linux-kernel - Re: [PATCH] cgroup: Relax restrictions on kernel threads moving out of root cpu cgroup

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Date:   Tue, 6 Apr 2021 12:15:24 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Pavan Kondeti <pkondeti@...eaurora.org>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Quentin Perret <qperret@...gle.com>, Wei Wang <wvw@...gle.com>
Subject: Re: [PATCH] cgroup: Relax restrictions on kernel threads moving out
 of root cpu cgroup

Hello,

On Tue, Apr 06, 2021 at 08:57:15PM +0530, Pavan Kondeti wrote:
> Yeah. The workqueue attrs comes in handy to reduce the nice/prio of a
> background workqueue if we identify that it is cpu intensive. However, this
> needs case by case analysis, tweaking etc. If there is no other alternative,
> we might end up chasing the background workers and reduce their nice value.

There shouldn't be that many workqueues that consume a lot of cpu cycles.
The usual culprit is kswapd, IO related stuff (writeback, encryption), so it
shouldn't be a long list and we want them identified anyway.

> The problem at our hand (which you might be knowing already) is that, lets say
> we have 2 cgroups in our setup and we want to prioritize UX over background.
> IOW, reduce the cpu.shares of background cgroup. This helps in prioritizing
> Task-A and Task-B over Task-X and Task-Y. However, each individual kworker
> can potentially have CPU share equal to the entire UX cgroup.
> 
> -kworker/0:0
> -kworker/1:0
> - UX
> ----Task-A
> ----Task-B
> - background
> ----Task-X
> ----Task-Y
> Hence we want to move all kernel threads to another cgroup so that this cgroup
> will have CPU share equal to UX.
> 
> The patch presented here allows us to create the above setup. Any other
> alternative approaches to achieve the same without impacting any future
> designs/requirements?

Not quite the same but we already have
/sys/devices/virtual/workqueue/cpumask which affects all unbound workqueues,
so maybe a global default priority knob would help here?

Thanks.

-- 
tejun