kernel/sched/fair.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
From: hupu <hupu@oppo.com>
I think the 'lag' calculation here is inaccurate.
Assume that delta needs to be subtracted from v_i to ensure that the
vlag of task i after placement is the same as before. At this time, the
vlag of task i after placement should be:
vl'_i = V' - (v_i - delta)
From the above formula, we know that vl'_i should be:
vl'_i = (vl_i * W)/(W + w_i)
That is to say:
V' - (v_i - delta) = (vl_i * W)/(W + w_i)
For a newly added entity, generally set v_i to V', and the above formula
can be converted into:
V' - (V' - delta) = (vl_i * W)/(W + w_i)
Therefore the value of delta should be as follows, where delta is the
'lag' in the code.
delta = (vl_i * W)/(W + w_i)
Signed-off-by: hupu <hupu@oppo.com>
---
kernel/sched/fair.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 03be0d1330a6..c5f74f753be8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5239,9 +5239,12 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
if (curr && curr->on_rq)
load += scale_load_down(curr->load.weight);
- lag *= load + scale_load_down(se->load.weight);
+ lag *= load;
+
+ load += scale_load_down(se->load.weight);
if (WARN_ON_ONCE(!load))
load = 1;
+
lag = div_s64(lag, load);
}
--
2.17.1
On Tue, Apr 23, 2024 at 07:44:16PM +0800, hupu@oppo.com wrote: > From: hupu <hupu@oppo.com> > > I think the 'lag' calculation here is inaccurate. > > Assume that delta needs to be subtracted from v_i to ensure that the > vlag of task i after placement is the same as before. Why ?!? v_i is the unkown, it makes no sense to complicate things by adding extra unknowns. > At this time, the > vlag of task i after placement should be: > vl'_i = V' - (v_i - delta) But but but, you can't have V' without knowing v_i. > From the above formula, we know that vl'_i should be: > vl'_i = (vl_i * W)/(W + w_i) > > That is to say: > V' - (v_i - delta) = (vl_i * W)/(W + w_i) > > For a newly added entity, generally set v_i to V', and the above formula > can be converted into: > V' - (V' - delta) = (vl_i * W)/(W + w_i) > > Therefore the value of delta should be as follows, where delta is the > 'lag' in the code. > delta = (vl_i * W)/(W + w_i) > > Signed-off-by: hupu <hupu@oppo.com> > --- > kernel/sched/fair.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 03be0d1330a6..c5f74f753be8 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -5239,9 +5239,12 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) > if (curr && curr->on_rq) > load += scale_load_down(curr->load.weight); > > - lag *= load + scale_load_down(se->load.weight); > + lag *= load; > + > + load += scale_load_down(se->load.weight); > if (WARN_ON_ONCE(!load)) > load = 1; > + > lag = div_s64(lag, load); You're making it: v_i = V - (W * vl_i) / (W + w_i) In direct contradiction to the giant comment right above this that explains why the code is as it is. > } > > -- > 2.17.1 >
On Thu, Apr 25, 2024 at 07:44:59PM +0800, hupu@oppo.com wrote: > > > From: hupu <hupu@oppo.com> > > > > > > I think the 'lag' calculation here is inaccurate. > > > > > > Assume that delta needs to be subtracted from v_i to ensure that the > > > vlag of task i after placement is the same as before. > > > > Why ?!? v_i is the unkown, it makes no sense to complicate things by > > adding extra unknowns. > > > > > At this time, the > > > vlag of task i after placement should be: > > > vl'_i = V' - (v_i - delta) > > > > But but but, you can't have V' without knowing v_i. > > > > Thank you for your patient guidance. I overlooked a important fact that > v_i is unknown in the process of proof. Below is the complete proof > process, and it turns out that you are correct. *phew*, thanks for checking!
© 2016 - 2024 Red Hat, Inc.