Hi Qais,
On 2023-05-18 12:30, Qais Yousef wrote:
Please CC sched maintainers (Ingo + Peter) next time as they should pick this
up ultimately and they won't see it from the list only.
Will do. I was using the get_maintainers script and I thought that gave
me all the CCs.
On 05/05/23 16:24, Hongyan Xia wrote:
The story in 5.2 about util_avg abruptly jumping from 300 when
Fmax/Fmin == 3 to 1024 when Fmax/Fmin == 4 hides some details about how
clock_pelt works behind the scenes. Explicitly mention it to make it
easier for readers to follow.
Signed-off-by: Hongyan Xia <hongyan.xia2@xxxxxxx>
Cc: Qais Yousef <qyousef@xxxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---
Documentation/scheduler/sched-util-clamp.rst | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/Documentation/scheduler/sched-util-clamp.rst b/Documentation/scheduler/sched-util-clamp.rst
index 74d5b7c6431d..524df07bceba 100644
--- a/Documentation/scheduler/sched-util-clamp.rst
+++ b/Documentation/scheduler/sched-util-clamp.rst
@@ -669,6 +669,19 @@ but not proportional to Fmax/Fmin.
p0->util_avg = 300 + small_error
+The reason why util_avg is around 300 even though it runs for 900 at Fmin is:
+Although running at Fmin reduces the rate of rq_clock_pelt() to 1/3 thus
+accumulates util_sum at 1/3 of the rate at Fmax, the clock period
+(rq_clock_pelt() now minus previous rq_clock_pelt()) in:
+
+::
+
+ util_sum / clock period = util_avg
+
+does not shrink to 1/3, since rq->clock_pelt is periodically synchronized with
+rq->clock_task as long as there's idle time. As a result, we get util_avg of
+about 300, not 900.
+
I feel neutral about these changes. It does answer some questions, but poses
more questions like what is clock_pelt. So we might end up in recursive
regression of explaining the explanation.
I don't think we have a doc about clock_pelt. Worth adding one and just add
a reference to it from here for those interested in understanding more details
on why we need to go to idle to correct util_avg? I think our code has
explanation, a reference to update_rq_clock_pelt() might suffice too.
Vincent, do you have an opinion here?
Sounds reasonable. I don't mind drafting a doc or just a couple of
paragraphs for clock_pelt (or all the different clocks like clock,
clock_task, clock_idle_*), if that's what we can agree on.
Hongyan
