| Message ID | 20240426152011.37069-1-richard120310@gmail.com (mailing list archive) |
|---|---|
| State | Changes Requested |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | tcp_bbr: replace lambda expression with bitwise operation for bit flip | expand |
On Fri, Apr 26, 2024 at 5:20 PM I Hsin Cheng <richard120310@gmail.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@gmail.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.
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@gmail.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@gmail.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.
On Sat, Apr 27, 2024 at 01:01:21AM +0800, I Hsin Cheng wrote: > 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. Out of curiosity, where do you see any lambda expressions in the entire thing?
On Fri, Apr 26, 2024 at 09:19:02PM +0100, Al Viro wrote: > On Sat, Apr 27, 2024 at 01:01:21AM +0800, I Hsin Cheng wrote: > > > 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. > > Out of curiosity, where do you see any lambda expressions in the entire > thing? Sorry, it's my fault to address the expression as "lambda expression", it should be called as "conditional" or "ternary" operator. Thanks for your remind.
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; } }
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@gmail.com> --- net/ipv4/tcp_bbr.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)