netdev - Re: [PATCH] tcp_bbr: replace lambda expression with bitwise operation for bit flip

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Date: Sat, 27 Apr 2024 01:01:21 +0800
From: I Hsin Cheng <richard120310@...il.com>
To: Eric Dumazet <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
Subject: Re: [PATCH] tcp_bbr: replace lambda expression with bitwise
 operation for bit flip

On Fri, Apr 26, 2024 at 05:32:57PM +0200, Eric Dumazet wrote:
> On Fri, Apr 26, 2024 at 5:20 PM I Hsin Cheng <richard120310@...il.com> wrote:
> >
> > 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;
> >                 }
> >         }
> 
> Or
> 
> bbr->extra_acked_win_idx ^= 1;
> 
> Note that C compilers generate the same code, for the 3 variants.
> 
> They do not generate branches for something simple like this.

I see, thanks for your explanation.
I thought the compilers behavior might alters due to different 
architecture or different compilers.
So would you recommend on the proposed changes or we should stick to
 the original implementation? 
Personally I think my version or your proposed change are both more 
understandable and elegant than the lambda expression.