linux-kernel - Re: RT-Scheduler/cgroups: Possible overuse of resources assigned via cpu.rt_period_us and cpu.rt_runtime

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Message-ID: <486090DC.7010705@qualcomm.com>
Date:	Mon, 23 Jun 2008 23:14:52 -0700
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"Daniel K." <dk@...no>, mingo@...e.hu,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul Jackson <pj@....com>,
	Gregory Haskins <ghaskins@...ell.com>
Subject: Re: RT-Scheduler/cgroups: Possible overuse of resources assigned
 via cpu.rt_period_us and cpu.rt_runtime_us

Peter Zijlstra wrote:
> On Wed, 2008-06-18 at 16:12 +0200, Daniel K. wrote:
>> mkdir /dev/cgroup
>> mount -t cgroup -o cpu,cpuset cgroup /dev/cgroup
>>
>> mkdir /dev/cgroup/0
>>
>> echo 3 > /dev/cgroup/0/cpuset.cpus
>> echo 0 > /dev/cgroup/0/cpuset.mems
>> echo 100000 > /dev/cgroup/0/cpu.rt_period_us
>> echo   5000 > /dev/cgroup/0/cpu.rt_runtime_us
>>
>> schedtool -R -p 1 -e burnP6 &
>> [1] 3309
>> echo 3309 > /dev/cgroup/0/tasks
>>
>> At this point I'd expect the burnP6 task to use 5% of the available CPU
>> resources in the cgroup (5000/100000), but the real CPU usage, as
>> reported by top, is 20% This is 4 times the expected result, and as I
>> have 4 cores, I think there is a strong hint of correlation there.
>>
>> Maybe with a 4 core system there really is 4 000 000 us available for
>> every 1 wall-time second?
> 
> Indeed. In effect each cpu (see below on specifics) gets the
> runtime/period you specify, and it moves unused runtime between cpus.
> 
>> However, I have only assigned one core (3) to _this_ cgroup, so I think
>> this cgroup is overusing its assigned resources.
>>
>> What do you think?
> 
> I think you're on to something :-)
> 
> It uses root domains, that is the largest domain this cpu is part of
> that has load-balancing enabled.
> 
> So while you have made your process part of the cgroup and the cpuset,
> there is no strong relation between them, that is to say, I could either
> mount the cpuset or cpu controller on a different mount point and add
> tasks to one but not the other.
Daniel is probably really confused by now :).

> So the relation I used is that of load-balance domains.
That's the key thing.

> So in order to get what you intended, do something like:
> 
> mount none /dev/cpuset cgroup -o cpuset
> mount none /cgroup/cpu cgroup -o cpu
> 
> mkdir /dev/cpuset/root
> mkdir /dev/cpuset/rt
> 
> #
> # this might not actually make the kernel happy
> # as it might attempt (and possibly succeed in)
> # moving cpu bound kernel threads
> #
> for i in `cat /dev/cpuset/tasks`; do
> 	echo $i > /dev/cpuset/root/tasks;
> done
It won't let you add tasks before adding cpus.

> echo 0-2 > /dev/cpuset/root/cpuset.cpus
> echo 3 > /dev/cpuset/rt/cpuset.cpus
> 
> echo 0 > /dev/cpuset/cpuset.sched_load_balance
> 
> mkdir /cgroup/cpu/foo
> echo 100000 > /cgroup/cpu/foo/cpu.rt_period_us
> echo   5000 > /cgroup/cpu/foo/cpu.rt_runtime_us
> 
> echo $$ > /dev/cpuset/rt/tasks
> echo $$ > /cgroup/cpu/foo/tasks
> 
> chrt -r -p 1 burnP6 &

That seems too complicated :). There is no need to mount them separately. The
only part that was missing from Daniel's example is the sched_load_balance
thingy otherwise he can still have a single cgroup unless I missing something.
In other words:

mkdir /dev/cgroup
mount -t cgroup -o cpu,cpuset cgroup /dev/cgroup

# Setup first domain (cpu 0-2)
mkdir /dev/cgroup/0
echo 0-2 > /dev/cgroup/0/cpuset.cpus
echo 0 > /dev/cgroup/0/cpuset.mems

# Setup second domain (cpu 3)
mkdir /dev/cgroup/1
echo 3 > /dev/cgroup/1/cpuset.cpus
echo 0 > /dev/cgroup/1/cpuset.mems

# Do not balance between domains
echo 0 > /dev/cpuset/cpuset.sched_load_balance

# Move all tasks into first domain if needed
...

# Setup RT bandwidth for second domain
echo 100000 > /dev/cgroup/1/cpu.rt_period_us
echo   5000 > /dev/cgroup/1/cpu.rt_runtime_us

schedtool -R -p 1 -e burnP6 &
[1] 3309
echo 3309 > /dev/cgroup/1/tasks

Max
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