The PELT decay_load comments are a bit confusing, first of all the 1/2^N should be (1/2)^N so that the reader doesn't get confused. Secondly, the y^N splitting into a 2-part decay factor deserves a better explanation. This patch improves the comments.
Cc: Paul Turner <[email protected]> Cc: Dietmar Eggemann <[email protected]> Cc: Juri Lelli <[email protected]> Cc: Patrick Bellasi <[email protected]> Cc: Peter Zijlstra <[email protected]> Signed-off-by: Joel Fernandes <[email protected]> --- kernel/sched/fair.c | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 6559d197e08a..1e1f2d77751e 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2761,11 +2761,15 @@ static __always_inline u64 decay_load(u64 val, u64 n) local_n = n; /* - * As y^PERIOD = 1/2, we can combine - * y^n = 1/2^(n/PERIOD) * y^(n%PERIOD) - * With a look-up table which covers y^n (n<PERIOD) - * - * To achieve constant time decay_load. + * As y^PERIOD = 1/2, we can decay the load by 1/2 + * for n/PERIOD number of PERIOD sized instances. Then + * we decay by the remaining windows in the final PERIOD, + * that is n%PERIOD. In other words, the decay factor + * y^N in terms of PERIOD becomes: + * y^n = (1/2)^(n/PERIOD) * y^(n%PERIOD). + * Since now we only need to compute powers of y where + * n < PERIOD, we use a look-up table for y^N (N<PERIOD). + * This helps achieve constant time decay_load. */ if (unlikely(local_n >= LOAD_AVG_PERIOD)) { val >>= local_n / LOAD_AVG_PERIOD; -- 2.12.0.246.ga2ecc84866-goog
