diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c index d2f49db70523..f2f9b75008bb 100644 --- a/net/sched/sch_cake.c +++ b/net/sched/sch_cake.c @@ -361,8 +361,24 @@ static const u8 besteffort[] = { static const u8 normal_order[] = {0, 1, 2, 3, 4, 5, 6, 7}; static const u8 bulk_order[] = {1, 0, 2, 3}; +/* There is a big difference in timing between the accurate values placed in the + * cache and the approximations given by a single Newton step for small count + * values, particularly when stepping from count 1 to 2 or vice versa. Hence, + * these values are calculated using eight Newton steps, using the + * implementation below. Above 16, a single Newton step gives sufficient + * accuracy in either direction, given the precision stored. + * + * The magnitude of the error when stepping up to count 2 is such as to give the + * value that *should* have been produced at count 4. + */ + #define REC_INV_SQRT_CACHE (16) -static u32 cobalt_rec_inv_sqrt_cache[REC_INV_SQRT_CACHE] = {0}; +static const u32 inv_sqrt_cache[REC_INV_SQRT_CACHE] = { + ~0, ~0, 3037000500, 2479700525, + 2147483647, 1920767767, 1753413056, 1623345051, + 1518500250, 1431655765, 1358187914, 1294981364, + 1239850263, 1191209601, 1147878294, 1108955788 +}; /* http://en.wikipedia.org/wiki/Methods_of_computing_square_roots * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2) @@ -388,47 +404,14 @@ static void cobalt_newton_step(struct cobalt_vars *vars) static void cobalt_invsqrt(struct cobalt_vars *vars) { if (vars->count < REC_INV_SQRT_CACHE) - vars->rec_inv_sqrt = cobalt_rec_inv_sqrt_cache[vars->count]; + vars->rec_inv_sqrt = inv_sqrt_cache[vars->count]; else cobalt_newton_step(vars); } -/* There is a big difference in timing between the accurate values placed in - * the cache and the approximations given by a single Newton step for small - * count values, particularly when stepping from count 1 to 2 or vice versa. - * Above 16, a single Newton step gives sufficient accuracy in either - * direction, given the precision stored. - * - * The magnitude of the error when stepping up to count 2 is such as to give - * the value that *should* have been produced at count 4. - */ - -static void cobalt_cache_init(void) -{ - struct cobalt_vars v; - - memset(&v, 0, sizeof(v)); - v.rec_inv_sqrt = ~0U; - cobalt_rec_inv_sqrt_cache[0] = v.rec_inv_sqrt; - - for (v.count = 1; v.count < REC_INV_SQRT_CACHE; v.count++) { - cobalt_newton_step(&v); - cobalt_newton_step(&v); - cobalt_newton_step(&v); - cobalt_newton_step(&v); - - cobalt_rec_inv_sqrt_cache[v.count] = v.rec_inv_sqrt; - } -} - static void cobalt_vars_init(struct cobalt_vars *vars) { memset(vars, 0, sizeof(*vars)); - - if (!cobalt_rec_inv_sqrt_cache[0]) { - cobalt_cache_init(); - cobalt_rec_inv_sqrt_cache[0] = ~0; - } } /* CoDel control_law is t + interval/sqrt(count)