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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <204d67f1-a21c-9d71-9b76-6f1a11c89092@arm.com>
Date:   Tue, 12 May 2020 14:39:13 +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 11/05/2020 10:01, Juri Lelli wrote:
> On 06/05/20 17:09, Dietmar Eggemann wrote:
>> 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:

[...]

>>> 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?
> 
> No. I was referring to the single CPU capacity. The capacity of the CPU
> where a task is running when setscheduler is called for it (and DL AC
> performed). See below, maybe more clear why I wondered about this case..

OK, got it! I was just confused since I don't think that this patch
introduced the issue.

Before the patch 'int cpus = dl_bw_cpus(task_cpu(p))' was used which
returns the number of cpus on the (default) rd (n). So for a single CPU
(1024) we use n*1024.

I wonder if a fix for that should be part of this patch-set?

[...]

>> ...
>> [  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
> 
> So, here you are checking new task bw against 1338 which is 3*L
> capacity. However, since load balance is disabled at this point for 3-5,
> once admitted the task will only be able to run on CPU 3. Now, if more
> tasks on CPU 3 are admitted the same way (up to 1338) I believe they
> will start to experience deadline misses because only 446 will be
> actually available to them until load balance is enabled below and they
> are then free to migrate to CPUs 4 and 5.
> 
> Does it makes sense?

Yes, it does.

So my first idea was to only consider the CPU (i.e. its CPU capacity) in
case we detect 'cpu_rq(cpu)->rd == def_root_domain'?

In case I re-enable load-balancing on cpuset '/', we can't make a task
in cpuset 'B' DL since we hit this in __sched_setscheduler():

4931           /*
4932            * Don't allow tasks with an affinity mask smaller than
4933            * the entire root_domain to become SCHED_DEADLINE.
...
4935            */
4936            if (!cpumask_subset(span, p->cpus_ptr) || ...

root@...o:~# echo 1 > /sys/fs/cgroup/cpuset/cpuset.sched_load_balance
root@...o:~# echo $$ > /sys/fs/cgroup/cpuset/B/tasks
root@...o:~# chrt -d --sched-runtime 8000 --sched-period 16000 -p 0 $$
chrt: failed to set pid 2316's policy: Operation not permitted

So this task has to leave 'B' first I assume.

[...]