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

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Date:   Thu, 25 Oct 2018 23:00:58 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
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

> 
> That's completely broken. Nothing in the numa balancing path uses that
> variable and afaict preemption is actually enabled where that's used, so
> using that per-cpu variable at all is broken.
> 

I can demonstrate that even without numa balancing, there are
inconsistent behaviour with isolcpus on.

> 
> Both of you are fixing symptoms, not the cause.
> 

Okay.

> 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?

> 
> 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
#

Note all the ebizzy and bash task that started it are on cpu 1. This happens
if the cpuset starts with an isolcpu, then all tasks in that cpuset might
only run in that cpu.  With a smaller cpuset, ebizzy always runs on cpu 1.
However, if I increase the cpuset, the chances of ebizzy spreading increases
but not always.

I only tried this on a powerpc kvm guest. I dont think there is anything to do
with arch/guest/host

I have something that seems to help out. Will post soon.

> isolcpus is just one case that goes wrong.
Similar to isolcpus, are there other cases that we need to worry about?

-- 
Thanks and Regards
Srikar Dronamraju