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SubjectRe: [RFC][PATCH] sched: attach extra runtime to the right avg
On Tue, Jul 04, 2017 at 12:13:09PM +0200, Ingo Molnar wrote:
>
> This code on the other hand:
>
> sa->last_update_time += delta << 10;
>
> ... in essence creates a whole new absolute clock value that slowly but surely is
> drifting away from the real rq->clock, because 'delta' is always rounded down to
> the nearest 1024 ns boundary, so we accumulate the 'remainder' losses.
>
> That is because:
>
> delta >>= 10;
> ...
> sa->last_update_time += delta << 10;
>
> Given enough time, ->last_update_time can drift a long way, and this delta:
>
> delta = now - sa->last_update_time;
>
> ... becomes meaningless AFAICS, because it's essentially two different clocks that
> get compared.

Thing is, once you drift over 1023 (ns) your delta increases and you
catch up again.



A B C D E F
| | | | | |
+----+----+----+----+----+----+----+----+----+----+----+


A: now = 0
sa->last_update_time = 0
delta := (now - sa->last_update_time) >> 10 = 0

B: now = 614 (+614)
delta = (614 - 0) >> 10 = 0
sa->last_update_time += 0 (0)
sa->last_update_time = now & ~1023 (0)

C: now = 1843 (+1229)
delta = (1843 - 0) >> 10 = 1
sa->last_update_time += 1024 (1024)
sa->last_update_time = now & ~1023 (1024)


D: now = 3481 (+1638)
delta = (3481 - 1024) >> 10 = 2
sa->last_update_time += 2048 (3072)
sa->last_update_time = now & ~1023 (3072)

E: now = 5734 (+2253)
delta = (5734 - 3072) = 2
sa->last_update_time += 2048 (5120)
sa->last_update_time = now & ~1023 (5120)

F: now = 6348 (+614)
delta = (6348 - 5120) >> 10 = 1
sa->last_update_time += 1024 (6144)
sa->last_update_time = now & ~1023 (6144)



And you'll see that both are identical, and that both D and F have
gotten a spill from sub-chunk accounting.


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