From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-m16.yeah.net (mail-m16.yeah.net [1.95.21.16]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F23663EFFD4; Tue, 1 Sep 2026 02:43:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=1.95.21.16 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788230644; cv=none; b=HZKcxAn6S7ouI8YzQUr7wX00KsGf37a8BixxFJTFhFYSHm2dhvzxW8TidIorih6nNJzIW/3P9X6ANZyk6zTndgBRy3j3umP8Q8wfy64CJ0oDa5OSEJRDYwK0Yt6Dv+r9KTVdh97mY/ypUc5B/IyM6wJp/S3r9GXtijpmcb5qCEU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788230644; c=relaxed/simple; bh=MWpT/0P1Af7d09VLzwF2GmeaI7uTka5XJYtq5CvCkps=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=biDE15jFqP1AK+Z3SF7MO6UCF/WjZJo487i2WsrigQoa97z59HDPBAiirAXzLG2GH3MRloDVdQr3OM43oHNgeCHTsterGJk7fCZQwy+nYG7O3rLKxddVFclfTsSkNG4DT4z0hxDmcJNWiT7xc5cLxlu27QFA0KGrv310WJgotxk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=yeah.net; spf=pass smtp.mailfrom=yeah.net; dkim=pass (1024-bit key) header.d=yeah.net header.i=@yeah.net header.b=hT3I+iRw; arc=none smtp.client-ip=1.95.21.16 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=yeah.net Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=yeah.net Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=yeah.net header.i=@yeah.net header.b="hT3I+iRw" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=yeah.net; s=s110527; h=From:To:Subject:Date:Message-ID:MIME-Version; bh=H3 1g108x3AGghVZiee+8fPu3Xnu7PnV0hFSaohOMJko=; b=hT3I+iRwvjJmeMQ0yn knE8hXfMlNB3B3wzUvkNpcazKLHrVw5wXbpV+CdSrBm+RKkbZibvJzp8BqcCwXtq Kvo1JLf2ONkwih2HtDXIHeaoFf4dV5HQG3EX/+EjK4YSUyi6cJ+NLWOiRl7Hgfxl VCXkqBqEn34VuEDqz2HFzN60Q= Received: from deepin (unknown []) by gzsmtp2 (Coremail) with UTF8SMTPA id Ms8vCgCnaf8zOpZqjCNzBQ--.36566S2; Tue, 01 Sep 2026 10:36:36 +0800 (CST) From: Ziran Zhang To: Eric Dumazet , Neal Cardwell , "David S . Miller" , Jakub Kicinski , Paolo Abeni Cc: Kuniyuki Iwashima , Simon Horman , linux-kernel@vger.kernel.org, netdev@vger.kernel.org, Ziran Zhang Subject: [PATCH net-next] tcp: fix mdev comment in tcp_rtt_estimator() Date: Tue, 1 Sep 2026 10:35:38 +0800 Message-ID: <20260901023539.6986-1-zhangcoder@yeah.net> X-Mailer: git-send-email 2.51.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-CM-TRANSID:Ms8vCgCnaf8zOpZqjCNzBQ--.36566S2 X-Coremail-Antispam: 1Uf129KBjvdXoWrurWDtF1DWFWDZFy7Cw4rKrg_yoWDGFcEka 4jkFWUGry8Xw42qw4DAF4YyFZ5CFs7A3WSgr1fGayqkw1UJF4fGan2yFy7X3W3CrsIkF93 Zwn0q34avr1fujkaLaAFLSUrUUUUjb8apTn2vfkv8UJUUUU8Yxn0WfASr-VFAUDa7-sFnT 9fnUUvcSsGvfC2KfnxnUUI43ZEXa7xR_UUUUUUUUU== X-CM-SenderInfo: x2kd0wpfrgv2o61htxgoqh3/1tbiIhTbnGqWOjT9awAA3V The old comment "mdev = 3/4 mdev + 1/4 new" only describes the common case. However, when RTT drops sharply, the code applies a finer gain of 1/32 instead of 1/4, resulting in: mdev = 31/32 mdev + 1/32 * new. Express both cases with a unified formula using variable gain g. Signed-off-by: Ziran Zhang --- net/ipv4/tcp_input.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 0f60a1dbf..2c9feb181 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -1108,7 +1108,10 @@ static void tcp_rtt_estimator(struct sock *sk, long mrtt_us) } else { m -= (tp->mdev_us >> 2); /* similar update on mdev */ } - tp->mdev_us += m; /* mdev = 3/4 mdev + 1/4 new */ + tp->mdev_us += m; /* + * mdev = (1-g)*mdev + g*new, g=1/4 normally, + * g=1/32 on sharp RTT drop + */ if (tp->mdev_us > tp->mdev_max_us) { tp->mdev_max_us = tp->mdev_us; if (tp->mdev_max_us > tp->rttvar_us) -- 2.51.0