Messages in this thread | | | Subject | Re: [PATCH v2 3/6] sched/deadline: Add dl_bw_capacity() | From | Dietmar Eggemann <> | Date | Tue, 12 May 2020 14:39:13 +0200 |
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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:
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>>> 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?
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>> ... >> [ 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@juno:~# echo 1 > /sys/fs/cgroup/cpuset/cpuset.sched_load_balance root@juno:~# echo $$ > /sys/fs/cgroup/cpuset/B/tasks root@juno:~# chrt -d --sched-runtime 8000 --sched-period 16000 -p 0 $$ chrt: failed to set pid 2316's policy: Operation not permitted
So this task has to leave 'B' first I assume.
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