linux-kernel - Re: [PATCH v2 3/9] sched: Remove checks against SD_LOAD

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Message-ID: <5decd96b-6fe0-3c35-4609-59378a0c8621@arm.com>
Date:   Mon, 23 Mar 2020 15:26:35 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        peterz@...radead.org, vincent.guittot@...aro.org
Subject: Re: [PATCH v2 3/9] sched: Remove checks against SD_LOAD_BALANCE

On 19.03.20 13:05, Valentin Schneider wrote:
> 
> On Thu, Mar 19 2020, Dietmar Eggemann wrote:
>> On 11.03.20 19:15, Valentin Schneider wrote:

[...]

> Your comments make me realize that changelog isn't great, what about the
> following?
> 
> ---
> 
> The SD_LOAD_BALANCE flag is set unconditionally for all domains in
> sd_init(). By making the sched_domain->flags syctl interface read-only, we
> have removed the last piece of code that could clear that flag - as such,
> it will now be always present. Rather than to keep carrying it along, we
> can work towards getting rid of it entirely.
> 
> cpusets don't need it because they can make CPUs be attached to the NULL
> domain (e.g. cpuset with sched_load_balance=0), or to a partitionned

s/partitionned/partitioned

> root_domain, i.e. a sched_domain hierarchy that doesn't span the entire
> system (e.g. root cpuset with sched_load_balance=0 and sibling cpusets with
> sched_load_balance=1).
> 
> isolcpus apply the same "trick": isolated CPUs are explicitly taken out of
> the sched_domain rebuild (using housekeeping_cpumask()), so they get the
> NULL domain treatment as well.
> 
> Remove the checks against SD_LOAD_BALANCE.

Sounds better to me:

Essentially, I was referring to examples like:

Hikey960 - 2x4

(A) exclusive cpusets:

root@...0:/sys/fs/cgroup/cpuset# mkdir cs1
root@...0:/sys/fs/cgroup/cpuset# echo 1 > cs1/cpuset.cpu_exclusive
root@...0:/sys/fs/cgroup/cpuset# echo 0 > cs1/cpuset.mems
root@...0:/sys/fs/cgroup/cpuset# echo 0-2 > cs1/cpuset.cpus
root@...0:/sys/fs/cgroup/cpuset# mkdir cs2
root@...0:/sys/fs/cgroup/cpuset# echo 1 > cs2/cpuset.cpu_exclusive
root@...0:/sys/fs/cgroup/cpuset# echo 0 > cs2/cpuset.mems
root@...0:/sys/fs/cgroup/cpuset# echo 3-5 > cs2/cpuset.cpus
root@...0:/sys/fs/cgroup/cpuset# echo 0 > cpuset.sched_load_balance

root@...0:/proc/sys/kernel# tree -d sched_domain

├── cpu0
│   └── domain0
├── cpu1
│   └── domain0
├── cpu2
│   └── domain0
├── cpu3
│   └── domain0
├── cpu4
│   ├── domain0
│   └── domain1
├── cpu5
│   ├── domain0
│   └── domain1
├── cpu6
└── cpu7

(B) non-exclusive cpuset:

root@...0:/sys/fs/cgroup/cpuset# echo 0 > cpuset.sched_load_balance

[ 8661.240385] CPU1 attaching NULL sched-domain.
[ 8661.244802] CPU2 attaching NULL sched-domain.
[ 8661.249255] CPU3 attaching NULL sched-domain.
[ 8661.253623] CPU4 attaching NULL sched-domain.
[ 8661.257989] CPU5 attaching NULL sched-domain.
[ 8661.262363] CPU6 attaching NULL sched-domain.
[ 8661.266730] CPU7 attaching NULL sched-domain.

root@...0:/sys/fs/cgroup/cpuset# mkdir cs1
root@...0:/sys/fs/cgroup/cpuset# echo 0-5 > cs1/cpuset.cpus

root@...0:/proc/sys/kernel# tree -d sched_domain

├── cpu0
│   ├── domain0
│   └── domain1
├── cpu1
│   ├── domain0
│   └── domain1
├── cpu2
│   ├── domain0
│   └── domain1
├── cpu3
│   ├── domain0
│   └── domain1
├── cpu4
│   ├── domain0
│   └── domain1
├── cpu5
│   ├── domain0
│   └── domain1
├── cpu6
└── cpu7