[RFC 1/1] sched/fair: Consider asymmetric scheduler groups in load balancer
Tobias Huschle
huschle at linux.ibm.com
Mon Jun 5 18:07:16 AEST 2023
On 2023-05-16 15:36, Vincent Guittot wrote:
> On Mon, 15 May 2023 at 13:46, Tobias Huschle <huschle at linux.ibm.com>
> wrote:
>>
>> The current load balancer implementation implies that scheduler
>> groups,
>> within the same domain, all host the same number of CPUs. This is
>> reflected in the condition, that a scheduler group, which is load
>> balancing and classified as having spare capacity, should pull work
>> from the busiest group, if the local group runs less processes than
>> the busiest one. This implies that these two groups should run the
>> same number of processes, which is problematic if the groups are not
>> of the same size.
>>
>> The assumption that scheduler groups within the same scheduler domain
>> host the same number of CPUs appears to be true for non-s390
>> architectures. Nevertheless, s390 can have scheduler groups of unequal
>> size.
>>
>> This introduces a performance degredation in the following scenario:
>>
>> Consider a system with 8 CPUs, 6 CPUs are located on one CPU socket,
>> the remaining 2 are located on another socket:
>>
>> Socket -----1----- -2-
>> CPU 1 2 3 4 5 6 7 8
>>
>> Placing some workload ( x = one task ) yields the following
>> scenarios:
>>
>> The first 5 tasks are distributed evenly across the two groups.
>>
>> Socket -----1----- -2-
>> CPU 1 2 3 4 5 6 7 8
>> x x x x x
>>
>> Adding a 6th task yields the following distribution:
>>
>> Socket -----1----- -2-
>> CPU 1 2 3 4 5 6 7 8
>> SMT1 x x x x x
>> SMT2 x
>
> Your description is a bit confusing for me. What you name CPU above
> should be named Core, doesn' it ?
>
> Could you share with us your scheduler topology ?
>
You are correct, it should say core instead of CPU.
One actual configuration from one of my test machines (lscpu -e):
CPU NODE DRAWER BOOK SOCKET CORE L1d:L1i:L2 ONLINE CONFIGURED
POLARIZATION ADDRESS
0 0 0 0 0 0 0:0:0 yes yes horizontal
0
1 0 0 0 0 0 1:1:0 yes yes horizontal
1
2 0 0 0 0 1 2:2:0 yes yes horizontal
2
3 0 0 0 0 1 3:3:0 yes yes horizontal
3
4 0 0 0 0 2 4:4:0 yes yes horizontal
4
5 0 0 0 0 2 5:5:0 yes yes horizontal
5
6 0 0 0 0 3 6:6:0 yes yes horizontal
6
7 0 0 0 0 3 7:7:0 yes yes horizontal
7
8 0 0 0 0 4 8:8:0 yes yes horizontal
8
9 0 0 0 0 4 9:9:0 yes yes horizontal
9
10 0 0 0 0 5 10:10:0 yes yes horizontal
10
11 0 0 0 0 5 11:11:0 yes yes horizontal
11
12 0 0 0 1 6 12:12:0 yes yes horizontal
12
13 0 0 0 1 6 13:13:0 yes yes horizontal
13
14 0 0 0 1 7 14:14:0 yes yes horizontal
14
15 0 0 0 1 7 15:15:0 yes yes horizontal
15
So, 6 cores / 12 CPUs in one group 2 cores / 4 CPUs in the other.
If I run stress-ng with 8 cpu stressors on the original code I get a
distribution
like this:
00 01 02 03 04 05 06 07 08 09 10 11 || 12 13 14 15
x x x x x x x x
Which means that the two cores in the smaller group are running into SMT
while two
cores in the larger group are still idle. This is caused by the
prefer_sibling path
which really wants to see both groups run the same number of tasks.
>>
>> The task is added to the 2nd scheduler group, as the scheduler has the
>> assumption that scheduler groups are of the same size, so they should
>> also host the same number of tasks. This makes CPU 7 run into SMT
>> thread, which comes with a performance penalty. This means, that in
>> the window of 6-8 tasks, load balancing is done suboptimally, because
>> SMT is used although there is no reason to do so as fully idle CPUs
>> are still available.
>>
>> Taking the weight of the scheduler groups into account, ensures that
>> a load balancing CPU within a smaller group will not try to pull tasks
>> from a bigger group while the bigger group still has idle CPUs
>> available.
>>
>> Signed-off-by: Tobias Huschle <huschle at linux.ibm.com>
>> ---
>> kernel/sched/fair.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 48b6f0ca13ac..b1307d7e4065 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -10426,7 +10426,8 @@ static struct sched_group
>> *find_busiest_group(struct lb_env *env)
>> * group's child domain.
>> */
>> if (sds.prefer_sibling && local->group_type == group_has_spare
>> &&
>> - busiest->sum_nr_running > local->sum_nr_running + 1)
>> + busiest->sum_nr_running * local->group_weight >
>> + local->sum_nr_running * busiest->group_weight
>> + 1)
>
> This is the prefer_sibling path. Could it be that you should disable
> prefer_siling between your sockets for such topology ? the default
> path compares the number of idle CPUs when groups has spare capacity
>
>
If I disable prefer_sibling (I played around with it a bit), I run into
the problem,
that the tasks are distributed s.t. each group has the same amount of
idle CPUs, which
yields distributions similar to this:
00 01 02 03 04 05 06 07 08 09 10 11 || 12 13 14 15
x x x x x x x x
Now both groups have 4 idle CPUs which fulfills the criteria imposed by
the load balancer,
but the larger group is now running SMT while the smaller one is just
idle.
So, in this asymmetric setup, both criteria seem to not work in an
optimal way. Going for
the same number of idle CPUs or alternatively for the same number of
running processes
both cause a sub-optimal distribution of tasks, leading to unnecessary
SMT.
It seems also to be possible to address the regular load balancing path
by aiming to have the
same unused capacity between groups instead of the same number of idle
CPUs. This seems to
have been considered in the past, but the choice went in favor of the
number of idle CPUs.
Since this decision was actively taken already, I focused on the
prefer_sibling path.
The question now would be how to address this properly (or if I'm
missing something here).
As mentioned in the cover letter, this was the most simplistic and least
invasive approach
I could find, others might be more sophisticated but also have some
side-effects.
I have a bit of a hard time leaving this one as-is, as it just
introduces two additional
multiplications with no effect for most architectures. Maybe an
architectures specific
inline function that the compiler can optimize away if not needed?
>> goto force_balance;
>>
>> if (busiest->group_type != group_overloaded) {
>> --
>> 2.34.1
>>
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