linux-kernel - Re: [PATCH 4/6] sched/deadline: Block DL tasks on non-exclusive cpuset if bandwitdh control is enable

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Message-ID: <4b37b32b-0e16-ffbc-ca6a-fbee935c0813@arm.com>
Date:   Thu, 14 Jan 2021 16:51:43 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org
Cc:     Marco Perronet <perronet@...-sws.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Li Zefan <lizefan@...wei.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Valentin Schneider <valentin.schneider@....com>,
        cgroups@...r.kernel.org
Subject: Re: [PATCH 4/6] sched/deadline: Block DL tasks on non-exclusive
 cpuset if bandwitdh control is enable

On 12/01/2021 16:53, Daniel Bristot de Oliveira wrote:
> The current SCHED_DEADLINE design supports only global scheduler,
> or variants of it, i.e., clustered and partitioned, via cpuset config.
> To enable the partitioning of a system with clusters of CPUs, the
> documentation advises the usage of exclusive cpusets, creating an
> exclusive root_domain for the cpuset.
> 
> Attempts to change the cpu affinity of a thread to a cpu mask different
> from the root domain results in an error. For instance:

[...]

> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 788a391657a5..c221e14d5b86 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2878,6 +2878,13 @@ int dl_task_can_attach(struct task_struct *p,
>  	if (cpumask_empty(cs_cpus_allowed))
>  		return 0;
>  
> +	/*
> +	 * Do not allow moving tasks to non-exclusive cpusets
> +	 * if bandwidth control is enabled.
> +	 */
> +	if (dl_bandwidth_enabled() && !exclusive)
> +		return -EBUSY;
> +
>  	/*
>  	 * The task is not moving to another root domain, so it is
>  	 * already accounted.
> 

But doesn't this mean you only have to set this cgroup exclusive/root to
run into the same issue:

with this patch:

cgroupv1:

root@...o:/sys/fs/cgroup/cpuset# chrt -d --sched-period 1000000000
--sched-runtime 100000000 0 sleep 500 &
[1] 1668
root@...o:/sys/fs/cgroup/cpuset# PID1=$!

root@...o:/sys/fs/cgroup/cpuset# chrt -d --sched-period 1000000000
--sched-runtime 100000000 0 sleep 500 &
[2] 1669
root@...o:/sys/fs/cgroup/cpuset# PID2=$!

root@...o:/sys/fs/cgroup/cpuset# mkdir A

root@...o:/sys/fs/cgroup/cpuset# echo 0 > ./A/cpuset.mems
root@...o:/sys/fs/cgroup/cpuset# echo 0 > ./A/cpuset.cpus

root@...o:/sys/fs/cgroup/cpuset# echo $PID2 > ./A/cgroup.procs
-bash: echo: write error: Device or resource busy

root@...o:/sys/fs/cgroup/cpuset# echo 1 > ./A/cpuset.cpu_exclusive

root@...o:/sys/fs/cgroup/cpuset# echo $PID2 > ./A/cgroup.procs

root@...o:/sys/fs/cgroup/cpuset# cat /proc/$PID1/status | grep
Cpus_allowed_list | awk '{print $2}'
0-5
root@...o:/sys/fs/cgroup/cpuset# cat /proc/$PID2/status | grep
Cpus_allowed_list | awk '{print $2}'
0

cgroupv2:

root@...o:/sys/fs/cgroup# echo +cpuset > cgroup.subtree_control

root@...o:/sys/fs/cgroup# chrt -d --sched-period 1000000000
--sched-runtime 100000000 0 sleep 500 &
[1] 1687
root@...o:/sys/fs/cgroup# PID1=$!

root@...o:/sys/fs/cgroup# chrt -d --sched-period 1000000000
--sched-runtime 100000000 0 sleep 500 &
[2] 1688
root@...o:/sys/fs/cgroup# PID2=$!

root@...o:/sys/fs/cgroup# mkdir A

root@...o:/sys/fs/cgroup# echo 0 > ./A/cpuset.mems
root@...o:/sys/fs/cgroup# echo 0 > ./A/cpuset.cpus

root@...o:/sys/fs/cgroup# echo $PID2 > ./A/cgroup.procs
-bash: echo: write error: Device or resource busy

root@...o:/sys/fs/cgroup# echo root > ./A/cpuset.cpus.partition

root@...o:/sys/fs/cgroup# echo $PID2 > ./A/cgroup.procs

root@...o:/sys/fs/cgroup# cat /proc/$PID1/status | grep
Cpus_allowed_list | awk '{print $2}'
0-5
root@...o:/sys/fs/cgroup# cat /proc/$PID2/status | grep
Cpus_allowed_list | awk '{print $2}'
0