linux-kernel - Re: [PATCH v2] sched/core: Don't mix isolcpus and housekeeping CPUs

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Date:   Fri, 26 Oct 2018 10:41:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Yi Wang <wang.yi59@....com.cn>, zhong.weidong@....com.cn,
        Yi Liu <liu.yi24@....com.cn>,
        Frederic Weisbecker <frederic@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] sched/core: Don't mix isolcpus and housekeeping CPUs

On Thu, Oct 25, 2018 at 11:00:58PM +0530, Srikar Dronamraju wrote:
> > But it doesn't solve the problem.
> > 
> > You can create multiple partitions with cpusets but still have an
> > unbound task in the root cgroup. That would suffer the exact same
> > problems.
> > 
> > Thing is, load-balancing, of any kind, should respect sched_domains, and
> > currently numa balancing barely looks at it.
> 
> Agreed that we should have looked at sched_domains. However I still believe
> we can't have task->cpus_allowed with a mix of isolcpus and non-isolcpus.
> won't it lead to inconsistent behaviour?

We currently can; and it depends on what you call inconsistent.

> > The proposed patch puts the minimal constraints on the numa balancer to
> > respect sched_domains; but doesn't yet correctly deal with hotplug.
> 
> I was also thinking about hotplug. Also your proposed patch and even my
> proposed patch don't seem to work well with the below scenario.
> 
> # cat /sys/devices/system/cpu/possible
> 0-31
> # cat /sys/devices/system/cpu/isolated
> 1,5,9,13
> # cat hist.sh
> echo 0 > /proc/sys/kernel/numa_balancing
> cd /sys/fs/cgroup/cpuset
> mkdir -p student
> cp cpuset.mems student/
> cd student
> echo "0-31" > cpuset.cpus
> echo $$ > cgroup.procs 
> echo "1-8" > cpuset.cpus
> /home/srikar/work/ebizzy-0.3/ebizzy -S 1000 &
> PID=$!
> sleep 10
> pidstat -p $! -t |tail -n +3 |head -n 10
> pidstat -p $$ -t |tail -n +3
> pkill ebizzy
> #
> # ./hist.sh
> 10:35:21  IST   UID      TGID       TID    %usr %system  %guest    %CPU   CPU  Command
> 10:35:21  IST     0      2645         -    8.70    0.01    0.00    8.71     1  ebizzy
> 10:35:21  IST     0         -      2645    0.01    0.00    0.00    0.01     1  |__ebizzy
> 10:35:21  IST     0         -      2647    0.14    0.00    0.00    0.14     1  |__ebizzy
> 10:35:21  IST     0         -      2648    0.13    0.00    0.00    0.13     1  |__ebizzy
> 10:35:21  IST     0         -      2649    0.13    0.00    0.00    0.13     1  |__ebizzy
> 10:35:21  IST     0         -      2650    0.13    0.00    0.00    0.13     1  |__ebizzy
> 10:35:21  IST     0         -      2651    0.13    0.00    0.00    0.13     1  |__ebizzy
> 10:35:21  IST     0         -      2652    0.13    0.00    0.00    0.13     1  |__ebizzy
> 10:35:21  IST     0         -      2653    0.13    0.00    0.00    0.13     1  |__ebizzy
> 10:35:23  IST   UID      TGID       TID    %usr %system  %guest    %CPU   CPU  Command
> 10:35:23  IST     0      2642         -    0.00    0.00    0.00    0.00     1  hist.sh
> 10:35:23  IST     0         -      2642    0.00    0.00    0.00    0.00     1  |__hist.sh
> #

That's correct and specified behaviour.

cpu1 has no sched domain (or the singleton domaon, which is the same)
and thus its tasks will not be migrated.

If there is a problem, it is with cpuset and its interaction with
isolcpus. I still have to look at the last version of cpuset-v2, but
iirc that was better in this regard.

I'm stil saying we should kill isolcpus entirely, its rubbish, has
always been.