This patch rewrites comments related task priorities and CPU usage along with an example to show how it works.
Cc: Juri Lelli <[email protected]> Cc: Patrick Bellasi <[email protected]> Cc: Dietmar Eggemann <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Ingo Molnar <[email protected]> Signed-off-by: Joel Fernandes <[email protected]> --- kernel/sched/core.c | 27 +++++++++++++++++++-------- 1 file changed, 19 insertions(+), 8 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index c56fb57f2991..2175bf663f3d 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -8823,16 +8823,27 @@ void dump_cpu_task(int cpu) } /* - * Nice levels are multiplicative, with a gentle 10% change for every - * nice level changed. I.e. when a CPU-bound task goes from nice 0 to - * nice 1, it will get ~10% less CPU time than another CPU-bound task - * that remained on nice 0. + * Nice levels are multiplicative, with a gentle 10% relative change + * for every nice level changed. I.e. if there were 2 CPU-bound tasks + * of equal nice value and one of them goes from a nice level of 0 to 1 + * then the task at nice level 1 will get ~5% less CPU time than before + * the change and the task that remained at nice level 0 will get ~5% + * more CPU time. * * The "10% effect" is relative and cumulative: from _any_ nice level, - * if you go up 1 level, it's -10% CPU usage, if you go down 1 level - * it's +10% CPU usage. (to achieve that we use a multiplier of 1.25. - * If a task goes up by ~10% and another task goes down by ~10% then - * the relative distance between them is ~25%.) + * if you go up 1 level, it's -10% relative CPU usage, if you go down + * by 1 level it's +10% CPU usage. To achieve that, we use a multiplier + * of 1.25. If a task goes up by ~5% and another task goes down by ~5% + * then the relative distance between their weights is ~25% as shown + * in the following example: + * + * Consider 2 tasks T1 and T2 which are scheduled within a sched_period + * of 10ms. Say T1 has a nice value 0 and T2 has a nice value 1, + * then their corresponding weights are 1024 for T1 and 820 for T2. + * + * The relative delta between their weights is ~25% (1.25 * 820 ~= 1024) + * T1's CPU slice = (1024 / (820 + 1024)) * 10 ~= 5.5ms (55% usage) + * T2's CPU slice = (820 / (820 + 1024)) * 10 ~= 4.5ms (45% usage) */ const int sched_prio_to_weight[40] = { /* -20 */ 88761, 71755, 56483, 46273, 36291, -- 2.12.0.246.ga2ecc84866-goog
