linux-kernel - Re: [PATCH v2 3/6] sched/deadline: Add dl_bw

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Date:   Wed, 6 May 2020 17:09:20 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Wei Wang <wvw@...gle.com>, Quentin Perret <qperret@...gle.com>,
        Alessio Balsini <balsini@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Qais Yousef <qais.yousef@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/6] sched/deadline: Add dl_bw_capacity()

On 06/05/2020 14:37, Juri Lelli wrote:
> On 06/05/20 12:54, Dietmar Eggemann wrote:
>> On 27/04/2020 10:37, Dietmar Eggemann wrote:

[...]

>> There is an issue w/ excl. cpusets and cpuset.sched_load_balance=0. The
>> latter is needed to demonstrate the problem since DL task affinity can't
>> be altered.
>>
>> A CPU in such a cpuset has its rq attached to def_root_domain which does
>> not have its 'sum_cpu_capacity' properly set.
> 
> Hummm, but if sched_load_balance is disabled it means that we've now got
> a subset of CPUs which (from a DL AC pow) are partitioned. So, I'd tend

Yes, the CPUs of the cpuset w/ cpuset.sched_load_balance=0 (cpuset B in
the example).

> to say that we actually want to check new tasks bw requirement against
> the available bandwidth of the particular CPU they happen to be running
> (and will continue to run) when setscheduler is called.

By 'available bandwidth of the particular CPU' you refer to
'\Sum_{cpu_rq(i)->rd->span} CPU capacity', right?

This is what this fix tries to achieve. Regardless whether cpu_rq(i)->rd
is a 'real' rd or the def_root_domain, dl_bw_capacity() will now always
return '\Sum_{cpu_rq(i)->rd->span} CPU capacity'

> If then load balance is enabled again, AC check we did above should
> still be valid for all tasks admitted in the meantime, no?
 
Example (w/ this fix) on Juno [L b b L L L], capacity_orig_of(L)=446 :

mkdir /sys/fs/cgroup/cpuset/A
echo 0 > /sys/fs/cgroup/cpuset/A/cpuset.mems
echo 1 > /sys/fs/cgroup/cpuset/A/cpuset.cpu_exclusive
echo 0-2 > /sys/fs/cgroup/cpuset/A/cpuset.cpus

mkdir /sys/fs/cgroup/cpuset/B
echo 0 > /sys/fs/cgroup/cpuset/B/cpuset.mems
echo 1 > /sys/fs/cgroup/cpuset/B/cpuset.cpu_exclusive
echo 3-5 > /sys/fs/cgroup/cpuset/B/cpuset.cpus

echo 0 > /sys/fs/cgroup/cpuset/B/cpuset.sched_load_balance
echo 0 > /sys/fs/cgroup/cpuset/cpuset.sched_load_balance

echo $$ > /sys/fs/cgroup/cpuset/B/tasks
chrt -d --sched-runtime 8000 --sched-period 16000 -p 0 $$

...
[  144.920102] __dl_bw_capacity CPU3 rd->span=3-5 return 1338
[  144.925607] sched_dl_overflow: [bash 1999] task_cpu(p)=3 cap=1338 cpus_ptr=3-5
[  144.932841] __dl_bw_capacity CPU3 rd->span=3-5 return 1338
...

echo 1 > /sys/fs/cgroup/cpuset/B/cpuset.sched_load_balance

echo $$ > /sys/fs/cgroup/cpuset/B/tasks
chrt -d --sched-runtime 8000 --sched-period 16000 -p 0 $$

...
[  254.367982] __dl_bw_capacity CPU5 rd->span=3-5 return 1338
[  254.373487] sched_dl_overflow: [bash 2052] task_cpu(p)=5 cap=1338 cpus_ptr=3-5
[  254.380721] __dl_bw_capacity CPU5 rd->span=3-5 return 1338
...

Regardless of 'B/cpuset.sched_load_balance'
'\Sum_{cpu_rq(i)->rd->span} CPU capacity' stays 1338 (3*446)

So IMHO, DL AC check stays intact.