linux-kernel - Re: [RFC v2 0/5] cgroup-aware unbound workqueues

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Message-ID: <20190605135319.GK374014@devbig004.ftw2.facebook.com>
Date:   Wed, 5 Jun 2019 06:53:19 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     hannes@...xchg.org, jiangshanlai@...il.com, lizefan@...wei.com,
        bsd@...hat.com, dan.j.williams@...el.com, dave.hansen@...el.com,
        juri.lelli@...hat.com, mhocko@...nel.org, peterz@...radead.org,
        steven.sistare@...cle.com, tglx@...utronix.de,
        tom.hromatka@...cle.com, vdavydov.dev@...il.com,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [RFC v2 0/5] cgroup-aware unbound workqueues

Hello, Daniel.

On Wed, Jun 05, 2019 at 09:36:45AM -0400, Daniel Jordan wrote:
> My use case for this work is kernel multithreading, the series formerly known
> as ktask[2] that I'm now trying to combine with padata according to feedback
> from the last post.  Helper threads in a multithreaded job may consume lots of
> resources that aren't properly accounted to the cgroup of the task that started
> the job.

Can you please go into more details on the use cases?

For memory and io, we're generally going for remote charging, where a
kthread explicitly says who the specific io or allocation is for,
combined with selective back-charging, where the resource is charged
and consumed unconditionally even if that would put the usage above
the current limits temporarily.  From what I've been seeing recently,
combination of the two give us really good control quality without
being too invasive across the stack.

CPU doesn't have a backcharging mechanism yet and depending on the use
case, we *might* need to put kthreads in different cgroups.  However,
such use cases might not be that abundant and there may be gotaches
which require them to be force-executed and back-charged (e.g. fs
compression from global reclaim).

Thanks.

-- 
tejun