linux-kernel - Re: [RFC PATCH] sched/fair: Make tg->load

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Date:   Wed, 29 Mar 2023 22:55:42 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
CC:     Aaron Lu <aaron.lu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Nitin Tekchandani <nitin.tekchandani@...el.com>,
        Waiman Long <longman@...hat.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] sched/fair: Make tg->load_avg per node

On 2023-03-29 at 14:36:44 +0200, Dietmar Eggemann wrote:
> On 28/03/2023 14:56, Aaron Lu wrote:
> > Hi Dietmar,
> > 
> > Thanks for taking a look.
> > 
> > On Tue, Mar 28, 2023 at 02:09:39PM +0200, Dietmar Eggemann wrote:
> >> On 27/03/2023 07:39, Aaron Lu wrote:
> 
> [...]
> 
> > Did you test with a v6.3-rc based kernel?
> > I encountered another problem on those kernels and had to temporarily use
> > a v6.2 based kernel, maybe you have to do the same:
> > https://lore.kernel.org/lkml/20230327080502.GA570847@ziqianlu-desk2/
> 
> No, I'm also on v6.2.
> 
> >> Is your postgres/sysbench running in a cgroup with cpu controller
> >> attached? Mine isn't.
> > 
> > Yes, I had postgres and sysbench running in the same cgroup with cpu
> > controller enabled. docker created the cgroup directory under
> > /sys/fs/cgroup/system.slice/docker-XXX and cgroup.controllers has cpu
> > there.
> 
> I'm running postgresql service directly on the machine. I boot now with
> 'cgroup_no_v1=all systemd.unified_cgroup_hierarchy=1' so I can add the
> cpu controller to:
> 
>   system.slice/system-postgresql.slice/postgresql@...main.service
> 
> where the 96 postgres threads run and to
> 
>   user.slice/user-1005.slice/session-4.scope
> 
> where the 96 sysbench threads run.
> 
> Checked with systemd-cgls and `cat /sys/kernel/debug/sched/debug` that
> those threads are really running there.
> 
> Still not seeing `update_load_avg` or `update_cfs_group` in perf report,
> only some very low values for `update_blocked_averages`.
> 
> Also added CFS BW throttling to both cgroups. No change.
> 
> Then I moved session-4.scope's shell into `postgresql@...main.service`
> so that `postgres` and `sysbench` threads run in the same cgroup.
> 
> Didn't change much.
> 
> >> Maybe I'm doing something else differently?
> > 
> > Maybe, you didn't mention how you started postgres, if you start it from
> > the same session as sysbench and if autogroup is enabled, then all those
> > tasks would be in the same autogroup taskgroup then it should have the
> > same effect as my setup.
> 
> This should be now close to my setup running `postgres` and `sysbench`
> in `postgresql@...main.service`.
> 
> > Anyway, you can try the following steps to see if you can reproduce this
> > problem on your Arm64 server:
> > 
> > 1 docker pull postgres
> > 2 sudo docker run --rm --name postgres-instance -e POSTGRES_PASSWORD=mypass -e POSTGRES_USER=sbtest -d postgres -c shared_buffers=80MB -c max_connections=250
> > 3 go inside the container
> >   sudo docker exec -it $the_just_started_container_id bash
> > 4 install sysbench inside container
> >   apt update and apt install sysbench
> > 5 prepare
> >   root@...tainer:/# sysbench --db-driver=pgsql --pgsql-user=sbtest --pgsql_password=mypass --pgsql-db=sbtest --pgsql-port=5432 --tables=16 --table-size=10000 --threads=224 --time=60 --report-interval=2 /usr/share/sysbench/oltp_read_only.lua prepare
> > 6 run
> >   root@...tainer:/# sysbench --db-driver=pgsql --pgsql-user=sbtest --pgsql_password=mypass --pgsql-db=sbtest --pgsql-port=5432 --tables=16 --table-size=10000 --threads=224 --time=60 --report-interval=2 /usr/share/sysbench/oltp_read_only.lua run
> 
> I would have to find time to learn how to set up docker on my machine
> ... But I use very similar values for the setup and sysbench test.
> 
> > Note that I used 224 threads where this problem is visible. I also tried
> > 96 and update_cfs_group() and update_load_avg() cost about 1% cycles then.
> 
> True, I was hopping to see at least the 1% ;-)
According to Aaron's description, the relatively high cost of update_load_avg() was
caused by cross-node access. If the task group is allocated on node0, but some tasks
in this task group are load balanced to node1, the issue could be triggered
easier? echo 0 > /sys/kernel/debug/sched/numa_balancing

thanks,
Chenyu