Re: [PATCH v3 01/10] tcp, util: Add function for scaling to linearly interpolated factor, use it

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Dec 08, 2025 at 08:20:14AM +0100, Stefano Brivio wrote:
> Right now, the only need for this kind of function comes from
> tcp_get_sndbuf(), which calculates the amount of sending buffer we
> want to use depending on its own size: we want to use more of it
> if it's smaller, as bookkeeping overhead is usually lower and we rely
> on auto-tuning there, and use less of it when it's bigger.
> 
> For this purpose, the new function is overly generic: @x is the same
> as @y, that is, we want to use more or less of the buffer depending
> on the size of the buffer itself.
> 
> However, an upcoming change will need that generality, as we'll want
> to scale the amount of sending buffer we use depending on another
> (scaled) factor.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  tcp.c  |  6 +-----
>  util.c | 38 ++++++++++++++++++++++++++++++++++++++
>  util.h |  1 +
>  3 files changed, 40 insertions(+), 5 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index bb661ee..026546a 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -788,11 +788,7 @@ static void tcp_get_sndbuf(struct tcp_tap_conn *conn)
>  		return;
>  	}
>  
> -	v = sndbuf;
> -	if (v >= SNDBUF_BIG)
> -		v /= 2;
> -	else if (v > SNDBUF_SMALL)
> -		v -= v * (v - SNDBUF_SMALL) / (SNDBUF_BIG - SNDBUF_SMALL) / 2;
> +	v = clamped_scale(sndbuf, sndbuf, SNDBUF_SMALL, SNDBUF_BIG, 50);
>  
>  	SNDBUF_SET(conn, MIN(INT_MAX, v));
>  }
> diff --git a/util.c b/util.c
> index f32c9cb..2232a24 100644
> --- a/util.c
> +++ b/util.c
> @@ -1223,3 +1223,41 @@ void fsync_pcap_and_log(void)
>  	if (log_file != -1)
>  		(void)fsync(log_file);
>  }
> +
> +/**
> + * clamped_scale() - Scale @x from 100% to f% depending on @y's value
> + * @x:		Value to scale
> + * @y:		Value determining scaling
> + * @lo:		Lower bound for @y (start of y-axis slope)
> + * @hi:		Upper bound for @y (end of y-axis slope)
> + * @f:		Scaling factor, percent (might be less or more than 100)
> + *
> + * Return: @x scaled by @f * linear interpolation of @y between @lo and @hi
> + *
> + * In pictures:
> + *
> + *                f % -> ,----   * If @y < lo (for example, @y is y0), return @x
> + *                      /|   |
> + *                     / |   |   * If @lo < @y < @hi (for example, @y is y1),
> + *                    /  |   |     return @x scaled by a factor linearly
> + * (100 + f) / 2 % ->/   |   |     interpolated between 100% and f% depending on
> + *                  /|   |   |     @y's position between @lo (100%) and @hi (f%)
> + *                 / |   |   |
> + *                /  |   |   |   * If @y > @hi (for example, @y is y2), return
> + * 100 % -> -----'   |   |   |     @x * @f / 100
> + *           |   |   |   |   |
> + *          y0  lo  y1  hi  y2   Example: @f = 150, @lo = 10, @hi = 20, @y = 15,
> + *                                        @x = 1000
> + *                                        -> interpolated factor is 125%
> + *                                        -> return 1250
> + */
> +long clamped_scale(long x, long y, long lo, long hi, long f)
> +{
> +	if (y < lo)
> +		return x;
> +
> +	if (y > hi)
> +		return x * f / 100;
> +
> +	return x - (x * (y - lo) / (hi - lo)) * (100 - f) / 100;
> +}
> diff --git a/util.h b/util.h
> index 17f5ae0..744880b 100644
> --- a/util.h
> +++ b/util.h
> @@ -242,6 +242,7 @@ int read_remainder(int fd, const struct iovec *iov, size_t cnt, size_t skip);
>  void close_open_files(int argc, char **argv);
>  bool snprintf_check(char *str, size_t size, const char *format, ...);
>  void fsync_pcap_and_log(void);
> +long clamped_scale(long x, long y, long lo, long hi, long f);
>  
>  /**
>   * af_name() - Return name of an address family
> -- 
> 2.43.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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