linux-kernel - [PATCH 4.4 111/312] tcp: do not set rtt

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Date:   Fri,  8 May 2020 14:31:42 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 4.4 111/312] tcp: do not set rtt_min to 1

From: Eric Dumazet <edumazet@...gle.com>

commit 372022830b06d9980c7e8b41fa0a4081cff883b0 upstream.

There are some cases where rtt_us derives from deltas of jiffies,
instead of using usec timestamps.

Since we want to track minimal rtt, better to assume a delta of 0 jiffie
might be in fact be very close to 1 jiffie.

It is kind of sad jiffies_to_usecs(1) calls a function instead of simply
using a constant.

Fixes: f672258391b42 ("tcp: track min RTT using windowed min-filter")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 net/ipv4/tcp_input.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2926,7 +2926,10 @@ static void tcp_update_rtt_min(struct so
 {
 	const u32 now = tcp_time_stamp, wlen = sysctl_tcp_min_rtt_wlen * HZ;
 	struct rtt_meas *m = tcp_sk(sk)->rtt_min;
-	struct rtt_meas rttm = { .rtt = (rtt_us ? : 1), .ts = now };
+	struct rtt_meas rttm = {
+		.rtt = likely(rtt_us) ? rtt_us : jiffies_to_usecs(1),
+		.ts = now,
+	};
 	u32 elapsed;

 	/* Check if the new measurement updates the 1st, 2nd, or 3rd choices */