linux-kernel - [PATCH] tcp_bbr: replace lambda expression with bitwise operation for bit flip

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Message-Id: <20240426152011.37069-1-richard120310@gmail.com>
Date: Fri, 26 Apr 2024 23:20:11 +0800
From: I Hsin Cheng <richard120310@...il.com>
To: edumazet@...gle.com
Cc: davem@...emloft.net,
	dsahern@...nel.org,
	kuba@...nel.org,
	pabeni@...hat.com,
	netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org,
	I Hsin Cheng <richard120310@...il.com>
Subject: [PATCH] tcp_bbr: replace lambda expression with bitwise operation for bit flip

In the origin implementation in function bbr_update_ack_aggregation(),
we utilize a lambda expression to flip the bit value of
bbr->extra_acked_win_idx. Since the data type of
bbr->extra_acked_win_idx is simply a single bit, we are actually trying
to perform a bit flip operation, under the fact we can simply perform a
bitwise not operation on bbr->extra_acked_win_idx.

This way we can elimate the need of possible branches which generate by
the lambda function, they could result in branch misses sometimes.
Perform a bitwise not operation is more straightforward and wouldn't
generate branches.

Signed-off-by: I Hsin Cheng <richard120310@...il.com>
---
 net/ipv4/tcp_bbr.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c
index 146792cd2..75068ba25 100644
--- a/net/ipv4/tcp_bbr.c
+++ b/net/ipv4/tcp_bbr.c
@@ -829,8 +829,7 @@ static void bbr_update_ack_aggregation(struct sock *sk,
 						bbr->extra_acked_win_rtts + 1);
 		if (bbr->extra_acked_win_rtts >= bbr_extra_acked_win_rtts) {
 			bbr->extra_acked_win_rtts = 0;
-			bbr->extra_acked_win_idx = bbr->extra_acked_win_idx ?
-						   0 : 1;
+			bbr->extra_acked_win_idx = ~(bbr->extra_acked_win_idx);
 			bbr->extra_acked[bbr->extra_acked_win_idx] = 0;
 		}
 	}
-- 
2.34.1