Re: [PATCH v6 5/6] sched: replace capacity_factor by usage

From: Wanpeng Li
Date: Sat Nov 22 2014 - 19:23:04 EST

Hi Vincent,
On 10/3/14, 8:50 PM, Vincent Guittot wrote:
On 3 October 2014 11:35, Morten Rasmussen <morten.rasmussen@xxxxxxx> wrote:
On Fri, Oct 03, 2014 at 08:24:23AM +0100, Vincent Guittot wrote:
On 2 October 2014 18:57, Morten Rasmussen <morten.rasmussen@xxxxxxx> wrote:
On Tue, Sep 23, 2014 at 05:08:04PM +0100, Vincent Guittot wrote:
Below are two examples to illustrate the problem that this patch solves:

1 - capacity_factor makes the assumption that max capacity of a CPU is
SCHED_CAPACITY_SCALE and the load of a thread is always is
SCHED_LOAD_SCALE. It compares the output of these figures with the sum
of nr_running to decide if a group is overloaded or not.

But if the default capacity of a CPU is less than SCHED_CAPACITY_SCALE
(640 as an example), a group of 3 CPUS will have a max capacity_factor
of 2 ( div_round_closest(3x640/1024) = 2) which means that it will be
seen as overloaded if we have only one task per CPU.
I did some testing on TC2 which has the setup you describe above on the
A7 cluster when the clock-frequency property is set in DT. The two A15s
have max capacities above 1024. When using sysbench with five threads I
still get three tasks on the two A15s and two tasks on the three A7
leaving one cpu idle (on average).

Using cpu utilization (usage) does correctly identify the A15 cluster as
overloaded and it even gets as far as selecting the A15 running two
tasks as the source cpu in load_balance(). However, load_balance() bails
out without pulling the task due to calculate_imbalance() returning a
zero imbalance. calculate_imbalance() bails out just before the hunk you
changed due to comparison of the sched_group avg_loads. sgs->avg_load is
basically the sum of load in the group divided by its capacity. Since
load isn't scaled the avg_load of the overloaded A15 group is slightly
_lower_ than the partially idle A7 group. Hence calculate_imbalance()
bails out, which isn't what we want.

I think we need to have a closer look at the imbalance calculation code
and any other users of sgs->avg_load to get rid of all code making
assumptions about cpu capacity.
We already had this discussion during the review of a previous version
and my answer has not changed; This patch is necessary to solve the 1
task per CPU issue of the HMP system but is not enough. I have a patch
for solving the imbalance calculation issue and i have planned to send
it once this patch will be in a good shape for being accepted by

I don't want to mix this patch and the next one because there are
addressing different problem: this one is how evaluate the capacity of
a system and detect when a group is overloaded and the next one will
handle the case when the balance of the system can't rely on the
average load figures of the group because we have a significant
capacity difference between groups. Not that i haven't specifically
mentioned HMP for the last patch because SMP system can also take
advantage of it
You do mention 'big' and 'little' cores in your commit message and quote
example numbers with are identical to the cpu capacities for TC2. That
By last patch, i mean the patch about imbalance that i haven't sent
yet, but it's not related with this patch

lead me to believe that this patch would address the issues we see for
HMP systems. But that is clearly wrong. I would suggest that you drop
This patch addresses one issue: correctly detect how much capacity we
have and correctly detect when the group is overloaded; HMP system

What's the meaning of "HMP system"?

Wanpeng Li

clearly falls in this category but not only.
This is the only purpose of this patch and not the "1 task per CPU
issue" that you mentioned.

The second patch is about correctly balance the tasks on system where
the capacity of a group is significantly less than another group. It
has nothing to do in capacity computation and overload detection but
it will use these corrected/new metrics to make a better choice.

Then, there is the "1 task per CPU issue on HMP" that you mentioned
and this latter will be solved thanks to these 2 patchsets but this is
not the only/main target of these patchsets so i don't want to reduce
them into: "solve an HMP issue"

mentioning big and little cores and stick to only describing cpu
capacity reductions due to rt tasks and irq to avoid any confusion about
the purpose of the patch. Maybe explicitly say that non-default cpu
capacities (capacity_orig) isn't addressed yet.
But they are addressed by this patchset. you just mixed various goal
together (see above)

I think the two problems you describe are very much related. This patch
set is half the solution of the HMP balancing problem. We just need the
last bit for avg_load and then we can add scale-invariance on top.
i don't see the link between scale invariance and a bad load-balancing
choice. The fact that the current load balancer puts more tasks than
CPUs in a group on HMP system should not influence or break the scale
invariance load tracking. Isn't it ?

Now, i could send the other patchset but i'm afraid that this will
generate more confusion than help in the process review.


IMHO, it would be good to have all the bits and pieces for cpu capacity
scaling and scaling of per-entity load-tracking on the table so we fit
things together. We only have patches for parts of the solution posted
so far.

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