From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-m16.yeah.net (mail-m16.yeah.net [1.95.21.14]) (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 75FE540F8E3; Thu, 3 Sep 2026 10:44:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=1.95.21.14 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788432290; cv=none; b=Im1xIAUMvX+7+Wb37oO/Wrw0I8/ToF0h/nC6i17Z2RbmZfi775b+yZz+Qiq/oqRHK5gtjUOdtWRkiLjTp2Tip0bY3ZryZOmfxlRd+9wEYnpLT0jnHMCdkGCS2YKRoIT+bzBtOB1kGlbCeCZulHAYQ5/YsVeztkIU2Q2evZEGvAY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788432290; c=relaxed/simple; bh=QpI/UgSWvuHbrm4um+SsKOLm27lIxK1ESWRukfP4YpY=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=gLhF+NeJHeLoDa/dwbTap9EGhM8LmdInn7STipKSLW6j3IL1ZrYGf2FchTs5mlEmkn3IdxJ+VsyJedhBeJI3xFPgfZXYl9p3Gpm0W9baNp9eDX3sqSOdYjLzgCB/5ImwBtb/a8i5qP1RR+iyadF+XWP7pEbps7sEe4hI1bweKN8= 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=E+gsez0X; arc=none smtp.client-ip=1.95.21.14 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="E+gsez0X" 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=Sr 3wCpan+elwvh65luVRE7s2DYBBu8bEJj0qSCuyV+c=; b=E+gsez0XpsNM6EA4jw bYhF0thBlJTD8sBn99I56vcrPTBg+ooAUFB9+6dNq9h9M8GW1QFWtFSirzU+hQNJ SZC/7dV0499LJn/G7R7tyHhJ14e849NpAD4IkjvpOyB+FdONuWttunFKk07Tzk+i 31ZZR/SKrO9qHRX730vOivHAQ= Received: from deepin (unknown []) by gzsmtp2 (Coremail) with UTF8SMTPA id Ms8vCgD3P1MvSJlqapWfBQ--.42003S2; Thu, 03 Sep 2026 18:13:04 +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 v2] tcp: fix mdev comment in tcp_rtt_estimator() Date: Thu, 3 Sep 2026 18:12:56 +0800 Message-ID: <20260903101256.8879-1-zhangcoder@yeah.net> X-Mailer: git-send-email 2.51.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-CM-TRANSID:Ms8vCgD3P1MvSJlqapWfBQ--.42003S2 X-Coremail-Antispam: 1Uf129KBjvdXoWrurWDtF45Jw4fKF13XrykuFg_yoWDCwb_Ca yjkFWDGry8X3Wayw4DAF4Yya98CFWxAa4Sgr1fGayDKw18GF4fGa92yF9rZ3W7Cr43KF95 Zws0q343trn3ujkaLaAFLSUrUUUUjb8apTn2vfkv8UJUUUU8Yxn0WfASr-VFAUDa7-sFnT 9fnUUvcSsGvfC2KfnxnUUI43ZEXa7xR_UUUUUUUUU== X-CM-SenderInfo: x2kd0wpfrgv2o61htxgoqh3/1tbiNhAf32qZSDBUWgAA3j The old comment "mdev = 3/4 mdev + 1/4 new" only describes the common case. However, when RTT decreases and |rtt - srtt| > mdev/4, 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 | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 0f60a1dbf..b4a6aba21 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -1108,7 +1108,11 @@ 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 RTT decrease when + * abs(rtt - srtt) > mdev/4 + */ 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