linux-kernel - Re: [memcg] 45208c9105: aim7.jobs-per-min -14.0% regression

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Message-ID: <CALvZod7mBLbkqZJzZz=XNGTDB4mxCd6dwM40aCv6t8fWbdUJdw@mail.gmail.com>
Date:   Mon, 13 Sep 2021 13:09:11 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Feng Tang <feng.tang@...el.com>, Hillf Danton <hdanton@...a.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Xing Zhengjun <zhengjun.xing@...ux.intel.com>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [memcg] 45208c9105: aim7.jobs-per-min -14.0% regression

On Mon, Sep 13, 2021 at 12:42 PM Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Mon, Sep 13, 2021 at 12:40:06PM -0700, Shakeel Butt wrote:
> > I did one more experiment with same workload but with system_wq
> > instead system_unbound_wq and there is clear difference in profile:
> >
> > With system_unbound_wq:
> > -    4.63%     0.33%  mmap  [kernel.kallsyms]  [k] queue_work_on
> >      4.29% queue_work_on
> >       - __queue_work
> >          - 3.45% wake_up_process
> >             - try_to_wake_up
> >                - 2.46% ttwu_queue
> >                   - 1.66% ttwu_do_activate
> >                      - 1.14% activate_task
> >                         - 0.97% enqueue_task_fair
> >                              enqueue_entity
> >
> > With system_wq:
> > -    1.36%     0.06%  mmap  [kernel.kallsyms]  [k] queue_work_on
> >      1.30% queue_work_on
> >       - __queue_work
> >          - 1.03% wake_up_process
> >             - try_to_wake_up
> >                - 0.97% ttwu_queue
> >                     0.66% ttwu_do_activate
> >
> > Tejun, is this expected? i.e. queuing work on system_wq has a
> > different performance impact than on system_unbound_wq?
>
> Yes, system_unbound_wq is putting the work item on the global shared
> workqueue while the system_wq is per-cpu, so on a loaded system, overhead
> difference showing up isn't too surprising.
>

Thanks a lot for the explanation. Are there any concerns to call
cgroup_rstat_flush_irqsafe(root_mem_cgroup->css.cgroup) in system_wq?
This will be called every 2 seconds, so, we can assume the updated
tree would be small most of the time.

Thanks,
Shakeel