...gle.com> > > Before commit a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB") > small tcp_rmem[1] values were overridden by tcp_fixup_rcvbuf() to accommodate various MSS. > > This is no longer the case, and Hazem Mohamed Abuelfotoh reported > that DRS would not work for MTU 9000 endpoints receiving regular (1500 bytes) frames. > > Root cause is that tcp_init_buffer_space() uses tp->rcv_wnd for upper limit > of rcvq_space.space computation, while it can select later a smaller > value for tp->rcv_ssthresh and tp->window_clamp. > > ss -temoi on receiver would show : > > skmem:(r0,rb131072,t0,tb46080,f0,w0,o0,bl0,d0) rcv_space:62496 rcv_ssthresh:56596 > > This means that TCP can not increase its window in tcp_grow_window(), > and that DRS can never kick. > > Fix this by making sure that rcvq_space.space is not bigger than number of bytes > that can be held in TCP receive queue. > > People unable/unwilling to change their kernel can work around this issue by > selecting a bigger tcp_rmem[1] value as in : > > echo "4096 196608 6291456" >/proc/sys/net/ipv4/tcp_rmem > > Based on an initial report and patch from Hazem Mohamed Abuelfotoh > https://lore.kernel.org/netdev/20201204180622.14285-1-abuehaze@amazon.com/ > > Fixes: a337531b942b ("tcp: up initial rmem to 128KB and SYN rwin to around 64KB") > Fixes: 041a14d26715 ("tcp: start receiver buffer autotuning sooner") > Reported-by: Hazem Mohamed Abuelfotoh <abuehaze@...zon.com> > Signed-off-by: Eric Dumazet <edumazet@...gle.com> Applied, thanks Eric.
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