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Date: Sat, 10 Dec 2022 18:45:56 +0100 From: Eric Dumazet <edumazet@...gle.com> To: Jakub Sitnicki <jakub@...udflare.com> Cc: Neal Cardwell <ncardwell@...gle.com>, Weiping Zhang <zhangweiping@...iglobal.com>, davem@...emloft.net, yoshfuji@...ux-ipv6.org, dsahern@...nel.org, kuba@...nel.org, pabeni@...hat.com, netdev@...r.kernel.org, zwp10758@...il.com Subject: Re: [RFC PATCH] tcp: correct srtt and mdev_us calculation On Tue, Dec 6, 2022 at 10:29 AM Jakub Sitnicki <jakub@...udflare.com> wrote: > Nifty. And it's documented. > > struct tcp_sock { > … > u32 srtt_us; /* smoothed round trip time << 3 in usecs */ > > Thanks for the hint. The >> 3 is all over the place... So even without a formal comment, anyone familiar with TCP stack would spot this... include/net/tcp.h:700: return usecs_to_jiffies((tp->srtt_us >> 3) + tp->rttvar_us); include/trace/events/tcp.h:286: __entry->srtt = tp->srtt_us >> 3; include/uapi/linux/bpf.h:6038: __u32 srtt_us; /* smoothed round trip time << 3 in usecs */ include/uapi/linux/bpf.h:6396: __u32 srtt_us; /* Averaged RTT << 3 in usecs */ et/ipv4/tcp.c:3906: info->tcpi_rtt = tp->srtt_us >> 3; net/ipv4/tcp.c:4045: nla_put_u32(stats, TCP_NLA_SRTT, tp->srtt_us >> 3); net/ipv4/tcp_bbr.c:273: if (tp->srtt_us) { /* any RTT sample yet? */ net/ipv4/tcp_bbr.c:274: rtt_us = max(tp->srtt_us >> 3, 1U); ...
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